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From "Stirling Chow (JIRA)" <>
Subject [jira] Created: (AMQ-3014) DemandForwardingBridgeSupport can send BrokerInfo to remote transport before local broker ID is known.
Date Wed, 03 Nov 2010 22:57:00 GMT
DemandForwardingBridgeSupport can send BrokerInfo to remote transport before local broker ID
is known.

                 Key: AMQ-3014
             Project: ActiveMQ
          Issue Type: Bug
          Components: Broker, Transport
    Affects Versions: 5.4.1
            Reporter: Stirling Chow

We have a production system that involves a set of Brokers connected in a demand-forwarding
Network-of-Brokers using HTTP-based bridges.  Each Broker periodically scans its list of peer
brokers by iterating over RegionBroker.getPeerBrokerInfos:

    public synchronized BrokerInfo[] getPeerBrokerInfos() {
        BrokerInfo[] result = new BrokerInfo[brokerInfos.size()];
        result = brokerInfos.toArray(result);
        return result;

This scanning code assumes that BrokerInfo.getBrokerId() is always non-null (since every broker
should have an ID).  However, we periodically noticed that BrokerInfo.getBrokerId() returned
a NULL value, which was very unexpected.

We analyzed the DemandForwardingBridgeSupport and noticed that when the remote bridge/transport
is started, it sends the local Broker's ID:

    protected void startRemoteBridge() throws Exception {

The local Broker's ID is not initially known until it is received from the local transport
and processed by DemandForwardingBridge.serviceLocalBrokerInfo(...):

    protected void serviceLocalBrokerInfo(Command command) throws InterruptedException {
        synchronized (brokerInfoMutex) {
            localBrokerId = ((BrokerInfo)command).getBrokerId();
            localBrokerPath[0] = localBrokerId;

The local Broker's ID is dispatched asynchronously when the local transport is started, as
seen in TransportConnection.start():

    public void start() throws Exception {
        starting = true;
        try {
            synchronized (this) {
                if (taskRunnerFactory != null) {
                    taskRunner = taskRunnerFactory.createTaskRunner(this, "ActiveMQ Connection
Dispatcher: "
                            + getRemoteAddress());
                } else {
                    taskRunner = null;
                active = true;

Because of the asynchronous dispatch, the remote bridge may be started before the local Broker's
ID is known.  This would be particularly evident when the local broker is under load processing
a lot of tasks.

We've attached a unit test that demonstrates how a slow asynchronous dispatch on the local
transport can cause the remote transport to transmit a null BrokerId.

DemandForwardingBridgeSupport already contains a  localBrokerIdKnownLatch, so starting the
remote transport should wait for this latch before accessing the local Broker's ID (see patch).

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