Session failover in the proxy server

The proxy server can use session failover to maintain sessions in case of disruption.

We can enable memory-to-memory replication on an application server to maintain session state in multiple servers. In this case, a private header is added to the response which identifies the backup servers for the session. The proxy server reads this header and maintains a list of backup servers for a session. If the proxy server fails to route to the primary server, it tries to route to the backup servers. If none of the backup servers are available, a deterministic algorithm selects one of the available servers; consequently, multiple proxy servers will route to the same server.

If the set of servers that are hosting a session changes, the private response header causes the proxy server to update its list of servers for the session. It is possible that the set of servers is updated, but the proxy server has not yet received an updated response header. In this case, the proxy server routes to a server that does not contain the session data. If this occurs, the backend server obtains the session data from a server containing the session data. There is no functional difference in this case; however, there is a performance difference due to the cost of obtaining the session data from another server.