Saturday, April 20, 2013 |
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Currently, it seems that most business activity in the cloud is focused on Infrastructure as a Service (IAAS) rather than Platform as a Service (PAAS).
Despite the obvious advantages of PAAS over IAAS, very few enterprises would see economic benefit in redesigning their applications for a PAAS platform. IAAS provides a means to allow them to use their already virtualized environments on someone else’s hardware. Ultimately, cloud computing is about economics. Even those moving to PAAS, for the most part, are treating it as a hosting platform. They are looking for a cheaper way to host, which as I have argued elsewhere is usually not the case.
In both cases, what people are doing is outsourcing the datacenter.
To get the true benefits of cloud computing, whether through IAAS or PAAS, you have to design explicitly for that platform. I have discussed this in many past blog posts. You have to design explicitly for failure, and you have to recognize that you are building a distributed system. Over the long term this approach will bring sustained success.
IAAS as currently applied is a dying niche. Of course any evolutionary extinction takes time. The long time span allows it to be unnoticed, or even if noticed, most people feel it will not affect them. Of course sometimes dynamic changes occur. Ask the dinosaurs, or even the railroads or minicomputer companies. See how the companies that compose the Dow Jones averages have changed over the past 100, 50, 30, or even 10 years.
Companies continue to do things the way they have always done until a successful company creates a new fad in corporate philosophy. Companies then run to become blind followers. One classic example is Deming’s work on modern manufacturing. Although he lived in the US, it was not until the Japanese had success with it that US industry started to adopt his ideas.
Someday, somewhere, somebody is going to build a widely successful company using PAAS. Then, everyone, like lemmings, will run to emulate them. The irony is that some will run to do it even if it makes no economic sense for them.
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Wednesday, July 25, 2012 |
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In order to build robust cloud applications, a client that calls a service has to handle four scenarios:
Complete Success
Partial Success (Success with conditions)
Transient Failure
Resource Failure
A partial success occurs when a service only accomplishes part of a requested task. This might be a query where you ask for the last 100 transactions, and only the last 50 are returned. Or the service only creates an order entry, but does not submit the order. Usually a reason is supplied with the partial success. Based on that reason the client has to decide what to do next.
Transient failures occur when some resource (like a network connection) is temporarily unavailable. You might see this as a timeout, or some sort of error information indicating what occurred. As discussed in a previous post , continually retrying to connect to a transient resource impedes scalability because resources a being held on to while the retries are occurring. Better to retry a few times and if you cannot access the resource, treat it as a complete failure.
With failure, you might try another strategy before you treat the resource access as a failure. You might relax some conditions, and then achieve partial success. You might access another resource that might be able to accomplish the same task (say obtain a credit rating) albeit at greater cost. In any case all failures should be reported to the client. You can summarize this responsibility in this diagram:
Bill Wilder helped me formulate these thoughts.
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Friday, June 08, 2012 |
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We have been talking about cloud failures. How likely are they?
Some outages occur all the time. Even with the excellent reliability of hardware, a cloud data center has an enormous number of components. Something is always failing. Cloud data centers are built to detect failure, and move applications to working hardware, and restart them. They have failure zones so that instances of the same service are kept on different sets of hardware.
What about other kinds of outages?
Amazon and Windows Azure have had interruptions in service, some longer than others. There could be major power outages such as in the Northeast United States in 1965 that left people without power for up to 12 hours. In 2003 there was a major power outage in the Northeastern and North central US as well as Ontario, Canada. The Japanese tsunami had a similar effect. Many smaller outages occur after storms. Even if a data center remains, external Internet connections might be interrupted. In 2009 Google had a major outage in Asia caused by a configuration error that caused problems even in the United States and Europe. The problem was analogous to the cause of the 1965 Northeast US power failure.
As we have discussed in previous posts, any software or service that you depend on is a possible source of an outrage, including the Internet itself. You don’t even need an outage; all you need is for a service to become less responsive. Remember when Michael Jackson died? It was difficult to get to any web site, because there was not enough bandwidth to accommodate everybody’s surfing to find out the news. It was the largest self-inflicted denial of service attack ever.
Are these outages really rare?
As we found out with the financial sector, black swan events can happen. Random events do occur. Small probability events happen. Kahneman and Tversky demonstrated that people reason about probability poorly. While this can be acceptable behavior with regard to personal decisions, it is very questionable when it comes to estimating the probabilities of rare engineering events
You cannot assume that any connection to a distributed service will always be available.
Netflix’s continual availability during the April 2011 Amazon outage is now legendary. The reason for their success was because they assumed failure was possible. They had stateless services. They restricted the use of relational data to where it was really necessary so they could switch to a hot standby. They degraded gracefully, only keeping alive services that were really necessary. You might not have been able to get your personalized movie list, but you could still find and play movies. They had enough excess capacity to deal with transient failures, and shifting loads.
Assume the rare can occur, because it will.
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Thursday, May 24, 2012 |
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To avoid a single point of failure you needed to have redundancy.
There are several power lines between a power station and a customer. When an overload is detected by a circuit breaker, power can be rerouted along another line. Each individual line is normally run at reduced capacity so it can handle the increased load.
Redundancy, however, introduces problems. Business applications have state. One electron is as good as another and can be easily rerouted; application state is tied to particular users, sessions, or transactions. I need to access parts of my current order. If the network connection I am on gets broken, substituting someone else's order does not work. Parallel processing of mathematical algorithms (such as MapReduce, or multi-threaded versions of Linq) cannot solve this problem.
You need to remember information about your users, or what products are on order. Some of this data changes rarely if at all; some of this data changes dynamically. Some of this data is very valuable, some of it is not. In a non-cloud application you might use data base mirroring. But you pay a performance penalty for a database mirror because the transaction must commit on both the original and the mirror. Imagine the cost (latency and throughput) of trying to keep redundant databases in different geographic areas in sync. Hence, you must deal with state explicitly.
You have to reduce the parts of your application that handle state to a minimum. Loss of a component means the loss of application state that it holds.
This is classic advice for scaling systems, but is more critical in the cloud. In the past we created stateless middle tier components for easier scalability, and relied on the data tier to handle scale. You then rely on clusters or technology built into the database (transaction logs, etc.). While stateless components are still an excellent idea, we cannot always rely on the database to scale, and the possibility of database failure is a large risk.
The business layer and the domain layer should be stateless, and to the greatest extent possible the clients should hold whatever state they can. Stateless services can be added or removed easily to handle changes in demand or the failure of a service instance, since you have decoupled functionality. Try to build services that are as atomic as possible because that makes it easier to scale or recover from failure by using redundancy. Here, atomic does not mean small.
Suppose you put customer information in the same service as your catalog service. If the customer information service goes down, so does the catalog. You should be able to check in people to your flight even if you cannot give the prices for tomorrow's flights.
There are other techniques you can use. For example, if the bill paying service is not available, you can usually use a transactional queue, and just retry the payment until the service is available. That is one of the reasons why banks, for example, say that they need 24 hours for an electronic payment that could be processed immediately. The odds that the payment service will be down for 24 hours are miniscule.
Also don't forget infrastructure pieces as to where logs are stored. You should probably back them up frequently.
So what about where state has to be stored, say in the data tier? This is a complicated problem which I have discussed this before in both a blog post and a presentation . It basically boils down to the question: what is the acceptable level of data loss? As it turns out, in many applications you do not have to be absolutely consistent in all places, and you can relax consistency constraints to get scalability and reliability. Now you may think this is ridiculous, data can never be lost. But think about how business is actually done.
Airline reservation systems separate out the flight query database, from the transactional database where flights are booked. As a result, occasionally a flight or a price you thought was available is not. But if they did not do that, the performance of making reservations would be very poor. More business would be lost under strict consistency than under relaxed consistency constraints. Why does Amazon use an email system for notifying you of your book order? To scale the user interface and to avoid performance problems due to abandoned shopping carts. You have to ask the question: What is the cost of an apology for the data loss?
What kind of availability are your customers actually willing to pay for, as opposed to what they say they want? Given a choice, do they really want absolute consistency in all cases? If you used replication over a short period of time to another data center, and a data center was destroyed by a hurricane, and a certain amount of data was lost, how terrible would that be?
Consider grouping your components in units of failure. If you have components of type α, β, and γ, you could put all α on a single host, all β on a separate host, or all γ on a separate host. Or you could put groups of α, β, γ on separate hosts. The latter would cause a complete failure on a loss assuming affinity to a given machine. The former would only result in a failure of α functionality.
What do you do if your components fail? Do you have to reroute traffic to another data store, or data center? Do you need to add more instances of a component? You have to monitor your components and understand why they fail. Under certain circumstances, you can degrade performance. Caching might help in keeping your application running under failure.
Degrade gracefully and predictably. Know what you can live without.
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Sunday, May 20, 2012 |
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Designing for Failure has been around a lot longer than cloud computing. As we have discussed in several other blog posts, cloud computing, as opposed to hosting in the cloud, is about the ability to acquire or release computing resources as necessary. Acquiring more resources allows you to keep up with demand, or to compensate for a failed instance of a resource.
You must examine every source of dependency in your application: third party libraries, hardware, software interfaces between parts of your own application, TCPIP ports, DNS servers, message queues, database drivers, database size, latencies, to name just a few. These include third party services such as credit card processors, fraud detection services, and geocoding services.
You also have to examine your queries because small queries can become large overnight as you scale. This is why search providers limit the result set that they return. See what kind of joins your ORM is producing when it handles inheritance. Look at the number of objects coming back from a DCOM or RMI call.
Any one of these could fail, or cause latency. As we discussed in a previous post, any potential long latency has to be treated as a potential failure. You need to avoid single points of failure because they are potential bottlenecks or failure points.
Acquiring more resources costs more money. So every strategy is a tradeoff between keeping the application responding (available and scalable) and how much it costs. This is driven of course, by what customers are willingness to pay. Every strategy has to undergo a cost benefit analysis.
The more you approach 100% availability, however, the more the law of diminishing return sets in. I cannot here tell you which problems you should solve, and which you can safely ignore. It depends on your application and your customers. I can tell you that every component is a potential source of failure.
Avoid single points of failure. Accept the fact that you have to build a distributed system.
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Sunday, May 13, 2012 |
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You have to make sure your cloud application is not brittle. Make your components more resistant to failure. Bridges can withstand more traffic than their largest anticipated load. Since you can add and remove resources in a cloud computing environment, your margin of safety can expand or contract as your load expands or contracts. Nonetheless, adding and removing resources is not instantaneous. You have to make sure that your system can handle a "normal load".
How do you determine your margin of safety?
Look at every resource you use in the system: database sizes, bandwidth, virtual memory, CPU, network latencies, and the response times of your software and your third party components. See how they respond under various types of commands, reports, and queries over time.
Because of the economic costs, and possible performance hits with handling failure, you want to ensure your application in its normal state of operations can handle the load. You might want to factor in some likely scenarios, for instance, and make the resources required larger than might be ordinarily needed.
Make sure all errors are handled, even unlikely ones. Return clear error codes that indicate what the problem is to the best of your ability. When problems occur, you might degrade performance rather than eliminate functionality. Determine what functionality is essential and what is not. During the Amazon outage last year Netflix turned off personalized movie lists, but you could still get lists of movies and play them.
Make reasonable SLA promises to your customers. So the UI can scale properly, Amazon sends confirmation emails for book orders.
A chain is as strong as its weakest link. If your web front end has limited capacity, or you run out of TCP/IP ports, it does not matter how strong your database server is.
Use a Margin of Safety when determining the resources needed for your application.
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Sunday, March 18, 2012 |
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In the simple example we have been discussing, the consequences of a failure appear immediately to the user. In a more complicated architecture there are many more tiers
and many more dependencies. With more dependencies, more problems can possibly result from poor decisions on how to handle failure. Those dependencies include
other applications in your
own shop, third party libraries that you don't control, the internet, etc. For example, if your order queues fail, you cannot do orders. If your customer service app fails,
you cannot retrieve member information. Unhandled failures propagate (like cracks) throughout your application.
Failures Cascade - a unhandled failure in one part of your system becomes a failure of your application.
In deciding on how to respond to failure we have to distinguish between two types of failure: transient failures and
resource failures. Transient failures are not due to component failure,
but a resource temporarily under load that cannot respond as fast as you had assumed. With resource failures you have to have an alternative strategy because a component is not available.
Transient failures occur for short periods of time. The typical response is to retry the operation after a short period of time. But questions still remain. How often do you retry?
