Supporting Session State with NLB

Some Web applications won’t work properly with Network Load Balancing (NLB) because they store persistent user data (called sessions) on Web servers. However, Application Center includes a Web server extension called the request forwarding component that enables some Web applications that use sessions to run unaltered in an NLB cluster.

The Problem of Sessions

IIS enables a Web application to maintain session data for a user that lasts across multiple requests by the user. For example, an online ordering application might maintain the user’s shopping cart as part of the user’s session. IIS uses “session cookies” stored on the user’s machine and supplied by the user’s browser in each request to keep track of which session belongs to which user.

Ideally, NLB would always route connections from a user with an active session to the server where that session is stored. However, NLB doesn’t know about sessions and doesn’t look at session cookies (or any other kind of cookies, for that matter) when deciding which Web server should handle an incoming request.