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From "Stirling Chow (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (AMQ-3014) DemandForwardingBridgeSupport can send BrokerInfo to remote transport before local broker ID is known.
Date Tue, 12 Jul 2011 01:02:05 GMT

    [ https://issues.apache.org/jira/browse/AMQ-3014?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13063632#comment-13063632
] 

Stirling Chow commented on AMQ-3014:
------------------------------------

I've added a revised unit test that does not rely on Broker.addBroker(...) to demonstrate
that the issue (transmitting BrokerInfo with null ID) is still present on trunk.

> DemandForwardingBridgeSupport can send BrokerInfo to remote transport before local broker
ID is known.
> ------------------------------------------------------------------------------------------------------
>
>                 Key: AMQ-3014
>                 URL: https://issues.apache.org/jira/browse/AMQ-3014
>             Project: ActiveMQ
>          Issue Type: Bug
>          Components: Broker, Transport
>    Affects Versions: 5.4.1
>            Reporter: Stirling Chow
>         Attachments: NullBrokerIdTest.java, NullBrokerIdTest.java, patch.txt
>
>
> Symptom
> ========
> 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.
> Cause
> ======
> 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 {
> ...
>                     brokerInfo.setBrokerId(this.localBrokerId);
>                     remoteBroker.oneway(brokerInfo);
>                 }
> 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;
>             localBrokerIdKnownLatch.countDown();
> 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;
>                 }
>                 transport.start();
>                 active = true;
>                 dispatchAsync(connector.getBrokerInfo());
> 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.
> Solution
> ======
> 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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