Pure Master Slave

A Pure Master Slave configuration provides a basic shared nothing, fully replicated topology which does not depend on a shared file system or shared database.

How Pure Master Slave works

• A slave of a master broker consumes all message states from the master - messages, acknowledgments and transactional states.
  Whilst a Slave is actively connected to the Master - it does not allow or start any network or transport connectors, it's sole purpose is to duplicate the state of the master.

• The master broker will only respond to a client when a message exchange has been successfully passed to the slave. For example, a commit
  in a clients transaction will not complete until the master and the slave have processed the commit.

• In the event the master fails (e.g. hardware failure) the slave has optionally two modes of operation
  1. starts all it's network and transport connectors - allowing clients connected to the master to resume on the slave.
  2. or can be configured to close down. In this mode, the slave is simply used to duplicate state for the master.

• clients should use a failover transport for connecting to the master broker first and then the slave. e.g. using a URL such as

failover://(tcp://masterhost:61616,tcp://slavehost:61616)?randomize=false

The randomize property just disables randomness so that the transport will always try the master first, then the slave if it can't connect to that. Note that the slave does not accept connections until it becomes the master

Limitations of Pure Master Slave

• Only one slave can be connected to the Master
• A failed master cannot be re-introduced without shutting down the the slave broker (no automatic failback)
• There is no automatic synchronization between brokers. This is a manual process.

Recovering a Pure Master Slave topology

This is a manual process - once a master has failed, the only sure way to ensure that the toplogy is synchronized again is manually:

• shutdown the slave broker (The clients do not need to be shutdown - they will wait until the topology is re-established if they are failover clients
• copy the data directory from the slave over the data directory of the master broker
• re-start the master and the slave

Configuring Pure Master Slave

A master broker doesn't need any special configuration - it's a normal broker until a slave broker attaches itself.
To identify a broker as a slave - there is just one property to set (see below) as this example shows - so configuration is nice and easy:

<broker masterConnectorURI="tcp://masterhost:62001" shutdownOnMasterFailure="false">
  <persistenceAdapter>
      <journaledJDBC journalLogFiles="5" dataDirectory="${activemq.base}/data/broker2" />
    </persistenceAdapter>

    <transportConnectors>
	  <transportConnector uri="tcp://slavehost:61616"/>
   </transportConnectors>
</broker>

Configuring the authentication of the Slave

In ActiveMQ 4.1 or later you can use a <masterConnector/> element as an alternative XML configuration mechanism as shown in the following example to configure the user and password that the slave will use to connect to the master

<broker brokerName="slave" useJmx="false"  deleteAllMessagesOnStartup="true"  xmlns="http://activemq.apache.org/schema/core">
    <services>
      <masterConnector remoteURI= "tcp://localhost:62001" userName="James" password="Cheese"/>
    </services>

    <transportConnectors>
      <transportConnector uri="tcp://localhost:62002"/>
    </transportConnectors>
  </broker>