Throttling in WCF

WCF throttling provides some properties that we can use to limit how many instances or sessions are created at the application level. Performance of the WCF service can be improved by creating proper instance.

When WCF was first released in .NET Framework 3.0, the default values for WCF Service throttling settings such as MaxConcurrentSessions and MaxConcurrentCalls were set very conservatively. For example, here are the default settings in 3.0:

• MaxConcurrentSessions (default: 10)
• MaxConcurrentCalls (default: 16)
• MaxConcurrentInstances (default: 26)

Attribute	Description
maxConcurrentCalls	Limits the total number of calls that can currently be in progress across all service instances. The default is 16.
maxConcurrentInstances	The number of InstanceContext objects that execute at one time across a ServiceHost. The default is Int32.MaxValue.
maxConcurrentSessions	A positive integer that limits the number of sessions a ServiceHost object can accept. The default is 10.

Service Throttling can be configured either Adminstractive or Programatically

Administrative(configuration file)

Using <serviceThrottling> tag of the Service Behavior, you can configure the maxConcurrentCalls, maxConcurrentInstances , maxConcurrentSessions property as shown below.

<system.serviceModel>
    <services >
      <service behaviorConfiguration="ServiceBehavior"  name="MyService">
        <endpoint address="" binding="wsHttpBinding" contract="IMyService">
          <identity>
            <dns value="localhost"/>
          </identity>
        </endpoint>
        <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange"/>
      </service>
    </services>
    <behaviors>
      <serviceBehaviors>
        <behavior name="ServiceBehavior">
          <serviceMetadata httpGetEnabled="true"/>
          <serviceDebug includeExceptionDetailInFaults="true "/>
          <serviceThrottling maxConcurrentCalls="500"
 maxConcurrentInstances ="100" 
maxConcurrentSessions ="200"/>
        </behavior>
      </serviceBehaviors>
    </behaviors>
  </system.serviceModel>

References

Nice information on throttling details 
by Mr. Kenny Wolf: http://kennyw.com/work/indigo/150. 

Nice article by Mr. Rick Rain in three parts which explains concurrency and 
throttling in depth: http://blogs.msdn.com/b/rickrain/archive/2009/06/15/wcf-instancing-concurrency-and-throttling-part-1.aspx. 

Mr. Michele Leroux Bustamante talks about settings which influence how 
throughput of WCF services can be increased 
using WCF concurrency: http://www.windowsitpro.com/article/net-framework2/concurrency-and-throttling-configurations-for-wcf-services.aspx. 

Justin Smith explains a sample code on how InstanceContextMode 
and ConcurrencyMode affect concurrency: http://www.wintellect.com/CS/blogs/jsmith/archive/2006/05/16/instancecontextmode-and-concurrencymode.aspx. 


Mr. Glav talks about throttling and 
legacy systems: http://weblogs.asp.net/pglavich/archive/2007/06/27/wcf-and-concurrent-usage-throttling.aspx. 

Default values for WCF throttling: http://blogs.msdn.com/b/wenlong/archive/2009/07/26/wcf-4-higher-default-throttling-settings-for-wcf-services.aspx. 
http://wcftutorial.net/WCF-Throttling.aspx