Even with synchronous replication, the client can read and receive the response before replication completes. This means they can send another request that potentially fails over and sees inconsistent session state.

Need to investigate how to cleanly prevent the sending of the response (or at least the final flush of the response body) until the replication completes. Preventing sending of response headers is probably too much to try to do; perhaps just preventing the close of the response output stream.

By "replication completes" I mean:

w/ instant mode and REPL_SYNC – the full synchronous replication with acknowledgement of receipt by the foreign nodes
w/ instant mode and REPL_ASYNC – until JGroups has pushed the replication message onto the wire
w/ interval mode – basically instantaneous. i.e. as soon as the session is marked for replication.