What is a short period of time? What do you with the data during the retry? On the other hand, remember that just as failures cascade, so do delays. While you are waiting or retrying,
scarce resources are being used (threads, memory, TCPIP ports, database connections) that cannot be used for other requests.
Go back to our
WCF example and look at the try/catch block. If we had to do a retry
how would we change the logic? We will have to adopt a whole different strategy to handle failure, a retry loop inside the catch handler would not be enough because other failures could
reoccur, and not every error would allow a retry. You have to design the entire routine around expecting failure, because as we discussed
in our last post, failures happen.
Since slow responses usually come from resource bottlenecks, you have to treat them as failures if you are going to have reasonable availability. Which means that transient
failure can soon look like resource failures. So what do you do? Retry for a limited amount time and then give up. In addition, never block on an I/O, timeout and assume failure.
From the point of view of architecture and design, there is really no such thing as a transient failure. If you have a transient failure, fail
fast and treat it as a resource failure.
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Friday, March 09, 2012 |
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Why is failure endemic to distributed systems?
In the past two blog posts we talked about a hypothetical ASP.NET application. Let's add a second tier to this app where we make a call to a web service.
We will then have some version of the following code fragment which resembles something everybody has written:
ClientProxy client = new ClientProxy();
int result = client.Do(a, b, c);
What's wrong with this?
We have assumed that the call would succeed. Why would it not succeed? At the very minimum you could have a network timeout.
You are assuming you have control over a resource that your really do not.
The fundamental concept in designing for failure is to understand that any interface between two components can fail.
So we rewrite the code as follows:
try
{
ClientProxy client = new ClientProxy();
int result = client.Do (a, b, c);
}
catch (Exception ex)
{
????
}
But now what do you do in the exception handler?
In this simple example, how many times do you retry?
When you give up do you cache the input, or do you make the user enter it over again?
Suppose the service on the other side stopped working? What happens when the underlying hardware crashes, and your application has to be restarted.
Where is the user data then?
What about total failure conditions? Do you "go to" out of the exception handler?
Where do you go to?
You cannot program your way out of a failure condition in code that is
based on the assumption that everything works properly. You have to architect
and design for failure conditions from the start.
The critical issue is how you respond to that failure.
Here is the fundamental principle of designing for failure:
Assume failure will occur. The question is how will the application respond to that failure. You cannot depend on the underlying infrastructure to achieve availability because it cannot make that guarantee.
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Monday, February 27, 2012 |
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Continuing out examination of hosting options from the last post, perhaps, the cheaper options have lower reliability. Here are the availability numbers for our providers:
| Provider |
Compute SLA (%) |
| Go Daddy |
99.9 |
| ORCS Web |
99.9 |
| Host Gator |
99.9 |
| Rackspace |
Repair within one hour |
| Amazon |
99.95 |
| Azure |
99.95 |
If Amazon and Azure are more expensive with a similar SLA, why use them?
Availability numbers do not tell the whole story.
The rate for cloud computing infrastructure is more expensive because it allows you to pay only for what you use. Not only does this allow you to use computing resources more economically, it allows you to design around outages. With a cloud computing infrastructure, you can reach, if you wish to pay for it,
very close to 100% availability.
This gets to the essence of the matter. You are doing cloud computing when you are interested in one of two things:
-
Paying for only the computing resources you use so you do not have to buy
enough hardware for peak scenarios that happen infrequently.
-
You want to achieve very high reliability, with almost no downtime. You have to design for failure.
We often refer to these goals as scalability and availability. Scalability is
making sure your application can handle increase load with reasonable
performance. Availability is making sure your application has reasonable
performance for a reasonable amount of time. What is reasonable, depends on the
economics of your business environment.
Technically they are different problems. Depending on your system load, you
could have reasonable availability for the vast majority of the times your
customer wants, and just be very unresponsive under very heavy loads. Or,
you could handle very large loads for small periods of time, and be very
unresponsive the remainder of the time.
But for most applications they are closely related.
To have high scalability requires you not only to be able to acquire more
computing resources, but it requires you to be able to detect and handle
failures of existing computing resources. High availability requires the
same thing.
Designing for failure is cloud computing.
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Thursday, January 12, 2012 |
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Lincoln said that calling a tail a leg does not make a horse a five legged animal. Hosting an application in a cloud computing environment does not mean that you are doing cloud computing.
One of the reasons that people are attracted to cloud computing is because they do not have to host their own infrastructure or run a data center. But that is true of traditional hosting. Now the feature sets of a cloud computing environment may make you want to host there, but that is only an enlightened hosting decision.
Let's make this clear with an example.
Suppose I decide that I do not want to run my traditional ASP.NET application in my data center. I have a web front end, and a back-end database.
Potentially, I have to pay for:
Hosting the application (pay for the machines virtual or otherwise that I need)
Local File storage
Database (relational or otherwise)
Bandwidth (in and out of the data center)
Let's assume this translates to the following configuration:
1 instance of a Web Site
Virtual Machine or Equivalent Hardware
One 1 GB SQL Server
Inboud and Outbound Bandwidth Costs
File System Costs
Researching some of the Service Providers I found the following costs for this configuration:
|
Provider |
$/ Monthly Cost |
|
Go Daddy |
10 |
|
Orcs Web |
69 |
|
Host Gator |
70 |
|
Rackspace |
117 |
|
Amazon |
86 + Bring Your Own SQL Server |
|
Windows Azure |
102 |
None of these providers are traditional hosters (provide your own machines, and
rent bandwidth, cooling, or electricity). Nor are ORCS Web's and Rackspace's
offerings to build a colocated data center considered.
Virtualizing your data center is not considered here as cloud computing because you still have to build out to maximum capacity. Nontheless, it might
be considered cloud computing from the point of view of your internal users if they can get resources elastically.
So we see there are cheaper options if you just want to host. Now if you want to some feature that one hosting company has that the other does not,
say blob storage, you could use that in conjunction with your host (say Amazon or Azure blobs).
Hosting is hosting no matter where you do it. Hosting an application in a cloud computing environment does not mean you are
doing cloud computing.
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Thursday, October 27, 2011 |
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InformIt has published my article on Azure Diagnostics. I explain how to use Azure diagnostics to audit, log, and monitor the health of your applications.
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Tuesday, December 14, 2010 |
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Windows Azure provides two storage mechanisms: SQL Azure and Azure Storage tables. Which one should you use?
Can Relational Databases Scale?
SQL Azure is basically SQL Server in the cloud. To get meaningful results from a query, you need a consistent set of data.
Transactions allow for data to be inserted according to the ACID principle: all related information is changed together.
The longer the database lock manager keeps locks, the higher the likelihood two transactions will modify the same data. As transactions wait for locks to clear, transactions will either be slower to complete, or transactions will time out and must be abandoned or retried. Data availability decreases.
Content distribution networks enable read-only data to be delivered quickly to overcome the speed of light boundary. They are useless for modifiable data. The laws of physics drive a set of diminishing economic returns on bandwidth. You can only move so much data so fast.
Jim Gray pointed out years ago that computational power gets cheaper faster than network bandwidth. It makes more economic sense to compute where the data is rather than moving it to a computing center. Data is often naturally distributed.
Is connectivity to that data always possible? Some people believe that connectivity will be always available. Cell phone connectivity problems, data center outages, equipment upgrades, and last mile problems indicate that is never going to happen.
Computing in multiple places leads to increased latency. Latency means longer lock retention. Increased locked retention means decreased availability.
Most people think of scaling in terms of large number of users: Amazon, Facebook, or Google. Latency also leads to scalability based on geographic distribution of users, transmission of a large quantity of data, or any bottleneck that lengthens the time of a database transaction.
The economics of distributed computing argue in favor of many small machines, rather than one large machine. Google does not handle its search system with one large machine, but many commodity processors. If you have one large database, scaling up to a new machine can cost hours or days.
The CAP Theorem
Eric Brewer’s CAP Theorem summarizes the discussion. Given the constraints of consistency, availability, and partitioning, you
can only have two of the three. We are comfortable with the world of single database/database cluster with minimal latency where we have consistency and availability.
Partitioning Data
If we are forced to partition our data should we give up on availability or consistency? Let us first look at the best way to partition, and then ask whether we want consistency or availability.
What is the best way to partition?
If economics, the laws of physics, and current technology limits argue in favor of partitioning, what is the best way to partition? Distributed objects, whether by DCOM, CORBA, or RMI failed for many reasons . The RPC model increases latencies that inhibit scalability. You cannot ignore the existence of the network. Distributed transactions fail as well because once you get beyond a local network the latencies with two-phase commit impede scalability.
Two better alternatives exist: a key value/type store such as Azure Storage Services, or partitioning data across relational databases without distributed transactions.
Storage Services allow multiple partitions of tables with entries. Only CRUD operations exist: no foreign key relations, no joins, no constraints, and no schemas. Consistency must be handled programmatically. This model works well with tens of hundreds of commoity processors, and can achieve massive scalability.
One can partition SQL Azure horizontally or vertically. With horizontal partitioning we divide table rows across the database. With vertical partitioning we divide table columns across databases. Within the databases you have transactional consistency, but there are no transactions across databases.
Horizontal partitioning works especially well when the data divides naturally: company subsidiaries that are geographically separate, historical analysis, or of different functional areas such as user feedback and active orders. Vertical partitioning works well when updates and queries use different pieces of data.
In all these cases we have to deal with data that might be stale or inconsistent.
Consistency or Availability?
Ask a simple question: What is the cost of an apology?
The number of available books in Amazon is a cached value, not guaranteed to be correct. If Amazon ran a distributed transaction over all your shopping cart orders, the book inventory system, and the shipping system, they could never build a massively scalable front end user interface. Transactions would be dependent on user interactions that could range from 5 seconds to hours, assuming the shopping cart is not abandoned. It is impractical to keep database locks that long. Since most of the time you get your book, availability is a better choice that consistency.
Airline reservation systems are similar. A database used for read-only flight shopping is updated periodically. Another database is for reservations. Occasionally, you cannot get the price or flight you wanted. Using one database to achieve consistency would make searching for fares. or making reservations take forever.
Both cases have an ultimate source of truth: the inventory database, or the reservations database.
Businesses have to be prepared to apologize anyway. Checks bounce, the last book in the inventory turns out to be defective,
or the vendor drops the last crystal vase. We often have to make records and reality consistent.
Software State is not the State of the World
We have fostered a myth that the state of the software has to be always identical to the state of the world. This often makes
software applications difficult to use, or impossible to write. Deciding what the cost of getting it absolutely right is a
business decision. As Amazon and the airlines illustrate, the cost of lost business and convenience sometimes offsets the occasional problems of inconsistent data. You must then design for eventual consistency.
Summary
Scalability is based on the constraints of your application, the volume of data transmitted, or the number and geographic distribution of your users.
Need absolute consistency? Use the relational model. Need high availability? Use Azure tables, or the partitioned relational
model. Availability is a subjective measure. You might partition and still get consistency.
If the nature of your world changes, however, it is not easy to shift from the relational model to a partitioned model.
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Monday, November 22, 2010 |
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Determining how to divide your Azure table storage into
multiple partitions is based on how your data is accessed. Here is an example
of how to partition data assuming that reads predominate over writes.
Consider an application that sells tickets to various
events. Typical questions and the attributes accessed for the queries are:
| How many tickets are left for an event? |
date, location, event |
|
What events occur
on which date?
|
date, artist,
location |
|
When is a particular
artist coming to town?
|
artist,
location |
|
When can I get a
ticket for a type of event?
|
genre
|
|
Which artists are
coming to town?
|
artist, location |
The queries are listed in frequency order. The most common
query is about how many tickets are available for an event.
The most common combination of attributes is artist or date
for a given location. The most common query uses event, date, and location.
With Azure tables you only have two keys: partition and row.
The fastest query is always the one based on the partition key.
This leads us to the suggestion that the partition key
should be location since it is involved with all but one of the queries. The
row key should be date concatenated with event. This gives a quick result for
the most common query. The remaining queries require table scans. All but one are
helped by the partitioning scheme. In reality, that query is probably location
based as well.
The added bonus of this arrangement is that it allows for geographic
distribution to data centers closest to the customers.
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Tuesday, September 28, 2010 |
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Popular consciousness creates popular myths. Here are some myths about cloud computing.
