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From John Lindwall <jlindw...@yahoo.com.INVALID>
Subject Re: Client hangs waiting for connection
Date Wed, 21 Jun 2017 00:23:32 GMT
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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