There are two general methods of connecting brokers into a conventional cluster: from the command line (using the -cluster option) or by setting the imq.cluster.brokerlist property in the cluster configuration file.
Whichever method you use, each broker that you start attempts to connect to the other brokers in the cluster every five seconds; the connection will succeed once the master broker is started up (if one is configured). If a broker in the cluster starts before the master broker, it will remain in a suspended state, rejecting client connections, until the master broker starts; the suspended broker then will automatically become fully functional. It is therefore a good idea to start the master broker first and then the others, after the master broker has completed its startup.
When connecting brokers into a conventional cluster, you should be aware of the following issues:
Mixed broker versions. A conventional cluster can contain brokers of different versions if all brokers have a version at least as great as that of the master broker. If the cluster is not configured to use a master broker, then all brokers must be of the same version.
Matching broker property values. In addition to cluster configuration properties, the following broker properties also must have the same value for all brokers in a cluster:
This restriction is particularly important when a cluster contains mixed broker versions that might contain properties with different default values. For example, If you are clustering a Message Queue version 4.1 or later broker together with those from earlier versions than Message Queue 4.1, you must set the value of the imq.autocreate.queue.maxNumActiveConsumers property, which has different default values before and after version 4.1 (1 and -1, respectively), to be the same. Otherwise the brokers will not be able to establish a cluster connection.
Multiple interface cards. On a multi-homed computer, in which there is more than one network interface card, be sure to explicitly set the network interface to be used by the broker for client connection services (imq.hostname) and for the cluster connection service (imq.cluster.hostname). If imq.cluster.hostname is not set, then connections between brokers might not succeed and as a result, the cluster will not be established.
Network loopback IP address. You must make sure that no broker in the cluster is given an address that resolves to the network loopback IP address (127.0.0.1). Any broker configured with this address will be unable to connect to other brokers in the cluster.
In particular, some Linux installers automatically set the localhost entry to the network loopback address. On such systems, you must modify the system IP address so that all brokers in the cluster can be addressed properly: For each Linux system participating in the cluster, check the /etc/hosts file as part of cluster setup. If the system uses a static IP address, edit the /etc/hosts file to specify the correct address for localhost. If the address is registered with Domain Name Service (DNS), edit the file DNS lookup is performed before consulting the local hosts file. The line in /etc/nsswitch.conf should read as follows: hosts: dns files/etc/nsswitch.conf to change the order of the entries so that
If you are using a master broker, identify it with the imq.cluster.masterbroker property in the configuration file.
Use the imqbrokerd command to start each broker.
If there is a master broker, start it first, then the others after it has completed its startup.
For example, the following command starts the broker as part of a cluster consisting of the brokers running at the default port (7676) on host1, at port 5000 on host2, and at port 9876 on the default host (localhost):
imqbrokerd -cluster host1,host2:5000,:9876
Once the master broker (if any) is running, start each of the other brokers in the cluster with the imqbrokerd command, using the same list of brokers with the -cluster option that you used for the master broker.
The value specified for the -cluster option must be the same for all brokers in the cluster.
For each broker in the cluster, set up SSL-based connection services, as described in Message Encryption.