WCF Service InstanceContextMode and ConcurrencyMode combinations

What does the InstanceContextMode and ConcurrencyMode attributes in a wcf service do?

A short MSDN description of this describes it as follows:

“A session is a correlation of all messages sent between two endpoints. Instancing refers to controlling the lifetime of user-defined service objects and their related InstanceContext objects. Concurrency is the term given to the control of the number of threads executing in an InstanceContext at the same time.” from http://msdn.microsoft.com/en-us/library/ms731193.aspx

I found a nice article and some test screenshots of different combinations of how InstanceContextMode andConcurrencyMode is used together.

InstanceContextMode can have one of InstanceContextMode enum values. InstanceContextMode specifies when an instance of a service is created and whether or not this instance will be a Singleton or not.

ConcurrencyMode specifies the concurrency accessebility of each seperate instance. For example. If ConcurrencyMode is set to Multiple, then the same instance of the service can service multiple requests simultaneously. When it’s set to Sinlge, different instances of the service can handle one request at a time, but simultaneously for each instance.

Couple of examples:

• InstanceContextMode = Single, ConcurrencyMode = Sinlge – Only a single instance of the service can be created and only one request can be serviced at a time for this instance.
• InstanceContextMode = Sinlge, ConcurrencyMode = Multiple – Only a sinlge instance of the service is created, but multiple request can be serviced at the same time by this one service.
• InstanceContextMode = PerSession, ConcurrencyMode = Single – An instance of the service is created for each session, these instances can service requests simultaneously, but only one request at a time per instance.
• InstanceContextMode = PerSession, ConcurrencyMode = Multiple – An instance is created for each session and each of these instances can perfom multiple tasks simultaneously.

From this, you can see that it will not be necessary to write thread safe code when InstanceContextMode = PerSession and ConcurrencyMode = Single. It will however be necessary to make your code thread safe if you’re doing some concurrent calls from within the service itself, or work with static objects in your application.

Read more: JD Stuart's Blog

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