Andy Coates created KAFKA-3918:
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Summary: Broker faills to start after ungraceful shutdown due to
non-monotonically incrementing offsets in logs
Key: KAFKA-3918
URL: https://issues.apache.org/jira/browse/KAFKA-3918
Project: Kafka
Issue Type: Bug
Components: core
Affects Versions: 0.9.0.1
Reporter: Andy Coates
Hi All,
I encountered an issue with Kafka following a power outage that saw a
proportion of our cluster disappear. When the power came back on several
brokers halted on start up with the error:
{noformat}
Fatal error during KafkaServerStartable startup. Prepare to shutdown”
kafka.common.InvalidOffsetException: Attempt to append an offset
(1239742691) to position 35728 no larger than the last offset appended
(1239742822) to
/data3/kafka/mt_xp_its_music_main_itsevent-20/00000000001239444214.index.
at
kafka.log.OffsetIndex$$anonfun$append$1.apply$mcV$sp(OffsetIndex.scala:207)
at kafka.log.OffsetIndex$$anonfun$append$1.apply(OffsetIndex.scala:197)
at kafka.log.OffsetIndex$$anonfun$append$1.apply(OffsetIndex.scala:197)
at kafka.utils.CoreUtils$.inLock(CoreUtils.scala:262)
at kafka.log.OffsetIndex.append(OffsetIndex.scala:197)
at kafka.log.LogSegment.recover(LogSegment.scala:188)
at kafka.log.Log$$anonfun$loadSegments$4.apply(Log.scala:188)
at kafka.log.Log$$anonfun$loadSegments$4.apply(Log.scala:160)
at
scala.collection.TraversableLike$WithFilter$$anonfun$foreach$1.apply(TraversableLike.scala:772)
at
scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:108)
at
scala.collection.TraversableLike$WithFilter.foreach(TraversableLike.scala:771)
at kafka.log.Log.loadSegments(Log.scala:160)
at kafka.log.Log.<init>(Log.scala:90)
at
kafka.log.LogManager$$anonfun$loadLogs$2$$anonfun$3$$anonfun$apply$10$$anonfun$apply$1.apply$mcV$sp(LogManager.scala:150)
at kafka.utils.CoreUtils$$anon$1.run(CoreUtils.scala:60)
at
java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
{noformat}
The only way to recover the brokers was to delete the log files that contained
non monotonically incrementing offsets.
I’ve spent some time digging through the logs and I feel I may have worked out
the sequence of events leading to this issue, (though this is based on some
assumptions I've made about the way Kafka is working, which may be wrong):
Given:
* A topic that is produced to using acks = 1
* A topic that is produced to using gzip compression
* A topic that has min.isr set to less than the number of replicas, (i.e.
min.isr=2, #replicas=3)
* Following ISRs are lagging behind the leader by some small number of
messages, (which is normal with acks=1)
* brokers are configured with fairly large zk session timeout e.g. 30s.
Then:
When something like a power outage take out all three replicas, its possible to
get into a state such that the indexes won’t rebuild on a restart and a broker
fails to start. This can happen when:
* Enough brokers, but not the pre-outage leader, come on-line for the partition
to be writeable
* Producers produce enough records to the partition that the head offset is now
greater than the pre-outage leader head offset.
* The pre-outage leader comes back online.
At this point the logs on the pre-outage leader have diverged from the other
replicas. It has some messages that are not in the other replicas, and the
other replicas have some records not in the pre-outage leader's log.
I’m assuming that because the current leader has a higher offset that the
pre-outage leader, the pre-outage leader just starts following the leader and
requesting the records it thinks its missing.
I’m also assuming that because the producers were using gzip, so each record is
actual a compressed message set, that when the pre-outage leader requests
records from the leader, the offset it requests just happened to be in the
middle of a compressed batch, but the leader returned the full batch. When the
pre-outage leader appends this batch to its own log it thinks all is OK. But
what has happened is that the offsets in the log are no longer monotonically
incrementing. Instead they actually dip by the number of records in the
compressed batch that were before the requested offset. If and when this
broker restarts this dip may be at the 4K boundary the indexer checks. If it
is, the broker won’t start.
Several of our brokers were unlucky enough to hit that 4K boundary, causing a
protracted outage. We’ve written a little utility that shows several more
brokers have a dip outside of the 4K boundary.
There are some assumptions in there, which I’ve not got around to confirming /
denying. (A quick attempt to recreate this failed and I've not found the time
to invest more).
Of course I'd really appreciate the community / experts stepping in and
commenting on whether my assumptions are right or wrong, or if there is another
explanation to the problem.
But assuming I’m mostly right, then the fact the broker won’t start is
obviously a bug, and one I’d like to fix. A Kafka broker should not corrupt
its own log during normal operation to the point that it can’t restart!
A secondary issue is if we think the divergent logs are acceptable? This may be
deemed acceptable given the producers have chosen availability over consistency
when they produced with acks = 1? Though personally, the system having
diverging replicas of an immutable commit log just doesn't sit right.
I see us having a few options here:
* Have the replicas detect the divergence of their logs e.g. a follower
compares the checksum of its last record with the same offset on the leader.
The follower can then workout that its log has diverged from the leader. At
which point it could either halt, stop replicating that partition or search
backwards to find the point of divergence, truncate and recover. (possibly
saving the truncated part somewhere). This would be a protocol change for
Kafka. This solution trades availability, (you’ve got less ISRs during the
extended re-sync process), for consistency.
* Leave the logs as they are and have the indexing of offsets in the log on
start up handle such a situation gracefully. This leaves logs in a divergent
state between replicas, (meaning replays would yield different messages if the
leader was up to down), but gives better availability, (no time spent not being
an ISR while it repairs any divergence).
* Support multiple options and allow it be tuned, ideally by topic.
* Something else...
I’m happy/keen to contribute here. But I’d like to first discuss which option
should be investigated.
Andy
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