1. Total reliance on the cloud is foolish or scary.
So is total reliance on the Internet or the electric grid, or the transportation network to get us our food. In fact, I imagine someone in 4000 BCE said: dwelling in cities is dangerous, and we should not let people farm to support them. Come to think of it, people are still saying it.
Dependency is a fact of life. It has been a fact of human existence since the first division of labor. Nonetheless, we should have contingency plans. Any organization that hosts any application should understand what the impact of an outage would be. It might be the cloud application itself is down, or the Internet connectivity is slow. Your contingency plan does depend on the nature of your application. After all, hospitals still have emergency generators for surgery, and we store a couple of day’s food in the refrigerator.
On the other hand, maybe you did build your own house, or sew your own clothing. Perhaps a day without email or applications (i.e. the Sabbath) might be a good idea after all.
Dependency, by itself is not an argument against Cloud Computing. It is the consequences of that dependency that matter. For most applications, even some of those considered the most critical; we could actual do without for a few hours.
2. Security is better/worse in the cloud
Data in the cloud is insecure. Data in the cloud is more secure. Nothing is quite like security for generating fear and myths.
The first question you always have to ask is: secure compared to what? Fort Knox? Money in your mattress? After all, the most secure computer is disconnected from the Internet. If you are really paranoid you can turn it off. Of course, it is now difficult to get any work done.
Is data in the cloud more or less secure? Is it secure compared to a corporate data center? There certainly have been some well publicized incidents of corporate data breaches. There are probably even more cases that have not been reported. Have there been any incidents in a cloud computing center? None yet, but there will be. If there are, they might be the fault of the application designers or owners. The same people who create insecure applications in their own data centers can certainly create them in the cloud. Cloud computing centers might be able to better focus on security (physical, data, and application) because that is part of their expertise.
On the other hand, with all that computation and storage focused in one place, people fear that cloud computing data centers may be an inviting target for attack. Employees of cloud computing centers may snoop. So can employees of a corporate data center. Will industrial espionage be easier in a cloud computing center? I am just waiting for the movie. Perhaps you are safer with people who specialize in keeping data centers secure, than a lot of smaller data centers. Bank robberies are not as frequent as they used to be.
Cloud computing centers may lack compliance certification, and that is a problem. On the other hand, as Berkeley researchers have
argued, cloud computing may make Denial of Service attacks economically unfeasible.
It is also currently unknown if security breaches in one virtual machine can cause a compromise of the underlying physical hardware.
As with any hosted application, the builders of the application share responsibility with the cloud providers. You might want to investigate how well capitalized, and what the security plans of your provider are.
The best security is to park your bicycle next to a better bicycle with a worse lock.
3. Cloud is reliable / unreliable
My electric utility only gets
99.98% uptime. So much for the vaunted four nines. How much uptime does Facebook really need? You need to understand exactly what your application requirements are, and the consequences of failure.
I do not know of a single cloud computing vendor that offers a service level agreement with real remediation in case of an outage. Don’t forget that as with any hosted application you are still subject to the vagaries of the external network connections. The data center may be fine, but when Michael Jackson died, the response time of the Internet slowed to a crawl: nothing like a self-inflicted denial of service attack.
Given the current fetish over net neutrality, the packets carrying the output of your pacemaker to your cardiologist get the same priority as someone streaming Lady Gaga’s latest hit.
First define the reliability requirements that your application needs, then decide the appropriate course of action.
4. Cloud computing requires no social infrastructure
Suppose your cloud computer provider goes bankrupt, and the machines are seized as collateral for the debt. What happens to your applications and data? We may need an FDIC-like organization to handle cloud computing provider insolvencies. Some regulation would need to be in place to handle continuity of service during takeovers.
The economies of scale may lead us down the same road that the electrical utilities and the water companies went. The small scale providers were eventually taken over by the larger providers and the resulting monopolies were regulated.
Companies, such as financial services, that operate in heavily regulated industries will be reluctant to use cloud computing providers unless there is some clarity to their legal responsibility for data in the cloud. On the other hand, Microsoft is selling its cloud computing fabric so that third parties might set up private clouds for various industries. Whether they are true computing clouds or just hosting services with flexible virtualization would depend on the actual scaling potential of the data center.
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Wednesday, September 15, 2010 |
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"Government, without popular information, or the means of acquiring it, is but a Prologue to a Farce or a Tragedy; or, perhaps both. Knowledge will forever govern ignorance."
James Madison
What is it?
Control over information is a societal danger similar to control over economic resources or political power. Representative government will not survive without the information to help us create meaningful policies. Otherwise, advocates will too easily lead us to the conclusion they want us to support.
How does one get access to this data?
Right now, it is not easy to get access to authoritative data. If you have money you search for it, purchase it, or do the research to obtain it. Often, you have to negotiate licensing and payment terms. Why can’t we shop for data the same way we find food, clothing, shelter, or leisure activities? None of these activities requires extensive searches or complex legal negotiations.
Why can’t we have a marketplace for data?
Microsoft Dallas is a marketplace for data. It provides a standard way to purchase, license, and download data. Currently it is a CTP, and no doubt will undergo a name change, but the idea will not.
The data providers could be commercial or private. Right now, they range from government agencies such as NASA or the UN to private concerns such as Info USA and NAVTEQ. You can easily find out their reputations so you know how authoritative they are.
As a CTP there is no charge, but the product offering will have either transaction/query or subscription based pricing. Microsoft has promised “easy to understand licensing”.
What are the opportunities?
There is one billing relationship in the marketplace because Microsoft will handle the payment mechanisms. Content Providers will not have to bill individual users. They will not have to write a licensing agreement for each user. Large provider organizations can deal with businesses or individuals that in other circumstances would not have provided a reasonable economic return. Small data providers can offer their data where it would have previously been economically unfeasible. Content Users would then be able to easily find data that would have been difficult to find or otherwise unavailable. The licensing terms will be very clear, avoiding another potential legal headache. Small businesses can create new business opportunities.
The marketplace itself is scalable because it runs on Microsoft Azure.
For application developers, Dallas is about your imagination. What kind of business combinations can you imagine?
How do you access the data?
Dallas will use the standard OData API. Hence Dallas data can be used from Java, PHP, or on an IPhone. The data itself can be structured or unstructured.
An example of unstructured data is the Mars rover pictures. The Associated Press uses both structured and unstructured data. The news articles are just text, but there are relationships between various story categories.
Dallas can integrate with the Azure AppFabric Access Control Service.
Your imagination is the limit.
The standard API is very simple. The only real limit is your imagining the possibilities for combining data together.
What kind of combinations can you think of?
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Wednesday, August 18, 2010 |
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Microsoft has published my five part introduction to the basics of partitioning and layering a software application. While there is a great deal of discussion about it in the literature on intermediate and advanced topics on software development, I have never found a good introduction that discusses the essentials. So I wrote one.
You can find it on the Microsoft's Visual C# Developer Center.
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Sunday, July 11, 2010 |
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Commodity hardware has gotten very cheap. Hence it often makes more economic sense to spread the load in the cloud over several cheap, commodity servers, rather than one large expensive server.
Microsoft's Azure data pricing makes this very clear. One Gigabyte of SQL Azure costs about $10 per month. Azure table storage costs $0.15 per GB per month.
The data transfer costs are the same for both. With Azure table storage you pay $0.01 for each 10,000 storage transactions.
To break even with the SQL Azure price you can get about 9,850,000 storage transactions per month. That is a lot of bandwidth!
Another way to look at the cost is to suppose you need only 2,600,000 storage transactions a month (1 a second assuming an equal time distribution over the day). That would cost you only $2.60. That means you could store almost 50 GB worth of data. To store 50 GB worth of data in SQL Azure would cost about $500 / month.
If you don't need the relational model, it is a lot cheaper to use table or blob storage.
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Thursday, December 24, 2009 |
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Thursday, November 05, 2009 |
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One of my clients,
ITNAmerica has become a Microsoft case study for the idea of software +
services. The idea behind software + services is that software should run where
ever it makes sense: in the cloud, on the desktop, or on a mobile device, not
just in a thin client such as a browser.
Latency, bandwidth
limits, and the need for software to
work if the connection to the cloud disappears makes this a logical
approach. Anybody who has tried to get a
cell phone signal should understand the issues about continual connectivity.
Curt Devlin, a
Microsoft evangelist, demonstrates another reason why this approach makes
sense. It makes the transition to a cloud provider such as Azure much simpler.
If you want some
further ideas on how to take a software + services application to a cloud
platform. Check out my recent ARCast on "Software + Services in the
Cloud."
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Wednesday, November 04, 2009 |
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The application delivery scenarios focus around software as
a service. Software as a service applications fall into three varieties: pure
service, and software + service, hosted application.
The hosted application scenario is similar to hosted
application delivery. Examples are SalesForce, or Hosted Microsoft Exchange.
People provide or buy an application that runs in the cloud. At the other
extreme is the pure service play. Providers create web services (SOAP or REST
based) that provide services used by other applications. Examples are credit
card approvals, or certain loan applications. Applications written by third
parties use this software to compose their applications in conjunction with
their own software. Then there is the mixed play. Providers create both web applications
and web services to be used by third parties. These applications consume the
same web services that are available to others to build their own applications.
This is often done to allow the provider to share the web services among
various offerings, or because they need to boot strap the application
marketplace. The need for rich clients does not necessarily disappear here. If
applications (such as emergency services) have to run with loss of internet
connectivity, stand alone apps may be necessary with synchronization software
used when connectivity is re-established. Transactional queuing is not enough
here because substantive work has to be done by the rich client app when
connectivity is absent.
Internet scale is the last class of application. The first
scaling factor is number of users. In order to achieve such scale you may to
use cloud features such as tables (Google Big Table, Azure Tables, Amazon
Simple DB) instead of or in addition to relational databases. Note that
transactional guarantees are often impossible to make here. The second scaling
factor is geographic distance. If your clients are geographically separated by
enough distance, the latency caused by the speed of light in fiber optic cable actually matters. You
may have to use the cloud features mentioned previously to achieve the responsiveness
for writeable data because transactions, especially distributed transactions
are not feasible to achieve scalability.
The next post in the series will start to discuss the architectural implications of these different types of applications.
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Wednesday, October 28, 2009 |
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Cloud computing is utility computing. No up front commitment
required. You buy only what you need, and when you do not need it any more you
do not pay for it.
There are three basic cloud computing scenarios:
infrastructure scenarios, application delivery scenarios, and scaling
scenarios. These scenarios are not independent, one or all of them can come into
play. Each, however, has different
technological implications.
The three basic scenarios are: infrastructure, application
delivery, or the need to reach internet scale.
Fundamentally, cloud computing is a software delivery
platform. Are the economics of working with the cloud cheaper than doing it
yourself? Doing it yourself could mean self-hosting, or traditional delivery of
desktop software. Self-hosting could be in your own data center, or in a
hosting facility.
Not needing to build to your peak capacity drives the
infrastructure scenarios. This is not an all or nothing proposition.
Some small and medium sized companies may decide they do not
want to run their own data centers. The savings in terms of not having to buy
machines and pay employees is enormous. This money could be put to use in
building better applications. This might be the entire compute infrastructure,
or just running an email server.
Other companies may have an occasional need for massive
computation. Say you have to do a stress analysis of a new airplane wing, or a
geographical routing of a complicated delivery, decide among alternative new financial models, or even a human genome search. Any of the classic grid
computations fall into this category. Your existing infrastructure is just fine, but for these not
every day scenarios (they might actually be frequent) it makes sense to rent
space in the cloud to do the computations.
A related scenario is cloud-bursting. You can handle your
everyday computing demands, but occasionally you get a burst of orders that
overwhelms your system. Ticket agencies are a classic example when tickets for a popular event first go on sale. So are stores
around the holidays. Here you use the cloud to handle the overflow so that
people wanting to order do not get unresponsive web pages, or busy signals on
the telephone.
Small divisions in large companies may find the cloud appealing
for prototyping, or even developing certain applications. Their central IT may
be unresponsive or slow to respond to their needs. It is well within the
capacity of a departmental budget to rent space in the cloud.
The next post will explore the other two scenarios, and look
at how the various vendor options would meet your needs.
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Tuesday, October 06, 2009 |
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Tuesday, August 11, 2009 |
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Microsoft has yet to release all the details of its Azure SLA, but it has said that you will have a 99.95 per cent up-time for compute and 99.9 per cent up-time for SQL Azure.
How does this compare with my electric utility?
