The following illustration shows salient features of a Message Queue broker cluster. Each of three brokers is connected to the other brokers in the cluster: the cluster is fully-connected. The brokers communicate with each other and pass messages by way of a special cluster connection service, shown in Figure 4–1 by the dashed lines.
Each broker typically has a set of messaging clients (producers and/or consumers) that are directly connected to that broker. For these client applications, the broker to which they are directly connected is called their home broker. Each client communicates directly only with its home broker, sending and receiving messages as if that broker were the only broker in the cluster.
Accordingly, a producer in the cluster produces messages to a destination in its home broker. The home broker is responsible for routing and delivering the messages to all consumers of the destination, whether these consumers are local (connected to the home broker) or remote (connected to other brokers in the cluster). The home broker works in concert with the other brokers to deliver messages to all consumers, no matter what brokers they are connected to.
To facilitate delivery of messages across the cluster, information about the destinations and consumers of each broker is propagated to all brokers in the cluster. Each broker therefore stores the following information:
The name, type, and properties of all physical destinations in the cluster
The name, location, and destination of interest of each message consumer
Changes in this information are propagated whenever one of the following events occurs:
A destination on one of the cluster’s brokers is created or destroyed.
There are minor variations in the propagation of destinations, depending on the kind of destination:
Admin-created destinations. When the destination is created, it is propagated across the cluster. When the destination is deleted on any broker in the cluster, it's deletion is propagated across the cluster.
Auto-created destinations. When a producer is created and the corresponding destination does not exist, the destination is auto-created on the producer's home broker, but is not immediately propagated across the cluster. By contrast, when a consumer is created and the corresponding destination does not exist, the destination is auto-created on the consumer's home broker and is propagated across the cluster (as part of the propagation of information about the consumer). An auto-created destination can be explicitly deleted by an administrator on each broker. Otherwise, the destination will be automatically deleted on each broker either when it has had no consumers and has contained no messages for two minutes, or when the broker restarts and there are no messages in the destination.
Temporary destinations. When the destination is programmatically created, it is propagated across the cluster. If the consumer of the temporary destination is set to automatically reconnect in the event of failure, then the destination is stored persistently, and propagated across the cluster as a persistent destination. When the consumer connection to the temporary destination closes, the destination is deleted, and it's deletion is propagated across the cluster. If the home broker of the consumer of a persistent temporary destination fails and is restarted, and if the consumer does not reconnect within a specific time interval, then it is assumed that the consumer has failed and the temporary destination is deleted, and it's deletion is propagated across the cluster.
The properties of a destination are changed.
A message consumer is registered with its home broker.
A message consumer is disconnected from its home broker (whether explicitly or through failure of the client, the broker, or the network).
The propagation of destination and consumer information across the cluster means that destinations and consumers are essentially global to the cluster. In the case of destinations, properties set for a physical destination (see Configuring Physical Destinations) apply to all instances of that destination in the cluster. Distributing producers across a cluster thus results in cumulative cluster-wide limits specified by destination properties such as the maximum number of messages, the maximum number of message bytes, and the maximum number of producers.
Despite the global nature of destinations and consumers in a cluster, a home broker has special responsibilities with respect to both its producers and consumers:
A producer’s home broker is responsible for persisting and routing messages originating from that producer, for logging, for managing transactions, and for processing acknowledgements from consuming clients across the cluster.
A consumer’s home broker is responsible for persisting information about consumers, for delivering remotely produced messages to the consumer, for letting a producer’s home broker know whether the consumer is still available, and for letting a producer's home broker know when each message has been successfully consumed.
The cluster connection service transports payload messages, when needed, from destinations on a home broker to destinations on remote brokers. It also transports control messages, such as client acknowledgements, from remote brokers back to a home broker. The cluster attempts to minimize message traffic across the cluster. For example, it only sends a message to a remote broker if the remote broker is home to a consumer of the message. If a remote broker has two identical consumers for the same destination (for example two topic subscribers), the message is sent over the wire only once. (You can further reduce traffic by setting a destination property specifying that delivery to local consumers has priority over delivery to remote consumers.)
If secure message delivery is required, you can configure a cluster to also provide secure, encrypted delivery of messages between brokers.
As a result of the cluster delivery mechanisms described above, each broker in a cluster stores different persistent messages and maintains different state information. If a broker fails, the mechanisms for recovering its persistent information depends on the cluster model being used, as described in subsequent sections.