Sagar Rao created KAFKA-14401:
---------------------------------
Summary: Connector/Tasks reading offsets can get stuck if
underneath WorkThread dies
Key: KAFKA-14401
URL: https://issues.apache.org/jira/browse/KAFKA-14401
Project: Kafka
Issue Type: Bug
Reporter: Sagar Rao
When a connector or task tries to read the offsets from the offsets topic, it
issues `OffsetStorageImpl#offsets` method. This method gets a Future from the
underneath KafkaBackingStore. KafkaBackingStore invokes
`KafkaBasedLog#readToEnd` method and passes the Callback. This method
essentially adds the Callback to a Queue of callbacks that are being managed.
Within KafkaBasedLog, there's a WorkThread which keeps polling over the
callback queue and executes them and it does this in an infinite loop. However,
there is an enclosing try/catch block around the while loop. If there's an
exception thrown which is not caught by any of the other catch blocks, the
control goes to the outermost catch block and the WorkThread is terminated.
However, the connectors/tasks are not aware of this and they would keep
submitting callbacks to KafkaBasedLog with nobody processing them. This can be
seen in the thread dumps as well:
```
"task-thread-connector-0" #6334 prio=5 os_prio=0 cpu=19.36ms elapsed=2092.93s
tid=0x00007f8d9c037000 nid=0x5d00 waiting on condition [0x00007f8dc08cd000]
java.lang.Thread.State: WAITING (parking)
at jdk.internal.misc.Unsafe.park([email protected]/Native Method)
- parking to wait for <0x000000070345c9a8> (a
java.util.concurrent.CountDownLatch$Sync)
at
java.util.concurrent.locks.LockSupport.park([email protected]/LockSupport.java:194)
at
java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt([email protected]/AbstractQueuedSynchronizer.java:885)
at
java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly([email protected]/AbstractQueuedSynchronizer.java:1039)
at
java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly([email protected]/AbstractQueuedSynchronizer.java:1345)
at
java.util.concurrent.CountDownLatch.await([email protected]/CountDownLatch.java:232)
at
org.apache.kafka.connect.util.ConvertingFutureCallback.get(ConvertingFutureCallback.java:98)
at
org.apache.kafka.connect.storage.OffsetStorageReaderImpl.offsets(OffsetStorageReaderImpl.java:101)
at
org.apache.kafka.connect.storage.OffsetStorageReaderImpl.offset(OffsetStorageReaderImpl.java:63)
```
We need a mechanism to restart the WorkThread if it dies. This could be done in
the outermost catch block for example.
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