With my latest electric bill, my local utility listed its 2008 average number of service interruptions per customer as 1.051, and the average number of minutes without power for a customer at 78.55 minutes. So my electric utility has an up-time of .9998. I guess they don't get 4 or 5 "9"s either.
I presume these numbers include outages due to winter storms, but I do not know what the utility regulators allow them to exclude. Microsoft, to my knowledge, has not stated whether the SLA percentages include planned downtime for upgrades.
How many outage minutes per year could we expect with Azure under the SLA? That comes to about 262.8 compute minutes per year, or about 4.36 hours. Of course when those outages occur matters, and whether they are concentrated in one or many interruptions.
For SQL Azure that SLA is on a per month basis. So for data you could loose access to it for 43.8 minutes per month.
Is 4 hours a long time? Could you live without data access for 45 minutes a month?
For Facebook probably, for emergency services you would need some sort of fallback just like they have backup generators now.
I wonder what a cloud computing brownout looks like?
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Sunday, July 05, 2009 |
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I just did an interview on .NET rocks about cloud computing.
We covered a whole bunch of topics including:
what is cloud computing
comparing the various offering of Google, Force.com, Amazon, and Microsoft
the social and economic environment required for cloud computing
the implications for transactional computing and the relational model
the importance of price and SLA for Microsoft whose offerring is different from Amazon and Google
the need for rich clients even in the world of cloud computing.
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Wednesday, June 24, 2009 |
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One of the big advantages of cloud computing is its utility computing model. Customers can use as much compute power or as little as they want without paying for what they do not need. Normally, most data centers have to be built for peak demand, with the servers unused when they are not needed.
Utility computing is based on the electric utility model. While this comparison has a lot of merit, there is one particular part of the analogy that really does not work.
Data are not electrons.
If someone steals some of your electric power by diverting it, you can get replacement power. If one part of the country's electric demand exceeds its generating ability, it can get power from another part of the grid. One electron is as good as another.
Data has identity, latency, and relationships to other pieces of data.
If someone steals your data, another piece of data cannot take its place. if your data is stolen, or even delayed it, can aversely affect you. Depending on your resolution of the CAP Theorem dilemma, your replication strategy might leave you with a window of vulnerability for data loss.
Curiously, the argument has been made that the utlity computing model makes denial of service attacks unfeasible because the economics of trying to get enough bot driven computers to assualt a hugh data center is prohibitive. Sooner or later, somebody is going to try to get the servers of one data center to attack the servers of another data center. Hopefully, the software that monitors the transactions would realize that somebody is exceeding their credit limit. |
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Tuesday, June 23, 2009 |
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It's time for me to be interviewed on .NET Rocks again!
Carl and Richard will interview me about Cloud Computing. The interview will be published on June 30 at http://www.dotnetrocks.com/.
Based on my previous show (and related DNR TV segments) it will be a lot of fun to do and to listen to.
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Wednesday, May 27, 2009 |
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Many people have misconceptions about cloud computing. For example, applications do not have to be built so they are all in the cloud. You can put the application in the cloud (to handle parallel computation), and have the database in your enterprise. I was interviewed at TechEd about some of the misconceptions about computing in the cloud. Other misconceptions discussed include what size business is right for the cloud, the role of the browser, guaranteed connectivity, and cloud security. |
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Wednesday, May 20, 2009 |
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Small or medium sized companies can have the advantages of being able to act as a big company while maintaining the advantages of being small.
A hosted solution has many advantages.
You no longer need the staff, or have to spend money on installing and upgrading software on your clients' machines. Your customers and clients can use your application anywhere, not just on their office computers. If you provide services as well as an application, third parties can easily use your solution as part of their offering. Sometimes these services can be used in your own applications such as portals, or future applications. Perhaps your customers can extend your application making it more valuable to them. Having your application in the cloud means that your intellectual property (your secret sauce) is better protected because it is not in the hands of your users.
All these arguments also apply to small business units within a large enterprise.
Nonetheless, small businesses very often do not have the financial ability to economically run, or even rent a significant hosted application solution beyond a small scale web application.
Cloud computing offers a way out of the dilemma.
Cloud computing offers businesses a utility model for computation. Host your application on a cloud platform and you pay only for what you use. With minimal initial investment, you can scale up or down as your customers use more or less of your application or services.
With many cloud vendors (Amazon being a major exception) you do not even know on what infrastructure your machine runs on. Scaling and failover happen in those environments with minimal work on the client's part.
Clearly the cost and reliability of the cloud provider is crucial. Google's most recent outage shows that this is not a unreasonable fear. Private IT centers also have had their outages, but they are not made public.
Microsoft, Amazon, Google and others are spending huge amounts of money to build cloud data centers. Clearly they see the opportunity.
Right now many large companies already have data centers that can offer cheaper compute power than the current generation of cloud providers. This will eventually change.
But right now, small companies, start-ups, and other similar organizations should think about cloud computing for their hardware infrastructure. |
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Friday, January 23, 2009 |
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I will be speaking at VSLive! San Francisco on February 25 on "Advanced Topics in Windows Workflow Foundation".
The conference will be at the Hyatt Regency Embarcadero from February 23-27. Workshops are offered on Feb 23 and 27. The conference sessions are on Feb 24, 25 and 26. If you register with promo code NS9F20 you will receive a $500 discount off the price. The event web site is vslive.com/2009/sf.
There is some great content that covers ALM and Development Tools, .NET, Data Management, Infrastructure, Rich Clients, Distributed Systems, and Web Development. I hope to see you there.
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Sunday, January 04, 2009 |
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"Once upon a time, we wrote a book called A Pattern Language and that is how we got our name. Now, a pattern is an old idea. The new idea in the book was to organize implicit knowledge about how people solve recurring problems when they go about building things. "
Christopher Alexander
What is a software pattern?
How do writers about software patterns decide what software artifacts are patterns? How do these writers decide what patterns are worthy of note?
Christopher Alexander is the writer most associated with originating the idea of a pattern as a design concept.1 As the above quotation makes clear patterns are about making explicit solutions to recurring problems that people have created.
So a pattern formalizes knowledge that the profession has already arrived at.
Alexander never defined the word pattern. Nothing wrong with that. In fact the whole idea that we should define all our terms before we use them is misguided. The best definitions emerge from discussion and debate. Relying on people's intuitive notions on what a pattern is, is better than trying to spend time defining what a pattern is. In fact philosophy has long realized that good definitions arrive over time and debate. 2
My dictionary has various definitions for pattern. To use "a model for making things" seems to be the most useful stake in the ground to start the discussion.
In other words, a pattern is not just a solution to a problem. It is an abstraction of a solution that can generate several possible implementations. This of course, corresponds to Alexander's use of the word. His patterns such as "agricultural valleys" or "house for one person" do not have one possible implementation.
So a good software pattern is not a software technology. Hence WS* and REST are not patterns. They are implementations of a standard.3 The standards operate the same way on different platforms. This is no different than a mold for a cup being used to cast a bronze or sliver cup.
Given this point of view, a looping construct is not a pattern. A linked list is not a pattern. What about file systems? Anybody who remembers JCL realizes that there is more than one way to work with disk sectors.
But don't looping constructs come in several flavors? Aren't they different ways to solve the problem of control of software programs. Way back in the early days of computing people had to come up with these various ways of handling control flow. They were not divine revelations; that had to be invented. Anybody who remembers the arguments over the use of "goto"s , whether programs should have single or multiple entry and endpoints, or whether co-routines were a good idea might think of all of these as control flow patterns.
It is just that we take them for granted now that we might not consider them as patterns, just as technological givens. So patterns do need a context. Whenever somebody discusses patterns you need to clarify the domain of discourse. There are certainly patterns in certain "application domains" such as "double-entry" bookkeeping in accounting.
In fact looping constructs, assignment constructs and the like perhaps should be considered patterns once again. The rise of multi-processors, and distributed computing force us to think once again about what it means to do an assignment statement. In a distributed environment, where there is a latency in updating the value of any value, saying "x=y" is not always simple.
Whenever you discuss patterns, you must state the context in which you are talking. A pattern in one context could be a foundational technology in another context.
- http://www.patternlanguage.com/leveltwo/caframe.htm?/leveltwo/../bios/douglea.htm
- http://www.sfu.ca/philosophy/swartz/definitions.htm
- The OAIS reference model for SOA (http://docs.oasis-open.org/soa-rm/v1.0/soa-rm.pdf) would consider WS* and REST as implementations.
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Wednesday, October 29, 2008 |
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At the PDC Microsoft announced its answer to Amazon and Google's cloud computing services.
This answer has two parts: the Azure platform and hosted applications. Unfortunately people confuse these two aspects of cloud computing although they do have some features in common.
The idea behind Azure is to have a hosted operating systems platform. Companies and individuals will be able to build applications that run on infrastructure inside one of Microsoft's data centers. Hosted services are applications that companies and individuals will use instead of running them on their own computers.
For example, a company wants to build a document approval system. It can outsource the infrastructure on which it runs by building the application on top of a cloud computing platform such as Azure. My web site and blog do not run on my own servers, I use a hosting company. That is an example of using a hosted application.
As people get more sophisticated about cloud computing we will see these two types as endpoints on a continuum. Right now as you start to think about cloud computing and where it makes sense, it is easier to treat these as distinct approaches.
The economics of outsourcing your computing infrastructure and certain applications is compelling as Nicholas Carr has argued.
Companies will be able to vary capacity as needed. They can focus scarce economic resources on building the software the organization needs, as opposed to the specialized skills needed to run computing infrastructure. Many small and mid-sized companies already using hosting companies to run their applications. The next logical step is for hosting on an operating system in the cloud.
Salesforce.com has already proven the viability of hosted CRM applications. If I am a small business and I need Microsoft Exchange, I have several choices. I can hire somebody who knows how to run an Exchange server. I can take one my already overburdened computer people and hope they can become expert enough on Exchange to run it without problems. Or I can outsource to a company that knows about Exchange, the appropriate patches, security issues, and how to get it to scale. The choice seems pretty clear to most businesses.
We are at the beginning of the cloud computing wave, and there are many legitimate concerns. What about service outages as Amazon and Salesforce.com have had that prevent us from accessing our critical applications and data? What about privacy issues? I have discussed the cloud privacy issue in a podcast. People are concerned about the ownership of information in the cloud.
All these are legitimate concerns. But we have faced these issues before. Think of the electric power industry. We produce and consume all kinds of products and services using electric power. Electric power is reliable enough that nobody produces their own power any more. Even survivalists still get their usual power from the grid.
This did not happen over night. Their were bitter arguments over the AC and DC standards for electric power transmission. Thomas Edison (the champion of DC power) built an alternating current electric chair for executing prisoners to demonstrate the "horrors" of Nikola Tesla's approach. There were bitter financial struggles between competing companies. Read Thomas Parke Hughes' classic work "Networks of power: Electrification in Western society 1880-1930". Yet in the end we have reliable electric power.
Large scale computing utilities could provide computation much more efficiently than individual business. Compare the energy and pollution efficiency of large scale electric utilities with individual automobiles.
Large companies with the ability to hire and retain infrastructure professionals might decide to build rather than outsource. Some companies may decide to do their own hosting for their own individual reasons.
You probably already have information in the cloud if you have ever used Amazon.com. You have already given plenty of information to banks, credit card companies, and other companies you have dealt with. This information surely already resides on a computer somewhere. Life is full of trust decisions that you make without realizing it.
Very few people grow their own food, sew their own clothes, build their own houses, or (even in these tenuous financial times) keep their money in their mattresses any more. We have learnt to trust in an economic system to provide these things. This too did not happen overnight.
I personally believe that Internet connectivity will never be 100% reliable, but how much reliability will be needed depends on the mission criticality of an application. That is why there will always be a role for rich clients and synchronization services.
Hosting companies will have to be large to have the financial stability to handle law suits and survive for the long term. We will have to develop the institutional and legal infrastructure to handle what happens to data and applications when a hosting company fails. We learned how to do this with bank failures and we will learn how to do this with hosting companies.
This could easily take 50 years with many false starts. People tend to overestimate what will happen in 5 years, and underestimate what will happen in 10-15 years.
Azure, the color Microsoft picked for the name of its platform, is the color of a bright, cloudless day. Interesting metaphor for a cloud computing platform. Is the future of clouds clear? |
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Monday, September 22, 2008 |
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"Software + Services" is Microsoft's representation of what a large part of the future of computing is going to be. Microsoft, however, has not done a great job of explaining what "Software + Services" is.
