Session Affinity

Accessible only to the web application that created the session, the Session ID but not the session data, can be shared across Web Applications.

Handled by a single JVM for that application at any one time

In a clustered environment, any HTTP requests associated with an HTTP session must be routed to the same Web application in the same JVM. This ensures that all of the HTTP requests are processed with a consistent view of the user’s HTTP session. The exception to this rule is when the cluster member fails or has to be shut down.

WebSphere is able to assure that session affinity is maintained in the following way: Each server ID is appended to the session ID. When an HTTP session is created, its ID is passed back to the browser as part of a cookie or URL encoding. When the browser makes further requests, the cookie or URL encoding will be sent back to the Web server. The Web server plug-in examines the HTTP session ID in the cookie or URL encoding, extracts the unique ID of the cluster member handling the session, and forwards the request.

The JSESSIONID cookie can be divided into four parts: cache ID, session ID, separator, clone ID, and partition ID (new in V6). JSESSIONID will include a partition ID instead of a clone ID when memory-to-memory replication in peer-to-peer mode is selected. Typically, the partition ID is a long numeric number.

For example the JSESSIONID cookie value of 0000HHAnbYWnNxGD-iVupvcArfr:14dtuueci is made up of these four parts

·                     Cache ID 0000

·                     Session ID: HHAnbYWnNxGD-iVupvcArfr

·                     separator :

·                     Clone ID: 14dtuueci

Note:  Session affinity can still be broken if the cluster member handling the request fails. To avoid losing session data, use persistent session management. In persistent sessions mode, cache ID and server ID will change in the cookie when there is a failover or when the session is read from the persistent store, so do not rely on the value of the session cookie remaining the same for a given session.