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POWCF Concurrent requests piling up on the server when using WSHttpBinding
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2009-05-07T04:21:15.643
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I have a WCF client/server app which is communicating over HTTP using the WSHttpBinding. Server Setup: self-hosting, using the standard WCF <code>ServiceHost</code>. My actual service class is attributed as: <pre><code>[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple, InstanceContextMode = InstanceContextMode.PerSession, UseSynchronizationContext = false)] </code></pre> Client Setup: using a visual-studio generated client proxy using synchronous service calls (<code>proxy.call_server_method</code> blocks until such time as the server has responded in full.) Scenario: I have one particular method call which takes 20 seconds to execute on the server. The client calls this method in a separate thread, so it is not being held up, and the <code>ConcurrencyMode.Multiple</code> means WCF should execute it in a separate thread on the server as well. This theory is supported by the fact that when I configure my app to use <code>NetTcpBinding</code>, everything works fine. Problem: If I configure the app to use <code>WSHttpBinding</code>, then this long method call causes the http requests to 'back up'. I have verified this behaviour both from inspecting my logs, and by debugging the HTTP requests using fiddler. Example: <ul> <li>Client initiates 20-second long request on a background thread</li> <li>Client initiates request B and C on foreground thread</li> <li>Requests B and C get sent to the server, which doesn't process them until it is done with the 20-second long request</li> </ul> But sometimes: <ul> <li>Requests B and C do not get sent (they don't even appear in fiddler) until the 20-second request comes back (this is rare). <ul> <li>Note: setting <code><add address="*" maxconnection="100"/></code> in the client's app.config made this (appear to) stop happening.</li> </ul></li> <li>Request B gets sent and receives a response immediately, while request C is held back until the 20-second one completes (this is rare)</li> </ul> Here's a timeline from fiddler demonstrating the problem: (click for bigger version) <a href="http://farm4.static.flickr.com/3311/3510636841_2a27435eec_o.png" rel="nofollow noreferrer"><img src="https://farm4.static.flickr.com/3311/3510636841_18f9b2b0a1.jpg" /></a> As you can see, the requests are all getting backed up at the server. Once the 20-second request completes, the responses all come flooding through, but note that there are some requests which aren't getting held up... So, Questions: <ul> <li>What the heck is going on here? Why does it work fine using <code>NetTcpBinding</code> and not work using <code>WSHttpBinding</code>?</li> <li>Why the inconsistent behaviour?</li> <li>What can I do to fix it?</li> </ul> Notes: <ul> <li>It's not locking on the server. I've set breakpoints and used <code>!syncblk</code> and it consistently reports no locks are being held.</li> <li>It's not my threading (NetTcpBinding shouldn't work otherwise)</li> <li>I have <code><serviceThrottling maxConcurrentCalls="1000" maxConcurrentInstances="1000" maxConcurrentSessions="1000" /></code> set in the server's app.config</li> <li>The 20-second call is just waiting on a timer, it's not thrashing the CPU or disk or network</li> <li>I'd prefer a solution that didn't involve re-architecting the application to use asynchronous calls... it's a big bunch of legacy code and I really don't want to be messing with stuff I don't understand.</li> </ul>
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<wcf><multithreading><concurrency><throttling>
Title
WCF Concurrent requests piling up on the server when using WSHttpBinding
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