Based on what I have read and heard, let me try to explain it as I see it.
The fundamental question that one has to ask is "Where does computation happen?"
The obvious answer to everyone today is: "Everywhere".
We compute on mobile devices, appliances, desktops and laptops, and remote computers. We communicate with text and voice.
Everybody understand this. The key question is: "Why?"
I think the answer is because "Hardware is cheap, and data is expensive to move."
The late Jim Gray did an analysis1 of the economics of distributed computing. His analysis came to two conclusions:
1. Put the computation near the data. Unless you have something that is very compute intensive, it is much cheaper to not move the data.
2. If you need data from multiple sites, push the processing closer to the data source by filtering the data early.
The assumption here is that telecommunication prices drop slower than Moore's Law. So far this has always been the case.
The natural conclusion is to do the computation where the data naturally resides. In other words: Do what makes sense. Some things will be in the cloud, some things will still be on the desktop. As long as Internet connectivity is not ubiquitous, and not always connected, you may have to cache data somewhere. Depending on the mission criticality of your application, a few seconds could be a long time.
As Ray Ozzie put it in his MIX Keynote, we live in a "World of small pieces loosely joined."
Software + Services means some things will be services in the cloud, others will be software as we know it today. That includes mobile devices and appliances that we are learning to love and hate, just as we have always done with traditional software.
1. MSR-TR-2003-24 "Distributed Computing Economics"
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Tuesday, September 09, 2008 |
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To further simplify
the example, let us assume that the we want to use the certificate to encrypt
a message from the client to the service. It is easy to apply what we discuss
here to other scenarios. As we discussed in
the previous post, we need to generate two certificates, the root certificate
that represents the Certificate Authority, and the certificate that represents
the identity of the client or service. We will also create a Certificate Revocation
List (CRL). We will use a tool
called makeCert to generate our certificates. Makecert, which ships with the
.NET platform, allows you to build an
X509 certificate that can be used for development and testing. It does three
things: Generates a public and private key It associates the key pair
with a name It binds the name with the
public key.
Many of the
published examples use makecert to both create and install the certificate. We
will do the installation in a separate step because this approach is closer to
the use of real certificates.
Separating the certificates also allows the certificates to be
installed on many machines instead of just one. This makes distributing
certificates to developer machines much easier. First we will
create the Root Certificate with the following command: makecert
-sv RootCATest.pvk -r -n "CN=RootCATest" RootCATest.cer -n specifies the
name for the root certificate authority. The convention is to prefix the name
with "CN=" where CN stands for "Common Name" -r indicates that
the certificate will be a root certificate because it is self-signed. -sv specifies the
file that contains the private key. The private key will be used for signing
certificates issued by this certificate authority. Makecert will ask you for a
password to protect the private key in the file. The file
RootCATest.cer will just have the public key. It is in the Canonical Encoding Rules (CER) format. This
is the file that will be installed on machines as the root of the trust chain. Next we will create
a certificate revocation list. makecert -crl -n
"CN=RootCATest" -r -sv RootCATest.pvk RootCATest.crl -crl indicates we
are creating a revocation list -n is the name of
the root certificate authority -r indicates that
this is the CRL for the root certificate, it is self-signed -sv indicates the
file that contains the private key RootCATest.crl is
the name of the CRL file. At this point we
could install the root certificate, but we will wait until we finish with the
certificate we will use in our scenario.
Here we need two files. We will need a CER file for the client machine
so that we can install the public key associated with the service. Then we
will create a PKCS12 format file that
will be used to install the public and private key in the service. The initial step is
: makecert -ic
RootCATest.cer -iv RootCATest.pvk -n "CN=TempCert" -sv TempCert.pvk -pe -sky exchange TempCert.cer -n specifies the
name for the certificate -sv specifies the
file for the certificate. This must be unique for each certificate created. If
you try to reuse a name, you will get an error message . -iv specifies the
name of the container file for the private key of the root certificate created
in the first step. -ic specifies the
name of the root certificate file created in the first step -sky specifies what
kind of key we are creating. Using the exchange option enables the certificate
to be used for signing and encrypting the message. -pe specifies that
the private key is exportable and is included with the certificate. For
message security is this required because you need the corresponding private
key. The name of the CER
file for the certificate is specified at the end of the command. Now we need to
create the PKCS12 file. We will use a the Software Publisher Certificate Test
Tool to create a Software Publisher's Certificate. You use this format to
create the PKCS12 file using the pvkimprt tool. cert2spc
TempCert.cer TempCert.spc pvkimprt -pfx
TempCert.spc TempCert.pvk We now have four
files: RootCATest.cer RootCATest.crl TempCert.cer TempCert.pvk The next step is to
install these on the appropriate machines. I could not get certmgr to work
properly to do an automated install.
The Winhttpcertcfg tool works for PKCS12 format files, but not CER
format files. We will use the MMC snap-in for this. Run the mmc snapin
tool (type mmc in the Run menu). First we will open the Certificates
snap-in. Choose: Add/Remove Snap-In.
When you add the
snap-in, choose local computer account for the computer you want to install
the certificate (usually the local one).
We want to install
the root certificate on both the client and service machines in the Trusted Root Certificate Store.
Select that store, right mouse click and
install both the RootCATest.cer and RootCATest.crl files. On the client side you want to install only
the public key in the TempCert.cer file.
On the service side only you want to install the PKCS12 format file
(TempCert.pvk) which has the private key for the certificate. Install that in
the Personal store. For private key installation you will have to provide the
password for the PKCS12 file. On the service
side, we need to give the identity of the running process (NETWORK SERVICE)
the rights to read the private key. We use two tools FindPrivateKey and cacls
to do this. Run the following command: for /F
"delims=" %%i in ('FindPrivateKey.exe My LocalMachine -n
"CN=TempITNCert" -a') do (cacls.exe "%%i" /E /G "NT
AUTHORITY\NETWORK SERVICE":R) Remember to delete
these certificates when you are finished with them.
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Sunday, August 24, 2008 |
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Working with X509
certificates can be very frustrating for WCF developers.
This is the first of
two posts. In this post I will explain just enough of the background for X509
certificates so that I can explain in the next post how to create and use
certificates during .NET development with WCF. The second post is here.
I do not know any
good books for a developer that explains how to use certificates. Even the
excellent books on WCF just give you the certificates you need to get the
sample code to work. They do not really explain to you why you are installing
different certificates into different stores, or how to generate the
certificates you need to get your software to work. Very often the examples run
on one machine with the client and service sharing the same store. This is not
a realistic scenario.
Obviously I cannot
explain all about certificates in one blog post. I just wish to share some
knowledge. Hopefully it will spare you some grief.
Here is the problem
I want to solve.
Suppose you have a
set of web services that is accessed by either an ASP.NET or rich client. The
service requires the client application to use an X509 certificate to access
the service. This could be to encrypt the data, to identify the client, to sign
the data to avoid repudiation, or for a number of other reasons. How do you
install the certificates on the client and service machines?
Certificate
technology is based on asymmetric
encryption.
In the encryption
scenario, the client would use the public key of the service to encrypt the
traffic. The service would use its
private key to decrypt the message. In
the identification scenario the service would use the public key of the client
to identify a message signed with the client's private key.
One of the key
issues is how you can be sure that the public key is associated with a given
identity. Perhaps somebody substituted their key for the one you should be
using. Perhaps somebody is hijacking
calls to the service, or you made a mistake in the address of the service. A classic example of these types of
vulnerabilities is the "man in the middle
attack". Another key issue is
that the private key cannot be read or modified by unauthorized parties.
Public Key
Infrastructure (PKI) is the name for a technology that uses a certificate
authority (CA) to bind the public key to an identity. This identity is unique
to the certificate authority. X509 is a standard for implementing a PKI. An X509 certificate represents an association
between an identity and a public key.
An X509 certificate
is issued by a given Certificate Authority to represent its guarantee that a
public key is associated with a particular identity. Depending on how much you
trust the CA, and the amount of identity verification the CA did, would determine how much trust you have in the certificate. For example VeriSign issues
different types of certificates depending on how much verification was done.
Sometimes organizations will be their own certificate authorities and issues
certificates because they want the maximum amount of control.
This relationship
between a CA and its issued certificates is represented in the "chain of
trust". Each X509 certificate is signed with the private key of the CA. In
order to verify the chain of trust you need the CA's public key. If you are your own CA authority you can
distribute the X509 certificate representing this "root
certificate". Some browsers and
operating systems install root certificates as part of their setup. So the
manufacturer of the browser or operating system is part of the chain of trust.
The X509 standard
also includes a certificate revocation list (CRL) which is a mechanism for
checking whether a certificate has been revoked by the CA. The standard does not specify how often this
checking is done. By default, Internet Explorer and Firefox do not check for certificate
revocation. Certificates also contain an expiration date.
Another approach to
trust is called "peer
to peer" trust, or "web of trust". Given the difficulties of peer trust it is
not practical for most Internet applications. It can, however, make development
scenarios simpler. Your development environment, however, should mimic your deployment
environment. Hence I do not recommend
using peer to peer trust unless that is practical for your deployed solution.
There are various
protocols for transmitting certificates.
We will be interested in two of them.
The Canonical
Encoding Rules (CER) protocol will be used to digitally transmit the public key
of a given identity. The PKCS12 protocol will be used to transmit the public
and private keys. The private key will be password protected.
The next post will
describe the mechanisms for creating and installing certificates in a .NET
development environment.
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Sunday, June 01, 2008 |
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On Friday, June 6 of Microsoft's Tech-Ed I will be hosting a Birds of a Feather Session on the topic "Software + Services is For Small Companies Too". It will be held in Room S330 E at noon. To continue the conversation, please add your comments and opinions to this blog post. If you are unable to attend feel free to add your thoughts as well here. Here are some questions to get you started thinking about the topic: What is Software + Services? Are small companies afraid of software + services? Are they afraid of cloud computing? Why? Doesn't cloud computing leverage the efforts of small companies? If cloud computing makes IT a commodity, doesn't this allow small companies to be even more nimble in their development efforts? What are the real advantages that large companies have over small companies? What about the innovators dillemma? How do large companies keep their current customers happy and assure future growth through innovation? Doesn't this help small companies. Doesn't cloud computing help small companies innovate even more?
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Friday, March 28, 2008 |
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Quick answer: When I don't know about it? When two experienced co-workers do not know also? I was working on a workflow code sample for an upcoming talk, when I started getting ridculous compilation errors. The compiler could not find the rules definition file when it was clearly available. The workflow designer could find it because I could associate it with a policy activity. The compiler falsely complained about an incorrect type association in a data bind, but it was clearly correct. Once again the designer had no problem doing the data bind. I tried to find an answer on Google with little success. After two hours of experimenting, I tried a different Google query and came up with the following link: https://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=612335&SiteID=1.The essence of the solution is the following: "this is a well-known
problem with code files that have desigable classes in them - the class
that is to be designed has to be the first class in the file. If you
do the same thing in windows forms you get the following error: the class Form1 can be designed, but is not the first class in the file. Visual Studio requires that designers use the first class in the file. Move the class code so that it is the first class in the file and try loading the designer again." It turns out I had changed a struct that was defined first in my file to a class. I moved that class to the end of the file and "mirabile dictu" everything worked. So if this is a well known problem, why can't we get an error message just like in the Windows Forms case?
While it was clearly my mistake, Microsoft has a share of the blame here. Clearly this requirement makes it easier to build the workflow designer. It would have been just as easy to check if this class was not defined first, and issue an error message.
