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From "Martin Murphy (JIRA)" <j...@apache.org>
Subject [jira] Created: (AMQ-2475) If tmp message store fills up, broker can deadlock due to while producers wait on disk space and consumers wait on acks
Date Tue, 03 Nov 2009 10:03:52 GMT
If tmp message store fills up, broker can deadlock due to while producers wait on disk space
and consumers wait on acks
-----------------------------------------------------------------------------------------------------------------------

                 Key: AMQ-2475
                 URL: https://issues.apache.org/activemq/browse/AMQ-2475
             Project: ActiveMQ
          Issue Type: Bug
          Components: Broker, Message Store, Transport
    Affects Versions: 5.3.0
         Environment: Tested on Windows XP with JDK 1.60_13, but fairly sure it will be an
issue on all platforms
            Reporter: Martin Murphy


I will attach a simple project that shows this. In the test the tmp space is set to 32 MB
and two threads are created. One thread will constantly produce 1KB messages and the other
consumes these, but sleeps for 100ms, note that producer flow control is turned off as well.
The goal here is to ensure that the producers block while the consumers read the rest of the
messages from the broker and catch up, this in turn frees up the disk space and allows the
producer to send more messages. This config means that you can bound the broker based on disk
space rather than memory usage.

Unfortunately in this test using topics while the broker is reading in the message from the
producer it has to lock the matched list it is adding it to. This is an abstract from the
Topic's point of view and doesn't realize that the file may block based on the file system.

{code}
    public void add(MessageReference node) throws Exception { //... snip ...
            if (maximumPendingMessages != 0) {
                synchronized (matchedListMutex) {   // We have this mutex
                    matched.addMessageLast(node); // ends up waiting for space
                    // NOTE - be careful about the slaveBroker!
                    if (maximumPendingMessages > 0) {
{code}
Meanwhile the consumer is sending acknowledgements for the 10 messages it just read in (the
configured prefetch) from the same topic, but since they also modify the same list in the
topic this waits as well on the mutex held to service the producer:
{code}
    private void dispatchMatched() throws IOException {       
        synchronized (matchedListMutex) {  // never gets passed here.
            if (!matched.isEmpty() && !isFull()) {
{code}
This is a fairly classic deadlock. The trick is now how to resolve this given the fact that
the topic isn't aware that it's list may need to wait for the file system to clean up.


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