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From "Gary Tully" <>
Subject Re: Race condition with transport listener and failover transports??
Date Mon, 22 Dec 2008 12:07:03 GMT
appended a little comment to the doc to use
Thanks :-)

2008/12/21 Jim Lloyd <>:
> To answer my own question, it turns out that ActiveMQConnectionFactory has a
> setTransportListener method that can be called before createTopicConnection
> is called.
> I also deduced that the race condition is unlikely in my previous example
> because the failover transport uses the initialReconnectDelay parameter to
> impose a sleep before starting the connection. I wanted to verify this so I
> did these tests:
>   1. Using essentially the same code I posted previously, I changed the
>   initialReconnectDelay to 1ms instead of the default 10ms. I inserted a sleep
>   of 5ms between the call to createTopicConnection and addTransportListener.
>   That was enough to trigger race condition problem.
>   2. I then added a call to setTransportListener on the factory before
>   creating the connection leaving the other code as is. That seemed to be
>   enough to fix the race condition.
> I haven't yet traced through the source to verify that using
> ActiveMQConnectionFactory.setTransportListener is enough to eliminate any
> chance of a race condition, but I expect it is. About the only thing I think
> would be useful at this point is if there was a note about this in the
> documentation, perhaps near the warning at the bottom of the
> failover-transport-reference<>page.
> Jim Lloyd
> On Thu, Dec 18, 2008 at 3:11 PM, Jim Lloyd <>wrote:
>> We're building a client library to install into our customer's applications
>> for publishing messages to our applications via ActiveMQ. We need to ensure
>> that our client library never blocks any of the customer's application's
>> threads. We also want to ensure that our client library is resilient to
>> problems connecting to a broker.
>> Our strategy is to use a failover transport. It looks like we can use the
>> default failover transport options as documented on the
>> failover-transport-reference<>page.
We're registering a TransportListener so that we can avoid blocking as
>> noted at the bottom of the failover transport reference page.
>> I have noticed a possible race condition and I wonder if there is something
>> I am overlooking.
>> Our class that manages our connection looks something like this:
>> class TopicPublisherClient implements TransportListener {
>>     private boolean mIsConnected = false;
>>     private TopicConnection mConnection;
>>     private TopicSession mSession;
>>     private TopicPublisher mPublisher;
>>     public void transportInterupted() { mIsConnected = false; }
>>     public void transportResumed() { mIsConnected = true; }
>>     private void initialize(String brokerURL) throws JMSException {
>>         ActiveMQConnectionFactory factory = new
>> ActiveMQConnectionFactory(brokerURL);
>>         mConnection = factory.createTopicConnection();
>>         ((ActiveMQConnection) mConnection).addTransportListener(this);
>>         mSession = mConnection.createTopicSession(false,
>>         createTopics(); // creates several Topic instances, details not
>> important here
>>         mPublisher = mSession.createPublisher(null);
>>     }
>> }
>> With the code as above, the transportResumed() callback is called before
>> initialize() exits. However the callback is called asynchronously, so as far
>> as I can tell we can't rely on when the callback will be called.
>> In fact, the race condition is even more problematic. If I insert a
>> Thread.sleep(1000) between the call to createTopicConnection() and the call
>> to addTransportListener(), then the callback is never called. If a sleep
>> causes this to happen, then even without the sleep it may happen. Is there
>> any way for us to guard against this? It seems to me that
>> createTopicConnection should have a variant that takes the
>> TransportListener. That variant should be able to be free of any race
>> conditions. Or am I missing some other solution?
>> Thanks,
>> Jim Lloyd


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