A shared JDBC master/slave group works by sharing a common database using the JDBC persistence adapter. Brokers automatically configure themselves to operate in master mode or slave mode, depending on whether or not they manage to grab a mutex lock on the underlying database table.
The advantage of this configuration are:
that a group can consist of more than two members. I group can have an arbitrary number of slaves.
that a failed node can rejoin the group without any manual intervention. When a new node joins, or rejoins, the group it automatically falls into slave mode until it can get a mutex lock on the database table.
The disadvantages of this configuration are:
The shared database is a single point of failure. This disadvantage can be mitigated by using a database with built in high availability(HA) functionality. The database will handle replication and fail over of the data store.
You cannot enable high speed journaling. This has a significant impact on performance.
Figure 3.6 shows the initial state of a JDBC master/slave group. When all of the brokers are started, one of them grabs the mutex lock on the database table and becomes the master. All of the other brokers become slaves and pause while waiting for the lock to be freed up. Only the master starts its transport connectors, so all of the clients connect to it.
Figure 3.7 shows the state of the group after the original master has shut down or failed. As soon as the master gives up the lock (or after a suitable timeout, if the master crashes), the lock on the database table frees up and another broker grabs the lock and gets promoted to master.
After the clients lose their connection to the original master, they automatically try all of the other brokers listed in the failover URL. This enables them to find and connect to the new master.
In a JDBC master/slave configuration, there is nothing special to distinguish a master broker from the slave brokers. The membership of a particular master/slave group is defined by the fact that all of the brokers in the cluster use the same JDBC persistence layer and store their data in the same database tables.
Example 3.7 shows the configuration used be a master/slave group that stores the shared broker data in an Oracle database.
Example 3.7. JDBC Master/Slave Broker Configuration
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:amq="http://activemq.apache.org/schema/core" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd http://activemq.apache.org/schema/core http://activemq.apache.org/schema/core/activemq-core-5.3.1.xsd"> <broker xmlns="http://activemq.apache.org/schema/core" brokerName="brokerA"> ... <persistenceAdapter> <jdbcPersistenceAdapter dataSource="#oracle-ds"/> </persistenceAdapter> ... </broker> <bean id="oracle-ds" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="oracle.jdbc.driver.OracleDriver"/> <property name="url" value="jdbc:oracle:thin:@localhost:1521:AMQDB"/> <property name="username" value="scott"/> <property name="password" value="tiger"/> <property name="poolPreparedStatements" value="true"/> </bean> </beans>
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The persistence adapter is configured as a direct JDBC persistence layer,
using the |
Clients of shared JDBC master/slave group must be configured with a failover URL
that lists the URLs for all of the brokers in the group.
Example 3.8 shows the client failover URL for a
group that consists of three brokers: broker1, broker2,
and broker3.
Example 3.8. Client URL for a Shared JDBC Master/Slave Group
failover:(tcp://broker1:61616,tcp://broker2:61616,tcp://broker3:61616)
For more information about using the failover protocol see Static Failover.
You can restart the failed node at any time and it will rejoin the group. It will rejoin the group as a slave because one of the other brokers already owns the mutex lock on the database table, as shown in Figure 3.8.
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