WCF Silverlight client getting 404 not found response for poll message

Question

Eventually WCF duplex Silverlight 4 client start getting 404 Not Found errors for poll messages, immediately after poll was send from WCF service to Silverlight

Answer 1

While investigating issue described on this StackOverflow post Static constructor called twice for PerSession WCF service I found that Polling Duplex start working stable when I switched IIS configuration for underlying AppPool to use single worker process rather than 2 as was specified before. I'm not sure why 2 was set before since I do not own this server, but anyway this is what I have now - multiple Silverlight clients launched on the same machine works stable and polls polls polls and no 404 errors, all clients reconnects in 1 attempt after IIS restarts and recycles...

See Performance Application Pool Settings for more details

TL;DR: When IIS hosted WCF resides in AppPool which has more than one worker process - polling duplex becomes unstable. So in case of high load IIS started second process and start creating WCF service instances in second process as well, so I ran into situation when client session was created in one process but it seems poll sometimes reached an other process whcih is not aware of current connection/session so start refusing such messages and entire connection fault.

So Polling Duplex by design is not scalable across multiple processes in scope of a single IIS server and AppPool, in other words if you have more than 1 worker process - this is WebGarden environment and duplex is not scalable across web farms and gardens