Chapter 4. Master/Slave and Broker Networks

Master/slave groups and broker networks represent different levels of organization. A network of brokers provides a symmetrical group of brokers that share information among all of the members in the group. They are useful for distributing the message processing load among many brokers.

Master/slave groups are asymmetrical> Only one member of the group is active at a time. They are useful for providing fault tolerance when data loss is unacceptable.

You can include a master/slave group as a node in a network of brokers. Using the basic principles of making a master/slave group a node in a broker network, you can scale up to an entire network consisting of master/slave groups.

When combining master/slave groups with broker networks there are two things to remember:

The network connection to a master/slave group needs to do two things:

The network connector's reconnect logic will handle the reconnection to the new master in the case of a network failure. The network connector's connection logic, however, attempts to establish connections to all of the specified brokers. To get around the network connector's default behavior, you use a masterslave URI to specify the list of broker's in the master/slave group. The masterslave URI only allows the connector to connect to one of brokers in the list which will be the master.

The masterslave protocol's URI is a list of the connections points for each broker in the master/slave group. The network connector will traverse the list in order until it establishes a connection.

Example 4.1, “Network Connector to a Master/Slave Group” shows a network connector configured to link to a master/slave group.

<networkConnectors>
  <networkConnector name="linkToCluster"
                uri="masterslave:(tcp://masterHost:61002,tcp://slaveHost:61002)"
                ... />            
</networkConnectors>

In order to scale up to a large fault tolerant broker network, it is a good idea to adopt a simple building block as the basis for the network. An effective building block for this purpose is the host pair arrangement shown in Figure 4.1, “Master/Slave Groups on Two Host Machines”.

The host pair arrangement consists of two master/slave groups distributed between two host machines. Under normal operating conditions, one master broker is active on each of the two host machines. If one of the machines should fail for some reason, the slave on the other machine takes over, so that you end up with two active brokers on the healthy machine.

When configuring the network connectors, you must remember not to open any connectors to brokers in the same group. For example, the network connector for brokerB1 should be configured to connect to at most brokerA1 and brokerA2.

You can easily scale up to a large fault tolerant broker network by adding host pairs, as shown in Figure 4.2, “Broker Network Consisting of Host Pairs”.

The preceding network consists of eight master/slave groups distributed over eight host machines. As before, you should open network connectors only to brokers outside the current master/slave group. For example, brokerA1 can connect to at most the following brokers: brokerB*, brokerC*, brokerD*, brokerE*, brokerF*, brokerG*, and brokerH*.

For detailed information on setting up a network of brokers see "Using Networks of Brokers".