ASP.NET session state and multiple worker processes

Question

I need to understand something about ASP.NET session state, as it applies to IIS 7 and ASP.net 3.5.

If an application is configured to use in-process session state, will that work OK if there are multiple worker processes? In other words, do worker processes share session state?

The default configuration for IIS 7 is to use in-process session state and to allocate a maximum of 10 worker processes. It would seem likely then, that this default configuration should work. I'm dealing with a company that has produced an ASP.NET MVC web app that is having some problems, they're blaming the server environment. The claim is that because I'm using the default settings of 10 worker processes, that is breaking their session state. I need to know whether this is in fact an accurate claim. I've never known an ASP.NET app to not work with the default configuration, so I'm a bit confused and need to have this clarified.

Solution 1

Having multiple worker processes and using InProc does not seem to be compatible.

See this:

If you enable Web-garden mode by setting the webGarden attribute to true in the processModel element of the application's Web.config file, do not use InProc session state mode. If you do, data loss can occur if different requests for the same session are served by different worker processes.

Solution 2

More than one worker process is a "web garden." In-process session state will not work correctly. You'll need to use either a single worker process for your web app, or use a session state server, or SQL Server for session state.

Solution 3

I may be wrong, but as far as I know, by default you only have 1 worker process per application domain with multiple worker threads to handle requests. In this case In-Proc Session State should work just fine (the default settings).

But if you do have multiple worker processes (not just worker threads, actual worker processes) you do need out of process session state.

I think having more than 1 worker process in ASP.NET is referred to web garden mode which you have to specifically enable and if you do, then you need out of process state management. See the comment box on this page under the In-Process Mode heading.

Solution 4

I experienced session lost problem and finally struggled to find the root cause.

Recently I received several bug reprot about the session lost. If the website load is low, everything is OK. If the website load is high, the session lost issue happens. This is very weird.

The root cause is between Worker process setting and Session state. Here we have 5 worker processes, which means it will have 5 independent processes running when the website load is high. While the session is stored in process, IIS cannot guarantee that a client user will use the same worker process. For example, the user client uses Process A when first visiting the web, and when he second visit the web, it may use Process B. There is no session stored in Process B, so his session is lost.

Why it is OK when the website load is low? Because IIS will only setup one worker process when the load is low. So the session lost issue will not happen. This explains why it is OK when I deploy a new version and test it OK at night, but the error happens again tomorrow morning. Because the website load is low at night.

Be careful to use session state in Process, it is unstable when your website load will be high and considering with mutiple worker processes. Try something like State Serversession state.

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