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Thursday, March 06, 2008 |
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I did a short podcast for Consortio Services about Software as a Service as part of their weekly techcast. I very briefly cover what SaaS is about and some of the critical issues facing organizations looking at delivering services using the SaaS model. |
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Tuesday, March 04, 2008 |
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I am going to be giving two talks and a workshop at VS Live! in San Francisco. The first talk is an "Introduction to Windows Workflow Foundation" where I explain both the business reasons why Microsoft developed Workflow Foundation as well as the technical fundamentals. This talk will help you understand not only how to build workflows, but when it makes sense to do so and when to use some other technology. The second is " Workflow Services Using WCF and WWF". WCF allows you to encapsulate business functionality into a service. Windows Workflow Foundation allows you to integrate these services into long running business processes. The latest version of the .NET Framework (3.5) makes it much easier to use these technologies together to build some very powerful business applications. On Thursday I will give a whole day tutorial on Workflow Foundation where will dive into the details of how to use this technology to build business applications. Other speakers will talk about VSTS, ALM, Silverlight, AJAX, .NET Framework 3.0 and 3.5, Sharepoint 2007, Windows WF, Visual Studio 2008, SQL Server 2008, and much more. If you have not already registered for VSLive San Francisco, you can receive a $695 discount on the Gold Passport if you register using priority code SPSTI. More at www.vslive.com/sf
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Tuesday, February 12, 2008 |
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One of the great features in Visual Studio is the ability to startup more than one project at the same time. You do not need to create two solutions, for example, for a client and a server to be able to debug them both. I thought everybody knew how to do this, but when I found out that two members of a project team I am working with did not, I decided to blog how to do this. Select the solution in the Solution Explorer, right mouse click and you will see the following menu:  Select the Set Startup Projects menu item, and a property page will appear that lists all the properties in the project. For example:  You can associate an action with each of the projects: None, Start, or Start without debugging.  When you start execution, the projects that you wanted to startup will begin execution. If you allowed debugging, and set breakpoints, the debugger will stop at the appropriate places. |
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Thursday, November 22, 2007 |
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The Windows Workflow
Foundation (WF) ships with a Policy Activity that allows you to execute a set
of rules against your workflow. This activity contains a design time rules
editor that allows you to create a set of rules. At run time, the Policy
Activity runs these rules using the WF Rules engine.
Among other
features, the rules engine allows you to prioritize rules and to set a chaining
policy to govern rules evaluation. The
rules engine uses a set of Code DOM expressions to represent the rules. These
rules can be run against any managed object, not just a workflow. Hence, the
mechanisms of the rules engine have nothing to do with workflow. You can
actually instantiate and use this rules engine without having to embed it
inside of a workflow. You can use this rules engine to build rules-driven .NET
applications.
I gave a talk at
the last Las Vegas VSLive! that demonstrates how to do this. The first sample
in the talk uses a workflow to demonstrate the power of the rules engine. The
second and third samples use a very simple example to demonstrate how to use
the engine outside of a workflow.
Two problems have to
be solved. You have to create a set of
Code DOM expressions for the rules. You have to host the engine and supply it
the rules and the object to run the rules against.
While the details
are in the slides and the examples, here is the gist of the solution.
To use the rules
engine at runtime, you pull the workflow rules out of some storage mechanism.
The first sample uses a file. A WorkflowMarkupSerializer instance deserializes
the stored rules to an instance of the RuleSet class. A RuleValidation instance validates the rules
against the type of the business object against which you will run the rules
against. The Execute method on the RuleExecution class is used to invoke the
rules engine and run the rules.
How do you create
the rules? Ideally you would use some domain language, or domain based
application, that would generate the rules as Code DOM expressions. If you were
masochistic enough, you could create those expressions by hand.
As an alternative,
the second sample hosts the Workflow rules editor dialog (RuleSetDialog class)
to let you create the rules. Unfortunately, like the workflow
designer, this is a programmer's tool, not a business analyst's tool. A WorkflowMarkupSerializer
instance is used to serialize the rules to the appropriate storage.
I would be
interested in hearing about how people use this engine to build rules driven
applications.
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Wednesday, October 31, 2007 |
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Meditation is supposed to develop awareness, help focus your attention, and relax while increasing your focus. At one of my current clients we are developing a Software as a Service (SaaS) application. We have developed the following "meditative principles": 1. It's not done until the tests are done. 2. If it's broke, fix it first. 3. If it's not in a script or code, it doesn't exist. 4. Don't explain, do it (but ask questions if you don't understand). And finally (with apologies to Bobby McFerrin), "Don't worry, be agile". Here is a little song I wrote You might want to sing it note for note Don't worry be agile In every software we have some trouble When you worry you make it double Don't worry, be agile Ain't got no place to lay your head Somebody came and took your machine Don't worry, be agile The manager say your code is late He may have to litigate Don't worry, be agile Look at me I refactor Don't worry, be agile Here I give you my url When you worry call me I make you agile Don't worry, be agile Ain't got no time ain't got no style Ain't got not money to make you smile But don't worry self organize Cause when you worry Your face will frown And that will bring everybody down So don't worry, be agile (now) There is this little song I wrote I hope you learn it note for note Like good little developers Don't worry, be agile Listen to what I say In your software expect some trouble But when you worry You make it double Don't worry, be agile Don't worry don't do it, be agile Put a smile on your face Don't bring everybody down like this Don't worry, it will soon pass Whatever it is Don't worry, be agile |
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Wednesday, June 28, 2006 |
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"You are so young; you stand before beginnings. I would like to beg of you,
dear friend, as well as I can, to have patience with everything that remains unsolved in your heart. Try to love the questions themselves, like locked rooms and like books written in foreign languages. Do not now look for the answers. They cannot now be given to you because you could not live them. At present you need to live the question."
- Rainer Maria Rilke, Letters to a Young Poet
I learned programming in high school from a Fortran IV manual, which was like learning how to drive a car from the owner's manual—unexciting. Later, after taking an operating systems course at MIT, I gave up entirely on programming as a profession. I did not want to spend my life doing the same thing over and over again.
What made me change my mind and become a professional programmer? In large part it was Gerald M. Weinberg's The Psychology of Computer Programming, which I first read in 1982. Weinberg not only demonstrated that programming was more than technology, it is a social activity, but he showed how the social element related to the technical. In essence, he identified and addressed the types of fundamental questions that Rilke advised the young poet Franz Kappus to study. Such as:
- “What does it mean when we say a program is good?” I learned from Weinberg that a good program is as much a matter of cultural fit as technological merit. A designer has to understand that tradeoffs are made not only among technical factors, but among technical, social and economic constraints. Too often, I have seen engineers try to build the “perfect” product while ignoring ease of use or budget constraints.
- “How do you get programmers to work together as a team?” One programmer cannot do it all. Different people have different skills, different skills are needed at different parts of the project.
- “What is leadership all about?” How do you manage change and performance? Why do many managers manipulate programmers and tread them poorly and then wonder why they get poor results?
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- “How do you find good programmers?” And just what does it mean to be a good programmer? Weinberg was one of the first to point out the stupidity of aptitude testing for programmers, and the importance of understanding individual psychology in dealing with programmers.
Weinberg confirmed my own intuition that software could have an enormous impact on society, and his discussion of programming as a social activity helped explain much of the “strange behavior” I saw around me as I began working on my first programs. For example, during one of my first projects, I saw that the inability of certain people to work together had more impact on the project’s outcome than the technological issues being debated.
Weinberg examined what a naïve programmer would consider just technical topics and demonstrated how the elements of human personality and interactions between people had just as much, if not more influence over the outcome of a computer programming project than the technical issues and debates.
By understanding the importance of questions such as these, even if not every question can be answered in every situation, my value as a programmer and designer transcends whatever today’s technology du jour happens to be. I would have to say that Weinberg’s book took years off my apprenticeship, and saved me much aggravation.
To this day, I view programming primarily as a human activity, with the technical merits secondary. This does not mean you can ignore the technical merits. What makes a programmer really great is not technical genius, but an understanding of the human context of what he or she is doing. Any programmer who creates a truly revolutionary and world-changing program understands this. Others did not, or did not care to, and their contributions are hidden behind or overwhelmed by others’ accomplishments.
It is incredible that a twenty-five year old programming text containing examples illustrated with technologies that many programmers today cannot even conceive of—I recently taught a programming class where not one of the students had any idea what a punched card or paper tape was—is still a great book. |
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Wednesday, June 21, 2006 |
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I have signed the petition at: http://www.mwdadvisors.com/resources/stop-the-madness.php. SOA does not need another buzz word. I think SOA 2.0 ranks belong even ESB on the buzz word list.
This is also an experiment. We have heard of viral marketing. Let us see if we can have viral common sense.
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How do workflow and service oriented architecture relate?
The real question is how service oriented architecture (SOA) and business processes relate.
Service orientation is about how to organize and utilize distributed capabilities that could be under the control of different owners.1 Business Process Management (BPM) is about modeling, designing, deploying and managing business processes.2 Business processes are the capabilities, or the users of those capabilities. Workflow is a technology that builds the automated part of a business process. It integrates human decision with synchronous and asynchronous software systems. Of course this is somewhat recursive because a workflow could use other services in its implementation.
For me, SOA and BPM are not in conflict. People talk about layering BPM on top of SOA. Or that SOA is for IT folks, and BPM is for business people. In today's world, business cannot afford to have people who just think IT, or just think business. Given the way the human mind works, multiple models are often needed to think about certain problems.3 SOA and BPM are two different ways to think about the same problem: how organizations can best accomplish their missions. Thinking about business process will transform how you architect your services. Architecting your services will impact how you model your business processes.
1 For more information about service oriented architecture take a look at the Reference Model that the OASIS TC that I am a member of has produced: http://www.oasis-open.org/committees/download.php/18486/pr-2changes.pdf
2 See http://ww6.infoworld.com/products/print_friendly.jsp?link=/article/06/02/20/75095_08FEbpmmap_1.html
3 See "Mental Models" by P.N. Johnson-Laird in Foundations of Cognitive Science edited by Michael I. Posner
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Monday, June 19, 2006 |
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I was interviewed by Carl Franklin and Richard Campbell on .NET Rocks: http://dotnetrocks.com/default.aspx?showID=183. Yes we talked about Workflow and SOA. But we touched on other topics such as the failure of technology to really make foreign language learning any better. |
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Tuesday, February 07, 2006 |
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(Apologies to Christopher Alexander)
Christopher Alexander, the architect who inspired the Design Patterns movement, wrote a two part article that appeared in the April and May 1965 issues of Architectural Forum entitled “The City is Not a Tree.” The tree in the title is not a biological tree, but refers to a hierarchy being used as a way to organize how modern cities are built.
We all try to organize the world into neat categories. It helps us make sense of the world. Unfortunately, those categories and subcategories force us to view the world as a set of hierarchical categories. Alexander argued that architects who think that way produce buildings and cities that are sterile and unlivable. For example, zoning that refuses to mix residential, industrial and commercial use has some very severe drawbacks in transportation, living conditions, and tax policy.
The world has too many interrelationships to be viewed as a hierarchy, it is really a semi-lattice. Now there are parts of the world that are hierarchies. But a hierarchy is a semi-lattice, but the reverse is not true. The point is that if you view the world as a hierarchy you miss the true picture.
Software often has to model some part of the world. The World Wide Web is a semi-lattice. Image what the Web would be like if it could only be structured as a hierarchical directory such as Yahoo. Don’t get me wrong; neat categories are often useful. But Search has become such an important part of the Web because it allows you to capture the relationships in a semi-lattice.
Take the classic example I used to give my software engineering students when teaching them about abstraction and object-oriented systems: How do you define a chair? Of course they start out with a standard definition. A chair has a back, a seat, and four legs. But what about a bean bag? Or even a table? In the end, what emerges is that a chair is about a relationship between a piece of anatomy and surface that can support it.It is a relationship, not an object with constraints.
This is what led Alexander to focus on patterns and not components. Of course, some patterns could become components. But components (software or otherwise) are packaging artifacts, not fundamental abstractions. This is why the authors of Design Patterns have the principle of "Favor object composition over class inheritance." Class inheritance is a hierarchy. Object composition allows you to build a semi-lattice if that is appropriate.
Focusing on relationships means you focus on behavior, on what happens in the real world. Systems built on behavior are more flexible and more scalable than those based on constrained objects. Of course not all systems have to be flexible and scalable. Flexible and scalable often conflict with other desired goals such as performance.
Service orientation is based on focusing on the relationships or behaviors between the capabilities of distributed services because ultimately, a service performs some action in the real world. In service oriented systems you do not focus on constrained objects. You try to model the world as the semi-lattice it really is. 1
[1] Look at http://polaris.gseis.ucla.edu/pagre/simon.html for another interesting perspective. |
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Saturday, November 19, 2005 |
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One of the dogmas of messaging technology is that the "truth is always on the wire." In the context of interoperability that is certainly true. The message, not the platform object model that generated the message, is all that really exists between a service provider and consumer.
Like all principles it has its limits. The statement the "truth is on the wire" only means that using an agreed upon message format is equivalent to a using a common syntax for a language such as English. It does not matter how you define the message format. XML Schema, RelaxNG, or just "ask Alice" are all equivalent. Humans are better at handling ambiguity than machines, hence English syntax can be a lot looser than a message format. Nonetheless, the point remains valid.
