The Other Foundation Technology

I mentioned last week that WF “is one of two foundation technologies that my project absolutely depends on”. Sam Gentile assumes the other foundation technology is WCF. It’s not.

As a quick reminder, my day job these days is to architect and deliver shared service-oriented infrastructure for Microsoft’s IT division. These services will be automating long running business operations. And when I say long running, I mean days, weeks or longer. While there will surely be some atomic or stateless services, I expect most of the services we build will be long running. Thus, the infrastructure I’m responsible for has to enable and support long running services.

The other foundation technology my project depends on is Service Broker. Service Broker was expressly designed for building these types of long running services. It supports several capabilities that I consider absolutely critical for long running services:

  • Service Initiated Interaction. Polling for changes is inefficient. Long running operations need support for the Solicit-Response and/or Notification message exchange patterns.
  • Durable Messaging. The first fallacy of distributed computing is that the network is reliable. If you need to be 100% sure the message gets delivered, you have to write it to disk on both sides.
  • Service Instance Dehydration. It’s both dangerous and inefficient to keep an instance of a long running service in memory when it’s idle. In order to maximize integrity (i.e. service instances survive a system crash) as well as resource utilization (i.e. we’re not wasting memory/CPU/etc on idle service instances), service instances must be dehydrated to disk.

In addition to these capabilities, Service Broker supports something called Conversation Group Locking, which turns out to be important when building highly scalable long running services. Furthermore, my understanding is that Conversation Group Locking is a feature unique to Service Broker, not only across Microsoft’s products but across the industry. Basically, it means that inbound messages for a specific long running service instance are locked so they can’t be processed on more than one thread at a time.

Here’s an example: let’s say I’m processing a Cancel Order message for a specific order when the Ready to Ship message arrives for that order arrives. With Conversation Group Locking, the Ready to Ship message stays locked in the queue until the Cancel Order message transaction is complete, regardless of the number of service threads there are. Without Conversation Group Locking, the Ready to Ship message might get processed by another service thread at the same time the Cancel Order message is being processed. The customer might get notified that the cancellation succeeded while the shipping service gets notified to ship the product. Oops.

There’s also an almost-natural fit between Service Broker and Windows Workflow. For example, a Service Broker Conversation Group and a WorkflowInstance are roughly analogous. They even both use a Guid for identification, making mapping between Conversation Group and WF Instance simple and direct. I was able to get prototype Service Broker / WF integration up and running in about a day. I’ll post more on that integration later this week.

Last but not least, Service Broker is wicked fast. Unfortunately, I don’t have any public benchmarks to point to, but the Service Broker team told me about a private customer benchmark that handled almost 9,000 messages per second! One of the reasons Service Broker is so fast is because it’s integrated into SQL Server 2005, which is is pretty fast in it’s own right. Since Service Broker is baked right in, you can do all your messaging work and your data manipulation within the scope of a local transaction.

Service Broker has a few rough areas and it lacks a supported managed api (though there is a sample managed api available). Probably the biggest issue is that Service Broker has almost no interop story. If you need to interop with a Service Broker service, you can use SQL Server’s native Web Service support. or the BizTalk adapter for Service Broker from AdapterWORX. However, I’m not sure how many of Service Broker’s native capabilities are exposed if you use these interop mechanisms. You would probably have to write a bunch of application code to make these capabilities work in an interop scenario.

Still, I feel Service Broker’s unique set of capabilities, its natural fit with WF and its high performance make it the best choice for building my project’s long running services. Is it the best choice for your project? I have no idea. One of the benefits of working for MSIT is that I get to focus on solving a specific problem and not on solving general problems. I would say that if you’re doing exclusively atomic or stateless services, Service Broker is probably overkill. If you’re doing any long running services at all, I would at least give Service Broker a serious look.