20.13. HornetQ Clustering

HornetQ clusters are used to create groups of HornetQ servers in order to share message processing load. Each active node in the cluster acts as an independent HornetQ server and manages its own messages and connections.
To form a cluster, each node (independent HornetQ server) declares cluster connections with another node with configuration parameters in server configuration files (standalone.xml and domain.xml).
In clustering, core bridges are used for bridging/routing messages from one cluster to another. Core bridges consume messages from a source queue and then forward these messages to a target HornetQ server (node) which may or may not be in the same cluster.
When a node forms a cluster connection with another node, it creates a core bridge internally. Each node creates an explicit core bridge and you do not need to declare it. These cluster connections allow transmission of messages between nodes in various clusters for balancing message processing load.
You can configure cluster nodes in server configuration files (standalone.xml and domain.xml).

Important

You can configure a node through server configuration files (standalone.xml and domain.xml) and copy this configuration to other nodes to generate a symmetric cluster. However you must be careful when you are copying the server configuration files. You must not copy the HornetQ data (i.e. the bindings, journal, and large messages directories) from one node to another. When a node is started for the first time it persists a unique identifier to the journal directory which is needed for proper formation of clusters.
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20.13.1. About Server Discovery

Servers use a mechanism called "server discovery" to:
  • Forward their connection details to messaging clients: Messaging clients intend to connect to servers of a cluster without specific details on the servers which are up and running at a given point of time
  • Connect to other servers: Servers in a cluster want to establish cluster connections with other servers without specific details on of all other servers in a cluster
Information about servers is sent to messaging clients via normal HornetQ connections and to other servers via cluster connections.
The initial first connection needs to be established and it can be established using dynamic server discovery techniques like UDP (User Datagram Protocol), JGroups or by providing a list of connectors.
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