20.14.6. HornetQ Message Replication
Only persistent messages are replicated. Any non-persistent messages do not survive failover.
Message replication between a live and a backup server is achieved via network traffic as the live and backup servers do not share the same data stores. All the journals are replicated between the two servers as long as the two servers are within the same cluster and have the same cluster username and password. All persistent data traffic received by the live server gets replicated to the backup server.
When the backup server comes online, it looks for and connects to a live server to attempt synchronization. While it is synchronizing, it is unavailable as a backup server. Synchronization can take a long time depending on the amount of data to be synchronized and the network speed. If the backup server comes online and no live server is available, the backup server will wait until the live server is available in the cluster.
To enable servers to replicate data, a link must be defined between them in the
standalone-full-ha.xml file. A backup server will only replicate with a live server with the same group name. The group name must be defined in the
backup-group-name parameter in the
standalone-full-ha.xml file on each server.
In the event of a live server failing, the correctly configured and fully synchronized backup server takes over its duties. The backup server will activate only if the live server has failed and the backup server is able to connect to more than half of the servers in the cluster. If more than half of the other servers in the cluster also fail to respond it would indicate a general network failure and the backup server will wait to retry the connection to the live server.
To get to the original state after failover, it is necessary to start the live server and wait until it is fully synchronized with the backup server. When this has been achieved, you can shutdown the backup server for the original live server to activate again. This happens automatically if the
allow-failback attribute is set to true.