Syntax tells you nothing about the semantics of the message. For those of you who abhor fancy terminology, semantics means nothing more or less than the real world actions that arise from processing the message.
Just like you can misunderstand an English sentence, you can "misunderstand" a SOAP message. This misunderstanding may be a programming error, or a misunderstood or mismatched policy.
For example, I send to my bank a correctly formatted message that says transfer $1000 from my cash reserve to my checking account. If the bank transfers the money from savings to checking, that is a programming error. The "wire truth" however was not violated.
Now suppose that the bank made the correct transfer, but the bank's policy (which I did not know of at the time) was to report such transfers to a credit bureau. My altered credit score resulted in a higher interest rate on the loan I was applying for. Understanding a service's policy is as important as understanding the message format.
Truth is not on the wire, truth is the real world effect of what happens when a SOAP message is processed. Truth is semantics.
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Monday, October 24, 2005 |
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Agile based software development methodologies often remind me of the story about the person who jumps off a 100 story building, and passing the 45th floor yells out "No problems yet!"
Agile based software methods have many good ideas. Their critique of the waterfall method has great merit. The best documentation is the code itself. Document based solutions do not work. But it does no good to demolish one myth only to have it be replaced by another.
To imagine that because the attempt to completely design everything up front is futile, the idea that you can iterate every few weeks and wind up with an adequate design is often wrong. That might work for a project that is strongly user interface or end-user driven. I doubt it would work for designing an air traffic control system, or system software such as Microsoft's Windows Communication Foundation. These kinds of projects have strong lifecycle requirements about safety, security, performance, or scalability. Often they require individuals to acquire new areas of knowledge or expertise.
Barry Boehm's spiral model of software development is a much better approach.1 The idea behind the spiral model is that at each choice point in the software development process one assess the risk that the project could fail to meet its goals. Based on that analysis the next step is to mitigate that risk. It might mean doing a prototype, refining the requirements, or doing more testing. Some of these tasks may be done concurrently. Analyzing the results of these steps might cause the development process to backtrack. In all cases, the views of all the project stakeholders (customers, developers, marketing, etc.) are considered at each analysis point.
Given this approach, the classic view (from Boehm's original paper) looks like a spiral:
Since the spiral model is a risk driven process, some circumstances might dictate an agile methodology. Other cases would require other approaches. By making risk the focus, rather than a manifesto of principles there is a higher probability of making the correct choices.
Let risk mitigation guide your development process.
1. Boehm's original paper appeared in IEEE Computer 21(5) 61-72 in 1988. In 2000 he updated the model at the "Spiral Development: Experience, Principles, and Refinements Spiral Development Workshop". |
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Friday, August 05, 2005 |
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One of the benefits of service oriented systems is that they are loosely coupled.
David Orchard
analyzes what loose coupling means from the perspective of the Web services stack. A human being can recognize that a field in a form is misplaced, software cannot. So for a particular message invocation, early binding is necessary. This is certainly true for standards. There needs to be a defined place for addresses and security tokens.
Orchard asks us to imagine Purchase Order system. A particular piece of information in a particular message must be bound to the appropriate programming types. If you need to know the name of the purchaser, you must early bind to the format of that name. Or to use fancy language, the service must understand its semantics. But it is only necessary for those programming types that the service needs to understand. Here is where building service interactions as messages rather than as remote procedure calls (RPC) is important.
If a service interaction is defined in terms of RPC, then if you change the semantics, you must change the service interface. As long as one type of the method call changes, the whole interface is broken. If you send messages (concretely XML messages), so long as the service can find the information it needs, the service is not bound to a particular message format. Other information can change, but the service does not care.
For example, if a service processing a message does not care about security, they can ignore the WS-Security SOAP headers. Those headers can change and the service can ignore all the security possibilities. The inventory service does not care if the credit information changes.
True, if XPath is used you are dependent on a certain structure to find information, but if you mark your documents with its version, or associated XML Schema, you could use the appropriate location path for the document. Or if you want to bind everything to type you can use the appropriate XML Schema instance to serialize the message to the appropriate programming types.
Loose coupling at the application level is about inserting levels of indirection to handle versioning (so what else is new?). But a message can do this because at the service interface the message is opaque. A RPC is not opaque.
At the application level loose coupling is how easy is to make a change that does not impact other parts of the system. With opaque messaging, a new version can be added without impacting other clients. If a service wants to reject a version it no longer supports, or does not yet support, it can do so without impacting other clients. In this restricted, but vitally important sense, semantic meaning in a Web service can be late bound.
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Tuesday, March 29, 2005 |
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Sarbanes-Oxley mandates that public companies should be able to produce all materially relevant transactions during an audit.
In the world of service oriented architecture, huge volumes of business documents flow freely as messages between services. These services are orchestrated (or choreographed if you wish) to produced business processes. To give you some idea of the volume, some people fear that the volume of XML is starting to take larger and larger fractions of network bandwidth. This is why some are starting to push the use of Binary XML for SOA messages.
In this world of huge stores of electronic messages and documents, how in the world do you find all the relevant ones? This is where XML Schema comes to the rescue. Your XML documents should be defined with schema, and hence subject to validation. Performance considerations may dictate that you do not validate your documents during message processing. Nonetheless, with schema definitions you should be able to query your messages to search and find the relevant documents.
For example, if you need to find all transactions with a given company worth over a certain threshold, you have to the tools to find it.
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Friday, December 24, 2004 |
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Grady Booch has
fired
another attacking missile in the great debate over software factories, and the idea's defenders have replied. Microsoft's view of the world is outlined in a series of articles by Jack Greenfield on the MSDN site:
http://msdn.microsoft.com/architecture/overview/softwarefactories/default.aspx?pull=/library/en-us/dnmaj/html/aj3softfac.asp
http://msdn.microsoft.com/architecture/overview/softwarefactories/default.aspx?pull=/library/en-us/dnbda/html/softwarefactwo.asp
http://msdn.microsoft.com/architecture/overview/softwarefactories/default.aspx?pull=/library/en-us/dnbda/html/softfact3.asp
The basic idea behind software factories is to move the production of software from a craft to an industry resembling manufacturing. This is not a new idea. In the second article, after reviewing why these efforts have failed in the past, Greenfield says:
"We are unable to achieve commercially significant levels of reuse beyond platform technology. The primary cause of this problem is that we develop most software products as individuals in isolation from other software products. We treat every software product as unique, although most are more similar to others than they are different from them. Return on investment in software development would be far higher if multiple versions or multiple products were taken into account during software product planning. Consequently, we rarely make commercially significant investments in identifying, harvesting, packaging and distributing reusable assets. The reuse that does occur is ad hoc, rather than systematic. Given the low probably that a component can be reused in a context other than the one for which it was designed, ad hoc reuse is almost an oxymoron. Reuse rarely occurs unless it is explicitly planned in advance."
I agree as far as he goes, but I do not think he fully comes to grips with why the idea of software factories has a long way to go. No doubt part of the reason is that the idea of a software factory resembles the idea of artificial intelligence. Every success redefines the goal. Nonetheless, I think there is a more fundamental reason.
Programming has always been a labor intensive activity. As a result, from the very start people have tried to figure out how to automate as much of the process as possible. Compilers were one of the very first attempts to automate software development. Today we take them for granted, but well into the 1970s people were still arguing over whether a good human assembly language programmer could code better than a compiler. Debuggers, linkers, loaders, file systems, operating systems, distributed transaction coordinators, were all invented to automate parts of the software development. How long did it take for the idea of a virtual machine (as in Java or .NET) to become practical for most software development?
You can, as Greenfield does, view these artifacts as improved abstractions. Abstractions are very critical to software development. I view these developments differently. I see them as automating what we understand how to automate. Code libraries such as for .NET and Java are in the same category. After years of experience we now understand enough of some of the critical elements of certain parts of software development to encapsulate them in libraries.
But software is not like other engineering pursuits such as bridge building. Most bridges, although they look different, are really one a few basic kinds. Because you cannot copy a bridge like you can copy a program, you need to build a new bridge at every place you need to cross a river. Hence you can much more easily replicate what you did before, or learn from experience.
This really came became clear to me when I was a graduate student in nuclear engineering. In the reactor design course final exam we were asked to design a cooling system for a nuclear reactor. We were not, however, to use our fundamental understanding of physics and engineering to do this. We were to apply the American Society of Mechanical Engineering (AMSE) standards for cooling systems to do the design.
Why has this been so difficult to do with software? Since software is easy to copy, you only need to create a new piece of software when you need to do something new. Automation is about understanding what you have done in the past. You cannot automate what you do not understand. So much of software development is done for things we have not done before. We do not know, or do not fully understand the domain models that Greenfield relies on for the idea of software factories. This is why the CASE tools, or the code generators of the past have not provided any reduction in cost and time. This is why I think UML based code generators will not be wildly successful either.
Of course you need to understand the domain models. It is just that in a dynamic economic environment, you do not arrive at the knowledge in time to automate the process until it becomes yesterday's understanding. As yesterday's understanding it will take a while to see if it is fundamental enough to be worth automating.
So long as software is about innovation, or doing what we do not yet understand how to do, it will always have a large craft component. Maybe automation will decrease the need for programmers, and thus reduce the labor cost. So far this has not happened in 60 years. People may use cheaper programmers, but that is another story for another time.
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Thursday, December 16, 2004 |
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Adam Bosworth has given a talk (discussed in his
blog
entry) that has received a lot of attention and comment. He argues that software programs and their tools are way too complex and should be simple.
The problem I have with his argument (and arguments similar to that) is that it posits a false binary choice: either be complex or simple. Complexity is a continuum. Bosworth argues against sophisticated abstractions. But it is sophisticated abstractions that make simplicity possible.
After all, the computer is just atomic particles. Does any programmer worry about that? Or the gotos/branches that are all over the microcode? What about the instruction pipeline? That is all abstracted away in the "hardware". How many programmers worry about exactly how the operating system scheduler works? The whole idea behind class libraries that come with Java and .NET is to allow the programmer to concentrate on the business logic and not worry about the "plumbing code".
Occasionally we have to break through that abstraction and worry about exactly how things work. I discovered that when I wrote my first test code to test the performance of the first MIPS machines back in the 1980s. I found that if I did not return a value from my test routine, the loops would be optimized out. Most of the time we can remain blissfully ignorant of the abstractions. Performance, scalability, and most important of all security, are problems that are classic examples of where we often have to worry about complexity and look at the abstractions. The solutions to those problems are sometimes simple, but more often than not messy.
You cannot divorce simplicity from abstraction. People dealing with complicated things need complicated abstractions. Engineers often make products and technologies that are too geeky, but sometimes things are too simple. After all, the Swiss Army knife comes in several sizes. You can match the level of simplicity that you need.
The Swiss Army knife analogy strikes at the heart of the issue for me. You need to keep it simple enough. Saint-Exupery's famous saying applies here. Perfection is achieved in design when there is nothing more to take away, not when you have nothing more to add. In other words you have to keep it simple, but it still has to accomplish the task. The issue is to make it simple enough for your user, whether they be a writer of a blog, or a user of a class library. But even the simple user to be effective has to understand the limits of the tool, or to be more sophisticated, the abstractions and assumptions used. This applies to all sophisticated problems whether they be the accuracy of a medical test, the stability of Social Security, or the usefulness of Atom or RSS.
Bosworth speaks about the virtue of "keeping it simple and sloppy and its effect on computing on the internet." Well if you have to be HIPPA compliant you cannot be sloppy and forgiving of human foibles and weaknesses. Human weaknesses and foibles are precisely the problem, and they cannot be abstracted or assumed away to achieve simplicity. If you do so, you will have a system so rigid, so bureaucratic, it would be unusable.
Bosworth concludes by talking about achieving simplicity in the information search space to avoid information overload. He talks about data mining, and machine learning as the potential solutions. But they all rely on abstractions about what is important, and what is not. Users better understand how they work. I cannot wait for the day when the social scientists start deconstructing data mining and machine learning for their social assumptions. At that point both humans and machines will prove once again what Hobbes argued so many years ago. Knowledge and the assumptions that go with it are the product of human actions. Knowledge is partly determined by our social relationships and what we assume. Simplicity results from assumptions and abstractions. But we cannot hide from the mess in the name of simplicity.
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Monday, November 01, 2004 |
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David Chappell,
in his latest newsletter
, argues that Service Oriented Architecture (SOA) promotes software reuse far better than objects do. For him, object reuse usually fails for two reasons. First, in an evolving business environment it is difficult to come up with a good definition of a business object such as a customer. Second, software developers seem to catch the not-invented-here plague fairly easily.
