Session Replication and Failover Operation

This section describes the session replication operation in detail.

At the end of a web request, the Web Server determines whether the session data needs to be copied through the session replication configuration that is stored in the server configuration file, the server.xml.

Consider a use case of four instances forming a cluster and the session replication is enabled on the Administration Server.

The session replication process in a Web Server cluster of four instances (A, B, C, and D) running on four nodes is as follows:

• Instance A is backup of D, B is backup of A, C is backup of B, and D is backup of C. This forms a complete backup ring.

• Each instance in the cluster keeps track of a static list of all the instances in the cluster and an active backup instance.

• Depending on the configuration, session data is sent to the backup instance synchronously at the end of each request.

The failover process in a Web Server clustered environment as follows:

• If instance A fails, the load balancer redirects all incoming web requests destined for instance A to the remaining instances in the cluster and the backup ring is re-configured as follows:

  • D detects that its backup A is down and selects the next instance to A on the ordered list as its new backup instance.

  • In this case, B is selected and D establishes a new backup connection to B. B now holds two backups: a read-only backup of A and an active backup of D.

  • The backup ring is now complete with B backing up to C, C backing up to D, and D backing up to B.

  • When the failed instance A is made available again, it rejoins the backup ring by sending its designated backup instance B a rejoin message and establishes a backup connection to B.

  • When D detects that A comes online by either receiving a successful ping return from A or by receiving a message from A.

  • D then establishes a backup connection to A and terminates its backup connection to B.

Web Server 7.0 does not support the following features in session replication:

• Recover simultaneous failures of two or more instances.

• The interval between two failures must be greater than the time needed for a resurrected instance to fully recover.

• Session backup to more than one instance. In normal operation, there are only two copies of any session: the primary session and a backup session.

• Session persistence: Sessions are only backed up in memory of another instance for the purpose of failover

• Web Server supports session replication for only Java web applications. If you are using non-Java applications such as CGI or PHP, the session data cannot be replicated.