Replicated caches replicate all changes to some or all of the other cache instances in the cluster. Replication can either happen after each modification (no transactions), or at the end of a transaction (commit time).

Replication can be synchronous or asynchronous . Use of either one of the options is application dependent. Synchronous replication blocks the caller (e.g. on a put() ) until the modifications have been replicated successfully to all nodes in a cluster. Asynchronous replication performs replication in the background (the put() returns immediately). JBoss Cache also offers a replication queue, where modifications are replicated periodically (i.e. interval-based), or when the queue size exceeds a number of elements, or a combination thereof.

Asynchronous replication is faster (no caller blocking), because synchronous replication requires acknowledgments from all nodes in a cluster that they received and applied the modification successfully (round-trip time). However, when a synchronous replication returns successfully, the caller knows for sure that all modifications have been applied to all cache instances, whereas this is not be the case with asynchronous replication. With asynchronous replication, errors are simply written to a log. Even when using transactions, a transaction may succeed but replication may not succeed on all cache instances.