ActiveMQ broker 集群, 静态发现和动态发现

下载 activemq 压缩包解压后，conf 目录下有各种示例配置文件，红线标出的是静态发现和动态发现的配置。

1. 静态配置

启动3个 broker，端口分别为61616，61618，61620，配置如下：

<networkConnectors></networkConnectors>
<transportConnectors>
    <transportConnector name="openwire" uri="tcp://localhost:61616"/>
</transportConnectors>

<networkConnectors>
    <networkConnector uri="static:(tcp://localhost:61616)" duplex="true"/>
</networkConnectors>
<transportConnectors>
    <transportConnector name="openwire" uri="tcp://localhost:61618"/>
</transportConnectors>

<networkConnectors>
    <networkConnector uri="static:(tcp://localhost:61616,tcp://localhost:61618)" duplex="true"/>
</networkConnectors>
<transportConnectors>
    <transportConnector name="openwire" uri="tcp://localhost:61620"/>
</transportConnectors>

3个 broker 组成了一张网，当 producer 发送消息给 broker：61616 后，broker：61618 的消费者可以收到该消息。消息从 broker：61616 流动到 broker：61618，底层原理是 broker：61618 是 broker：61616 的一个消费者。

2. 动态配置

同样地， 启动3个 broker，端口分别为61616，61618，61620，配置如下：

<networkConnectors>
    <networkConnector uri="multicast://default"/>
</networkConnectors>
    <transportConnector name="openwire" uri="tcp://0.0.0.0:61616" discoveryUri="multicast://default" />
</transportConnectors>

<networkConnectors>
    <networkConnector uri="multicast://default"/>
</networkConnectors>
<transportConnectors>
    <transportConnector name="openwire" uri="tcp://0.0.0.0:61618" discoveryUri="multicast://default" />
</transportConnectors>

<networkConnectors>
    <networkConnector uri="multicast://default"/>
</networkConnectors>
<transportConnectors>
    <transportConnector name="openwire" uri="tcp://0.0.0.0:61620" discoveryUri="multicast://default" />
</transportConnectors>

使用多播协议，把 3 个 broker 动态地组成了一张网。

// 省略其他代码
public class MulticastDiscoveryAgent implements DiscoveryAgent, Runnable {

    public static final String DEFAULT_DISCOVERY_URI_STRING = "multicast://239.255.2.3:6155";
    public static final String DEFAULT_HOST_STR = "default";
    public static final String DEFAULT_HOST_IP  = System.getProperty("activemq.partition.discovery", "239.255.2.3");
    public static final int    DEFAULT_PORT  = 6155; 

    private static final Logger LOG = LoggerFactory.getLogger(MulticastDiscoveryAgent.class);
    private static final String TYPE_SUFFIX = "ActiveMQ-4.";
    private static final String ALIVE = "alive.";
    private static final String DEAD = "dead.";
    private static final String DELIMITER = "%";
    private static final int BUFF_SIZE = 8192;
    private static final int DEFAULT_IDLE_TIME = 500;
    private static final int HEARTBEAT_MISS_BEFORE_DEATH = 10;

    public void run() {
        byte[] buf = new byte[BUFF_SIZE];
        DatagramPacket packet = new DatagramPacket(buf, 0, buf.length);
        while (started.get()) {
            // 发送多播数据
            doTimeKeepingServices();
            try {
                // 接收多播数据
                mcast.receive(packet);
                if (packet.getLength() > 0) {
                    String str = new String(packet.getData(), packet.getOffset(), packet.getLength());
                    processData(str);
                }
            } catch (SocketTimeoutException se) {
                // ignore
            } catch (IOException e) {
                if (started.get()) {
                    LOG.error("failed to process packet: " + e);
                }
            }
        }
    }
}

3. broker 集群的原理

如上图，61616和61618组成集群，当61618加入一个consumer时，61618向61616发送一条ConsumerInfo消息，这样61618就成为了61616的consumer。

ConsumerInfo 示例：

ConsumerInfo {
    commandId = 4,
    responseRequired = false,
    consumerId = dynamic-broker1->dynamic-broker2-1872-1524494145961-2:1:1:1,
    destination = queue://TEST.BAT,
    prefetchSize = 1,
    maximumPendingMessageLimit = 0,
    browser = false,
    dispatchAsync = true,
    selector = null,
    clientId = ID:USER-20140617MT-1882-1524494166015-0:1,
    subscriptionName = null,
    noLocal = false,
    exclusive = true,
    retroactive = false,
    priority = -5,
    brokerPath = [ID:USER-20140617MT-1877-1524494148767-0:1],
    optimizedAcknowledge = false,
    noRangeAcks = false,
    additionalPredicate = org.apache.activemq.command.NetworkBridgeFilter@413249b
}