Sun Java System Message Queue 4.3 Technical Overview

Cluster Message Delivery

A broker cluster facilitates the delivery of messages between client applications that are connected to different brokers in the cluster.

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.

Figure 4–1 Message Queue Broker Cluster

Diagram showing basic elements of a broker cluster. Figure
explained in the text.

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.

Propagation of Information Across a Cluster

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:

Changes in this information are propagated whenever one of the following events occurs:

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.

Message Delivery Mechanisms

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:

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.