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From Abraham Fine <af...@apache.org>
Subject Re: Client hangs waiting for connection
Date Wed, 21 Jun 2017 05:42:06 GMT
Would it be possible to include the rest of the jstack, it appears that
is just the thread waiting on the latch and doesn't tell us why the
latch has not been counted down. Also, did ZK produce any interesting
logs?

Thanks,
Abe

On Tue, Jun 20, 2017, at 17:23, John Lindwall wrote:
> Thanks for the reply! I forgot to include the thread dump that I have 
> collected.  This process has been hung for almost a day so I'm guessing 
> it'll never connect properly ;-)  We actually had 2 such processes hung 
> today with the same stack trace (at least the same root cause as I show 
> below).  Please note that this problem is rare but supremely not good 
> when it does happen if we fail to detect it. We've been running this 
> code for many months now and this issue has only recently occurred.
> 
> Thread 4396: (state = BLOCKED)
> 
> - sun.misc.Unsafe.park(boolean, long) @bci=0 (Interpreted frame)
> 
> - java.util.concurrent.locks.LockSupport.park(java.lang.Object) @bci=14, 
> line=186 (Interpreted frame)
> 
> - 
> java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt() 
> @bci=1, line=834 (Interpreted frame)
> 
> - 
> java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(int)

> @bci=72, line=994 (Interpreted frame)
> 
> - 
> java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(int)

> @bci=24, line=1303 (Interpreted frame)
> 
> - java.util.concurrent.CountDownLatch.await() @bci=5, line=236 
> (Interpreted frame)
> 
> - com.mycode.ZooKeeperFactory.connect(java.lang.String, int) @bci=34, 
> line=59 (Interpreted frame)
> ...
> [remainder of stack trace omitted]
> 
> John
> 
> 
> Michael Han wrote:
> > Sounds like a dead lock on client library. One idea is to instrument your
> > client code and dump the thread stack when the wait timeouts. The stack
> > will hopefully contain the states of various threads and provide some
> > insights on what to look for next.
> >
> > On Tue, Jun 20, 2017 at 3:14 PM, John Lindwall<jlindwall@yahoo.com.invalid>
> > wrote:
> >
> >> We are seeing some occasional incidents where a zookeeper java client will
> >> hang in CountDownLatch.await() while waiting for a connection to be
> >> established.  Our connect() code is pretty standard I think and it similar
> >> to this:
> >>
> >>      private ZooKeeper connect(String hosts, int sessionTimeout) throws
> >> IOException, InterruptedException {
> >>          final CountDownLatch connectedSignal = new CountDownLatch(1);
> >>
> >>          ZooKeeper zk = new ZooKeeper(hosts, sessionTimeout, new Watcher() {
> >>              @Override
> >>              public void process(WatchedEvent event) {
> >>                  if (event.getState() == Event.KeeperState.SyncConnected) {
> >>                      connectedSignal.countDown();
> >>                  }
> >>              }
> >>          });
> >>
> >>          connectedSignal.await();
> >>          return zk;
> >>      }
> >>
> >> Has anyone else had an issue with the await() blocking forever like this?
> >> Any advice?
> >>
> >> As a "fix" I am considering adding a timeout to the CountDownLatch await()
> >> call; if we fail to connect within that timeout then retry the connection
> >> attempt. After, say, 3 retries, give up entirely.
> >>
> >> Thanks!
> >> --
> >> John Lindwall
> >>
> >>
> >
> >
> 
> -- 
> John Lindwall
> 

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