ActiveMQ 配置jdbc主从

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：