使用 jdbc 方式配置主从模式，持久化消息存放在数据库中。
在同一时刻，只有一个 master broker，master 接受客户端的连接，slave 不接受连接。
当 master 因为关机而下线后，其中一个 slave 会提升为 master，然后接受客户端连接。但原来 master 的非持久消息丢失了，而持久消息保存在数据库中。
broker xml 配置：使用 MySQL 数据源
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<!--
Use JDBC for message persistence
For more information, see: http://activemq.apache.org/persistence.html You need to add Derby database to your classpath in order to make this example work.
Download it from http://db.apache.org/derby/ and put it in the ${ACTIVEMQ_HOME}/lib/optional/ folder
Optionally you can configure any other RDBM as shown below To run ActiveMQ with this configuration add xbean:conf/activemq-jdbc.xml to your command e.g. $ bin/activemq xbean:conf/activemq-jdbc.xml
-->
<beans
xmlns="http://www.springframework.org/schema/beans"
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.xsd"> <broker useJmx="false" brokerName="jdbcBroker" xmlns="http://activemq.apache.org/schema/core">
<persistenceAdapter>
<jdbcPersistenceAdapter dataDirectory="${activemq.base}/data" dataSource="#mysql-ds"/>
</persistenceAdapter> <transportConnectors>
<transportConnector name="default" uri="tcp://0.0.0.0:61616"/>
</transportConnectors>
</broker> <!-- Embedded Derby DataSource Sample Setup -->
<!-- <bean id="derby-ds" class="org.apache.derby.jdbc.EmbeddedDataSource">
<property name="databaseName" value="derbydb"/>
<property name="createDatabase" value="create"/>
</bean> --> <!-- Postgres DataSource Sample Setup -->
<!--
<bean id="postgres-ds" class="org.postgresql.ds.PGPoolingDataSource">
<property name="serverName" value="localhost"/>
<property name="databaseName" value="activemq"/>
<property name="portNumber" value="0"/>
<property name="user" value="activemq"/>
<property name="password" value="activemq"/>
<property name="dataSourceName" value="postgres"/>
<property name="initialConnections" value="1"/>
<property name="maxConnections" value="10"/>
</bean>
--> <!-- MySql DataSource Sample Setup --> <bean id="mysql-ds" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close">
<property name="driverClassName" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://192.168.40.8:3306/db_zhang?relaxAutoCommit=true"/>
<property name="username" value="root"/>
<property name="password" value="root"/>
<property name="maxActive" value="200"/>
<property name="poolPreparedStatements" value="true"/>
</bean> <!-- Oracle DataSource Sample Setup -->
<!--
<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="maxActive" value="200"/>
<property name="poolPreparedStatements" value="true"/>
</bean>
--> </beans>
客户端配置，producer 和 consumer 是一样的：
new ActiveMQConnectionFactory("failover:(tcp://localhost:61616,tcp://localhost:61618)");
以 MySQL 数据库为例，启动 broker 后，MySQL 中会创建 3 张表：
ACTIVEMQ_MSGS 存放持久消息
ACTIVEMQ_LOCK 表中只有一条记录，broker 执行 SELECT * FROM ACTIVEMQ_LOCK FOR UPDATE 获取锁，获得锁的 broker 是 master
ACTIVEMQ_ACKS
broker 获取锁的调用栈如下：
// org.apache.activemq.store.jdbc.DefaultDatabaseLocker
public void doStart() throws Exception { LOG.info("Attempting to acquire the exclusive lock to become the Master broker");
// SELECT * FROM ACTIVEMQ_LOCK FOR UPDATE
String sql = getStatements().getLockCreateStatement();
LOG.debug("Locking Query is "+sql); while (true) {
try {
connection = dataSource.getConnection();
connection.setAutoCommit(false);
lockCreateStatement = connection.prepareStatement(sql);
// 执行 SELECT * FROM ACTIVEMQ_LOCK FOR UPDATE
// 如果成功，则跳出循环。如果超时，则抛异常
lockCreateStatement.execute();
break;
} catch (Exception e) {
try {
if (isStopping()) {
throw new Exception(
"Cannot start broker as being asked to shut down. "
+ "Interrupted attempt to acquire lock: "
+ e, e);
}
if (exceptionHandler != null) {
try {
exceptionHandler.handle(e);
} catch (Throwable handlerException) {
LOG.error( "The exception handler "
+ exceptionHandler.getClass().getCanonicalName()
+ " threw this exception: "
+ handlerException
+ " while trying to handle this exception: "
+ e, handlerException);
} } else {
LOG.debug("Lock failure: "+ e, e);
}
} finally {
// Let's make sure the database connection is properly
// closed when an error occurs so that we're not leaking
// connections
if (null != connection) {
try {
connection.rollback();
} catch (SQLException e1) {
LOG.debug("Caught exception during rollback on connection: " + e1, e1);
}
try {
connection.close();
} catch (SQLException e1) {
LOG.debug("Caught exception while closing connection: " + e1, e1);
} connection = null;
}
}
} finally {
if (null != lockCreateStatement) {
try {
lockCreateStatement.close();
} catch (SQLException e1) {
LOG.debug("Caught while closing statement: " + e1, e1);
}
lockCreateStatement = null;
}
} LOG.info("Failed to acquire lock. Sleeping for " + lockAcquireSleepInterval + " milli(s) before trying again...");
try {
Thread.sleep(lockAcquireSleepInterval);
} catch (InterruptedException ie) {
LOG.warn("Master lock retry sleep interrupted", ie);
}
} LOG.info("Becoming the master on dataSource: " + dataSource);
}
broker 执行 SELECT * FROM ACTIVEMQ_LOCK FOR UPDATE 获取锁，获取成功，则成为 master，如果失败，则睡眠一段时间后，继续获取锁。
超时而抛出的异常：
