Within a major version, releases of JBoss Cache are meant to be compatible and interoperable. Compatible in the sense that it should be possible to upgrade an application from one version to another by simply replacing the jars. Interoperable in the sense that if two different versions of JBoss Cache are used in the same cluster, they should be able to exchange replication and state transfer messages. Note however that interoperability requires use of the same JGroups version in all nodes in the cluster. In most cases, the version of JGroups used by a version of JBoss Cache can be upgraded.
As such, JBoss Cache 2.x.x is not API or binary compatible with prior 1.x.x versions. However, JBoss Cache 2.1.x will be API and binary compatible with 2.0.x.
A configuration attribute, ReplicationVersion, is available and is used to control the wire format of inter-cache communications. They can be wound back from more efficient and newer protocols to "compatible" versions when talking to older releases. This mechanism allows us to improve JBoss Cache by using more efficient wire formats while still providing a means to preserve interoperability.
A compatibility matrix is maintained on the JBoss Cache website, which contains information on different versions of JBoss Cache, JGroups and JBoss AS.