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HTTP sessions in WebSphere Application server can be replicated from one server instance to another to prevent single point of failures. This memory-to-memory replication uses the Data Replication Service (DRS) of WAS. Another option for HTTP session state is to persist session data in DB2.
The Web server plug-in keeps a list of the primary and possible backup appservers in the replication domain. When an HTTP session request fails on the primary server, the Web server plug-in sends the request to one of the backup appservers where the session is replicated (or which can retrieve the session from the database).
Application servers on z/OS that are enabled for HTTP session memory-to-memory replication can store replicated HTTP session data in the controller and replicate data to other appservers.
The HTTP session data that is stored in a controller is retrievable by any of the servants of that controller. HTTP session affinity is still associated to a particular servant. However, if that servant should fail, any of the other servants can retrieve the HTTP session data stored in the controller and establish a new affinity.
To handle sessional HTTP requests, the Web server plug-ins for all HTTP servers need to use static IP addresses. In addition, for sessionless HTTP requests, the plug-ins should also use a cell level IP address (DVIPA) so Sysplex Distributor can balance these requests between all systems in the sysplex.
