chia7712 commented on a change in pull request #8657:
URL: https://github.com/apache/kafka/pull/8657#discussion_r479679930
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File path: core/src/main/scala/kafka/server/DelayedOperation.scala
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@@ -100,41 +99,22 @@ abstract class DelayedOperation(override val delayMs: Long,
def tryComplete(): Boolean
/**
- * Thread-safe variant of tryComplete() that attempts completion only if the
lock can be acquired
- * without blocking.
*
- * If threadA acquires the lock and performs the check for completion before
completion criteria is met
- * and threadB satisfies the completion criteria, but fails to acquire the
lock because threadA has not
- * yet released the lock, we need to ensure that completion is attempted
again without blocking threadA
- * or threadB. `tryCompletePending` is set by threadB when it fails to
acquire the lock and at least one
- * of threadA or threadB will attempt completion of the operation if this
flag is set. This ensures that
- * every invocation of `maybeTryComplete` is followed by at least one
invocation of `tryComplete` until
- * the operation is actually completed.
+ * There is a long story about using "lock" or "tryLock".
+ *
+ * 1) using lock - There was a lot of cases that a thread holds a group lock
and then it tries to hold more group
+ * locks to complete delayed requests. Unfortunately, the scenario causes
deadlock and so we had introduced the
+ * "tryLock" to avoid deadlock.
+ *
+ * 2) using tryLock - However, the "tryLock" causes another issue that the
delayed requests may be into
+ * oblivion if the thread, which should complete the delayed requests, fails
to get the lock.
+ *
+ * Now, we go back to use "lock" and make sure the thread which tries to
complete delayed requests does NOT hold lock.
+ * The approach is that ReplicaManager collects all actions, which are used
to complete delayed requests, in a queue.
+ * KafkaApis.handle() and the expiration thread for certain delayed
operations (e.g. DelayedJoin) pick up and then
+ * execute an action when no lock is held.
Review comment:
It is dangerous to complete delayed actions by ```DelayedOperation``` as
most methods of ```DelayedOperation``` are executed with locking. We, now,
depend on ```KafkaApi. handler``` to complete the delayed action produced by
someone who can't complete delayed action due to locking. For example,
```DelayedJoin.onComplete``` can produce an new action and the new action can't
be completed by ```DelayedJoin``` itself due to group locking.
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