17.13. Clustering with ModeShape

ModeShape has the ability to have a cluster of JcrEngine instances distributed across multiple processes while behaving as though everything was happening in a single process. With clusters, the workload can be distributed across multiple machines, increasing tolerance against failure while allowing ModeShape to scale out to handle more workload.

ModeShape clustering uses the powerful, flexible and mature JGroups library to handle all network communication within the cluster. JGroups provides a wealth of capabilities, including automatically detecting new engines in the cluster (called discovery), reliable multicast communication, and automatic determination of the master node in the cluster. JGroups has a flexible protocol stack, works across firewalls, WANs and LANs, and supports multiple transport protocols, failure detection, reliable unicast and multicast message transmission, and encryption.

By default, clustering is not enabled. This means that each JcrEngine instance is self-contained and will not be aware of changes made in other JcrEngine instances. This is perfect in many lightweight or embedded scenarios, because it does not introduce any overhead associated with network communication.

However, clustering ModeShape is very easy and requires only a few simple steps: Your JCR-based application does not need to change in any other ways. Any EventListener implementations registered in Sessions on any of the engines will be notified of all events, regardless of whether those events were due to changes in the local or remote engines.

It also does not matter how many Repository instances are defined in the configuration and managed by each JcrEngine instance: each engine in the cluster can manage multiple named repositories. ModeShape ensures that all Sessions for a named repository see the changes made to that repository, regardless of where those sessions are located in the cluster. Likewise, those same changes will not be visible to the sessions for any other named repository.