Out of the box .NET sessions are stored on the web server in memory, which is perfect for single-server environments. Load balanced environments with multiple servers however require a little more.

The alternatives:

1. Use a database - Regardless of which database camp you're on, using it for this purpose is extremely inefficient and should not be deployed in any production environment.
2. L7 load balance - Not extremely, but also inefficient. Requires special hardware. Prevents continuous balancing for a single user.

Memcached is an open-source distributed memory caching system that fits the bill perfectly:

• It's lightweight
• It's in-memory so it's fast
• It's distributed and very easy to scale

Duh. Many articles have been written about setting up Memcached. Use the one you like the most.

The Memcached client used in this project is BeITMemcached. It's lean, fast, and easy to work with.

A compiled version (dll) is included for simplicity and can be dropped into you /bin folder. The complete source is available on the project's homepage at http://code.google.com/p/beitmemcached/. Pick the way you like better and get BeIT into your project.

Defining a pool assigns one or more memcached servers a friendly name that can be used programmatically.

Add the following to your Global.asax file. Feel free to change DefaultPool.

void Application_Start(object sender, EventArgs e) {
  BeIT.MemCached.MemcachedClient.Setup("DefaultPool", new String[] { "server:port" });
}

Add MemcachedSessionStateStore.cs and LockInfo.cs to your App_Code folder.

Update the SessionState section of your Web.Config file as follows:

<sessionState customProvider="Memcached" mode="Custom" regenerateExpiredSessionId="true">
  <providers>
    <add name="Memcached" type="MemcachedSessionStateStore" ApplicationName="[application]" description="" pool="[pool]"/>
  </providers>
</sessionState>

Where:

• [application] is a unique name for your application. This is used to ensure multiple applications can run on the same Memcached server.
• [pool] is the name of the pool you defined in the previous step.

That's it. Your sessions should now be stored in Memcached.

• Memcached was/is not really meant to run on Windows. While there are ways to set it up none are recommended for production environments. Run your Memcached on Linux!.
• Memcached stores everything in memory which means a restart of the daemon or the server will result in the loss of all sessions. If your application can't sustain such loss you'll need to find a more persistent place to store your sessions!