ActiveMQ-cpp Broker URI with PrefetchPolicy has no effect

Question

I am using activemq-cpp 3.7.0 with VS 2010 to build a client, the server is ActiveMQ 5.8. I have created a message consumer using code similar to the following, based on the CMS configurations mentioned here. ConnClass is a ExceptionListener and a MessageListener. I only want to consume a single message before calling cms::Session::commit().

void ConnClass::setup()
{

    // Create a ConnectionFactory
    std::tr1::shared_ptr<ConnectionFactory> connectionFactory(
        ConnectionFactory::createCMSConnectionFactory(
            "tcp://localhost:61616?cms.PrefetchPolicy.queuePrefetch=1");

    // Create a Connection
    m_connection = std::tr1::shared_ptr<cms::Connection>(
        connectionFactory->createConnection());

    m_connection->start();
    m_connection->setExceptionListener(this);

    // Create a Session
    m_session = std::tr1::shared_ptr<cms::Session>(
        m_connection->createSession(Session::SESSION_TRANSACTED));


    // Create the destination (Queue)
    m_destination = std::tr1::shared_ptr<cms::Destination>(
        m_session->createQueue("myqueue?consumer.prefetchSize=1"));

    // Create a MessageConsumer from the Session to the Queue
    m_consumer = std::tr1::shared_ptr<cms::MessageConsumer>(
        m_session->createConsumer( m_destination.get() ));

    m_consumer->setMessageListener( this );

}

void ConnClass::onMessage( const Message* message )
{
    // read message code ...
    // schedule a processing event for
    // another thread that calls m_session->commit() when done
}

The problem is I am receiving multiple messages instead of one message before calling m_session->commit() -- I know this because the commit() call is triggered by user input. How can I ensure onMessage() is only called once before each call to commit()?

score 0 · Accepted Answer · answered Jul 19 '13 at 20:02

0

It doesn't work that way. When using async consumers the messages are delivered as fast as the onMessage method completes. If you want to consume one and only one message then use a sync receive call.

For an async consumer the prefetch allows the broker to buffer up work on the client instead of firing one at a time so you can generally get better proformance, in your case as the async onMessage call completes an ack is sent back to the broker an the next message is sent to the client.

answered Jul 19 '13 at 20:02

Tim Bish

17,475
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But why can't the number of messages be controlled by the prefetch "size" options? Or have I understood the prefetch options wrongly? – devil Jul 20 '13 at 04:53
You aren't understanding the option. Its simply an indication to the broker of how much work to try and keep stacked up on the client end. The word async in async consumer should be your clue. – Tim Bish Jul 20 '13 at 10:45
I used async so that I do not need to manage an extra thread which I will have to explicitly do so with sync. I am trying to limit the number of messages I receive before I commit the transaction. Are you saying that this cannot be done using the size parameter in the prefetch policy for the async method? Does it mean I should force onMessage to block at the end of the method until I commit if I only want a single message? By the way, I am using SESSION_TRANSACTED mode so I use commit instead of ACKing the messages. – devil Jul 21 '13 at 09:02
I'm not sure how many more ways to say prefetch is not a limit. Its async so you can invent some way to block it or you can just use a sync consumer and make you life much simpler. – Tim Bish Jul 21 '13 at 13:05
I see, the prefetch size must be an internal library buffer setting then. Thank you for being patient. I will try a sync consumer. – devil Jul 22 '13 at 01:29

Pingmin Fenlly Liu · Answer 2 · 2017-12-08T09:29:16.647

0

Yes, I find this too. However, when I use the Destination URI option ( "consumer.prefetchSize=15" , http://activemq.apache.org/cms/configuring.html#Configuring-DestinationURIParameters ) for the asynchronous consumer, It works well.

BTW, I just use the latest ActiveMQ-CPP v3.9.4 by Tim , and ActiveMQ v5.12.1 on CentOS 7.

Thanks!

edited Dec 08 '17 at 09:29

answered Dec 08 '17 at 08:49

Pingmin Fenlly Liu

51
2

ActiveMQ-cpp Broker URI with PrefetchPolicy has no effect

2 Answers2