I certainly agree with this.
One of my favorite questions when discussing object oriented design with students is to ask for the definition of a chair. Invariably, the answers will include legs. Then I ask them if an ottoman or a bean bag chair fits their definition. Coming up with good class definitions is hard. I have been involved in software development long enough to have frequently seen the not-invented-here syndrome.
He also argues that the best examples of reuse occur with applications such as PeopleSoft or SAP. SOA reuse resembles application reuse. I certainly agree with this as well.
But, in my opinion, the fundamental reason that reuse with SOA will be more frequent than with objects is that SOA reuse is loosely-coupled black box reuse, most object reuse is tightly-coupled white box reuse. SOA is a design pattern that has benefited from our struggles with object-oriented approaches.
When you treat a component as a black box you interact with it through its external properties, or as we say in the programming world, its interfaces. You do not have to understand how a fork is built or what it is composed of. If you do not like your two-pronged desert fork, you get a three-pronged fork to eat your peas. You do not try to modify the two-pronged fork. The real world is loosely coupled.
With inheritance you must understand the object's implementation. You must always be aware of the fragile base class problem. You can change the behavior of existing programs if you are not careful. This only reinforces the not-invented-here syndrome because it is perceived to be easier to write classes from scratch, than to try to understand another programmer's code. Objects are tightly coupled because they are connected through a stack and a linker/loader.
Modern object oriented practitioners have realized this. The design patterns community emphasizes composition/delegation over inheritance, and interfaces over implementation. You use the component through its interface treating it as a black box. Interfaces produce looser coupling.
SOA is a design pattern for building business process in the real world. It does not tell you how to architect your application. It does not provide an implementation. It does tell you to construct your services completely independent of each other. Services are independently deployed. Code or database tables are not shared between services. Services in a SOA interact through a loosely coupled interface that is defined as a series of messages.
SOA is a continuation in the large of what good object oriented designs have started to become: less emphasis on the classes, and more emphasis on the loose coupling provided by interfaces. Obviously objects provide the actual implementation, but that is not how the users of those implementations view them. They view them as black box interfaces.
Class frameworks such as J2EE and .NET have been successful. Both those frameworks, encapsulate stable, well-understood problems that the software world has been working on for over 50 years. Not only are the problems of business not well understood, the environment is quite dynamic. Services in a SOA are loosely coupled black boxes because they reflect the loosely coupled real world.
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Wednesday, August 11, 2004 |
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Everybody talks about how the New England Patriots Super Bowl win last year was a team effort. Whether it was the backup quarterback imitating Peyton Manning on the scout team, the statisticians, the players, the coach, or the personnel guy, everybody contributed.
Teamwork, of course, is one the perennial topics du jour in the software world. Demarco and Lister’s classic Peopleware talks about it, introducing the concept of a “jelled” team. All the variants of Extreme Programming rave about it.
But what makes a team good? It is difficult to have a good discussion about software teams because there are not enough public concrete case studies. Could sports teams provide a basis for such a discussion?
Peter Drucker, the management expert, thought so. In a Wall Street Journal essay written several years ago, he discussed three paradigmatic teams: baseball, football and tennis.
What type of team does your organization have? As Drucker makes clear in his essay these teams are distinct alternatives. They have unique strengths and weaknesses, but attempts to combine parts of each are a recipe for disaster.
I bet a lot of traditional software organizations have baseball style teams. You do not play as baseball team, you are a member of the team. The third baseman never pitches, the tester does not do development. Designers do not have much interaction with developers. This is the old-style Detroit assembly line. The big advantage to this team is that it makes it easy to train and evaluate personnel. Everybody can be a "star" no matter how difficult they are to get along with. On many plays, certain players are not important. The left fielder does not do much on a routine ground ball to the second baseman. Symphony orchestras are like this as well.
This approach works well when the task is well understood and can be reduced to "routine". Here one can understand the drive to outsource and offshore software tasks. If things are well defined, and competence exists elsewhere, then price drives all.
The problem comes when you need to innovate quickly.
Football teams are more flexible. There is no equivalent to the halfback option pass in baseball. On almost every play, every player is necessary, if for no other reason then to prevent some other player from getting to the ball carrier, or the quarterback. Everybody works in parallel. Unlike baseball, everybody has to follow the coach's orders or else the team will not win. You have to train together to be effective.
How do you evaluate and train people? An individual's value is often related to how they complement the rest of the team, not only on their individual strengths and weaknesses. Play without a linebacker or a safety, and you will fail just as if you played without a quarterback. But why do quarterbacks get more money? Why do they both get more money than teachers? This is the old diamond-water paradox in economics. Here you have to reward people based on their marginal value to your team, not on some absolute scale. It is much harder to outsource, much less offshore if you live in this world.
You still need, as Drucker points out, a score to evaluate how well the team is doing. Though as any football fan will tell you, the score does not relate well to how an individual player is doing. That is why you have to "watch film" to evaluate players, something baseball sabermetricians ( http://www-math.bgsu.edu/~albert/papers/saber.html ) do not have to do. Software managers in this world really have to understand what is going on in order to evaluate and train their people.
Finally there are tennis doubles teams (or jazz combos). There are no clear players, only roles to be filled by different team members at different times. This is the ultimate in flexibility and adoptability to changing circumstances. But there really has to be a fit here. How do you train and compensate in such a world where you can succeed or fail, but there is no score. It is hard to relate the end result to the individual. You certainly cannot outsource or offshore here.
As Drucker says, teams are tools, and you have to understand your environment and pick the appropriate approach. This is what management is all about.
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Wednesday, June 30, 2004 |
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One of the recently discovered Internet Explorer bugs allows malicious sites to install key stroke recording code on your system. This certainly has got a lot of press and deservedly so because of the widespread presence of IE as a browser.
Every time this happens I wonder, does open source produce more secure code? Do “more eyeballs” reviewing the code produce better code? Looking over the list of vulnerabilities on the US-Cert Issues Advisory list makes me doubt that this is true.
Based on my experience, too many reviewers often make for poorer reviews. Remember the last time you had to sign off on a document with a long list of reviewers? The early reviewers glance at the document knowing more reviewers will look at it later. The later reviewers assume the early reviewers did most of the work already. The result is a lackadaisical, poor job of reviewing. You cannot tell me that the open source community is immune from the natural tendencies of human nature.
Approval by committee is no different than design by committee. Just because the committee is larger does not automatically make the review better.
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Monday, June 14, 2004 |
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Driving on the highway around Boston I was wondering about its virtual counterpart, the Information Superhighway. Massachusetts’s accident rate is the highest in the country. People mutter in frustration, “You can’t get there from here” as they navigate streets that look as if cow meanderings determined their path. Yet people and commerce move with an ease and openness that can only imagined on the Information Superhighway.
What makes one so open and the other not? Some might proclaim “Heed the Three Opens of Modern Enterprise Architecture: Open Source, Open Standards, and Open Data.” In my mind I compared the concrete and virtual parallels.
The most obvious analogy is Open Standards. Traffic laws allow for vehicles to travel. Vehicles must be able to signal turns. Vehicles have to stay in lane. They have stop lights and backup lights. In fact any vehicle that follows these standards is allowed on the road. Much to the chagrin of many drivers, following these standards allows bicycles on the road. Standards can even allow for varying defaults. Everywhere but New York City has “right turn on red” as the default. When vehicles arrive at their destination then the work begins.
Vehicles are similar to messages. Open standards define the contents of a message and allow them to get to their proper destination. When the messages arrive at their destination, the actual work begins. Here Web Services standards (SOAP, WSDL, WS-Security, etc.) seem to have reached critical mass. While much more work needs to be done, the industry seems to understand what must be done (routing, federated security, etc.), although in some areas, such as transactions, it is not clear what the right approach is.
Applications create these messages. Viewed in this light, the dispute over Open Source does not seem as important as Open Standards. How the vehicles are built is not as important as their ability to interoperate on the open road. Yes, both the real and virtual counterparts have to be reliable and economic. You have to be able to upgrade and maintain them. But how that is accomplished is not critical to either superhighway. Some drive a BMW, others a Ford Escort. Different cars perform differently, they just have to perform. The success of the Information Superhighway does not depend on the success or failure of Open Source.
What does matter is what happens when the message or vehicle arrives at its destination. This is where commerce, recreation, or whatever occurs. In the real world, human beings can interpret the ambiguity of their interaction. To sign into a building, a security guard can judge whether the picture on your driver’s license (your federated security id) matches the person in front of them. A human can interpret the way you write out your address, or whether you put dashes or dots in your phone number. Data need not be strongly typed in the real world.
The data that moves on the Information Superhighway is different. If two applications have a different way of encoding an address, or a list of drug interactions in a data structure, these applications cannot interoperate even if they can exchange messages. Without Open Data information cannot easily move.
There is much sound and fury over Open Source, much love and singing kumbaya with Open Standards, and confusion over Open Data. Open Source and Open Standards people understand. But what is this “Open Data” concept? Look at one of the great intellectual popularity contests of our generation, Google (6/4/2004) by searching on the terms “open source”, “open standards” and “open data” and see the quality of what comes back, the first two are understood terms, the latter is not.
XML by itself does not help here. A customer record, or an address, or a list of drug interactions can be encoded in any one of several posssible sets of XML elements. Open Data requires XML Schema so that XML can be typed. If organizations can agree on the appropriate schemas they will be able to transform the content of their messages into their applications data structures.
Open Data is the missing link to make the Information Superhighway a reality. How can you integrate business services unless you have Open Data? You can talk about Service Oriented Architecture (SOA) until you are blue in the face, but without Open Data it will all be pointless. SOA is a way to build flexible, evolvable applications, but it is the moving of data that makes the building of services a useful endeavor.
It will take a while before enterprises learn how to achieve Open Data. On the other hand, do not be overly discouraged. Our automotive superhighway was like that once. Imagine what driving across the country in the late 19th or early 20th century was like. There were only 150 miles of paved road in the US in 1903. It was an adventure. Read books such as “Horatio’s Drive: America’s First Road Trip” by Dayton Duncan and Ken Burns, or “Coast to Coast by Automobile” by Curt McConnell and compare those experiences with ours today. We tend to forget how far we came, and how long it took us.
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Sunday, March 14, 2004 |
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Over lunch the other day, a programmer mentioned that Gamma, et.al’s book Design Patterns revolutionized his thinking about software development. He asked me what programming books revolutionized my thinking. I agreed that Design Patterns changed the way I think about software, but I did not consider it revolutionary.
Thinking about this later, I realized that the most important books that I had read were not about the mechanics of programming itself, or about designing, or architecting software. They were about the cultural anthropology of programmers.
Cultural anthropology studies the patterns of human relationships in areas such as language, communication, socialization, relationships, and politics. Cultural anthropologists try to relate the organization of a person’s mind to their behavior. Books that helped me understand the programmer’s mind, and how it relates to their behavior have revolutionized the way I think about software development.
The success or failure of programming projects that I have been part of usually has less to do with the technology, than of the patterns of human relationships. One of my favorite examples is the software project whose architecture followed the corporate structure – the application groups designed the applications, the graphics group the graphics, the database group the database, and the user interface group the user interface. The resulting program was like feudal Europe; it did not work well together.
Here, in no particular order, are the books that have really influenced me.
The Mythical Man-Month by Frederick P. Brooks - pure wisdom about why software projects succeed or fail. See my review of it
here
.
The Psychology of Computer Programming by Gerald M. Weinberg - the first book that got me to see programming as a human activity, and why understanding human behavior is important to understanding how to build better software.
Both these books have been republished with new material. Get the latest editions.
Peopleware by Tom Demarco and Timothy Lister - a great book on the workplace and software teams. This is a book your supervisor should read as well. There is a second edition of the book, but I have not read it.
Donald E. Knuth wrote two essays that strongly influenced me. While dated, and I do not recommend them as strongly as the others, I feel obliged to mention them. The first, "Structured Programming with goto statements", written in 1974, made me realize the importance of thinking about software structure, and not language constructs. The point of the article is not that gotos are great things; but that the correct level of abstraction is critical to writing good programs. The second, “Literate Programming”, written in 1984, got me to realize that software programs could be written clearly using literary concepts. Programs that are clear to human readers are better programs because they are clear about what they want to accomplish. Both essays have been reprinted in the book Literate Programming.
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