WCF Design: using both: session and per call mechanism?

Question

I am to design a webservice using WCF that yields methods that don't require a session (like static calls, eg: giving back some information about the webservice itsself) and other methods that require a session.

furthermore, the session based methods are using Workflows that are supposed to be able to be changed at runtime.

my current design would look like this:

there is a singleton service that runs in IIS that handles all the per call methods which also works as a host for the session based services. that way the singleton class knows about all the sessions and can halt the running workflows to exchange them.

is this a good/possible design choice?
is this a common scenario that uses a common design?
would be happy about any reading hints as the msdn help wasn't such a help to me.

thanks for your answers
-redoced

Answer 1

Using a singleton WCF service class is almost never a good idea - unless you really have just one single (physical) resource which you want to protect from concurrent access, it doesn't really make sense.

Because: either it's not multi-threading capable, but in this case, it becomes a huge bottleneck - requests are handled strictly sequentially - one after another. Not very good for performance.

Or then you need to make the whole service multi-threading aware - and let me tell you, making this properly, safely, and efficiently isn't for the faint of heart. It's really really hard to get this right, and make it perform well.

I really don't see any need for this, at all.

• leave you "static" message calls (as you call them) be per-call services - those are easy to program, work well, perform well, never cause any multithreading issues

• those few service calls (hopefully!) that do require a session - put them on their own service endpoint(s), make them per-session, use the "SessionId" in your session to identify them. It's a bit more work than per-call - but still nowhere near as complicated and error-prone as multi-threaded programming

Resources for WCF sessions:

• MSDN article on Using Sessions in WCF
• WCF Sessions blog post

Resources for WCF durable services (that persist their state between calls):

• Blog post on "durable services"
• WCF Durable Services
• Orcas durable services
• Screencast by Mike Taulty on durable WCF Services

Resources for WCF Workflow Services:

• Foundataions: Workflow Services
• Channel9 webcast: Intro to Workflow Services
• Webcast: Intro to workflow services in WCF 4.0