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From "Mario Siegenthaler (JIRA)" <j...@apache.org>
Subject [jira] Created: (AMQ-1925) JDBC-Master/Slave Failover - Consumer stop after 1000 Messages
Date Tue, 02 Sep 2008 17:39:52 GMT
JDBC-Master/Slave Failover - Consumer stop after 1000 Messages
--------------------------------------------------------------

                 Key: AMQ-1925
                 URL: https://issues.apache.org/activemq/browse/AMQ-1925
             Project: ActiveMQ
          Issue Type: Bug
          Components: Broker
    Affects Versions: 5.1.0
            Reporter: Mario Siegenthaler
         Attachments: heapdump-1220373534484.hprof, threaddump-1220371256910.tdump

In a JDBC-Master/Slave Environment with ActiveMQ 5.1.0 (+patches for 1710 und 1838) the failover
for consumers works, the consumers resume to get messages after the failover but then the
suddenly stop after approx. 1000 messages (mostly 1000, one got to 1080). The consumers are
using transacted sessions.

The thread dump look unsuspicious, everybody is waiting on the Socket
   java.lang.Thread.State: RUNNABLE
	at java.net.SocketInputStream.socketRead0(Native Method)
	at java.net.SocketInputStream.read(SocketInputStream.java:129)
	at org.apache.activemq.transport.tcp.TcpBufferedInputStream.fill(TcpBufferedInputStream.java:50)
	at org.apache.activemq.transport.tcp.TcpBufferedInputStream.read(TcpBufferedInputStream.java:58)
	at java.io.DataInputStream.readInt(DataInputStream.java:370)
	at org.apache.activemq.openwire.OpenWireFormat.unmarshal(OpenWireFormat.java:269)
	at org.apache.activemq.transport.tcp.TcpTransport.readCommand(TcpTransport.java:203)
	at org.apache.activemq.transport.tcp.TcpTransport.doRun(TcpTransport.java:195)
	at org.apache.activemq.transport.tcp.TcpTransport.run(TcpTransport.java:183)
	at java.lang.Thread.run(Thread.java:619)

A memory dump from the consumers shows that they've really run out of messages and are waiting
for the broker to deliver new ones. I've attached both the thread dump and the heap dump to
this issue (or better: I'll do so :)

The broker doesn't do anything (also waits on the transport-socket), the queue has a full
page-in buffer (100 messages) but obviously fails to do anything with it. If I manually trigger
a doDispatch of all pagedIn messages (via the debugger, just a try to revive the thing) it
returns doing nothing at all, since all subscriptions are full (s.isFull). I further investigated
the issue and was confused to see the prefetchExtension field of the PrefetchSubscription
having a value of -1000 (negative!). This explains why it was considered full:
  dispatched.size() - prefetchExtension >= info.getPrefetchSize()
  0 - (-1000) >= 1000
quite nasty.. so even though the dispatched size was zero the client didn't receive any new
messages.
The only place this value can become negative is inside acknowledge, where it's decremented
(prefetchExtension--), all other places do a Math.max(0, X).

So here's my guess what happened: The client had a full (1000 messages) prefetch buffer when
I killed my master. As soon as the slave was done starting they reconnected and started processing
the messages in the prefetch and acknowleding them. This gradually decremented the counter
into a negative value because the slave never got a chance to increment the prefetchExtension
since it didn't action delivery those messages.

Possible solutions:
- clear the prefetch buffer on a failover
- just don't allow this value to become smaller than zero (not sure if that covers all bases)

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