Session persistence and failover

In order to make a session fail over successfully, you need to persist the session and to make the session information available in the adoptive server. WebSphere V5 adds the new feature of memory-to-memory session replication, in addition to database session persistence.

When session affinity and session persistence are both enabled, HTTP sessions are held in-memory of the appserver containing the Web application, and are also periodically persisted to the database or published to the broker(s). The plug-in will route multiple requests for the same HTTP session to the same server. This server can then retrieve the information from its in-memory session cache.

If this server is unavailable, the request is routed to another server, which reads the session information from a database or receives the session information from the broker locally (if available) or goes one more hop to retrieve the session information from a remote broker. The current WebSphere V5 HTTP WLM routing has no information about which servers have session information locally.

Another new and nice feature is that in WebSphere V5, the session manager can be configured at various levels:

1. Web module level

2. Enterprise application level

   All Web modules inside the enterprise application inherit the configuration defined at this level.

3. Appserver Web container level

   All Web modules in the server's Web container have the same configuration defined at this level. This is the default in WebSphere V5; WebSphere V4 supports only this level.

Therefore, you can have different persistent mechanisms in the Web containers at the same time to tune failover for each Web module. How much session information might be lost depends on the frequency of the session persistence, which is configurable on the appserver from the Administrative Console. This is discussed in the following section.

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IBM is a trademark of the IBM Corporation in the United States, other countries, or both.