DemandForwardingBridgeSupport can send BrokerInfo to remote transport before
local broker ID is known.
------------------------------------------------------------------------------------------------------
Key: AMQ-3014
URL: https://issues.apache.org/activemq/browse/AMQ-3014
Project: ActiveMQ
Issue Type: Bug
Components: Broker, Transport
Affects Versions: 5.4.1
Reporter: Stirling Chow
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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