What is "Session Migration" ?
- Persisting the sessions, typically to a database
- Storing the sessions in-memory on multiple servers, typically a primary server and a backup server
In the latter case, all session data is redundantly stored in-memory on multiple servers within a cluster. The session state is replicated in-memory between servers within a cluster using IP multicasting. In this case, if one server crashes, its backup server now becomes the primary server for all sessions the failed server was maintaining.
In either situation, this type of session management allows for server failure without losing users' state.