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From "Jun Rao (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (KAFKA-3693) Race condition between highwatermark-checkpoint thread and handleLeaderAndIsrRequest at broker start-up
Date Wed, 15 Jun 2016 22:02:09 GMT

    [ https://issues.apache.org/jira/browse/KAFKA-3693?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15332684#comment-15332684
] 

Jun Rao commented on KAFKA-3693:
--------------------------------

[~maysamyabandeh], that by itself may not be a bad idea. We will have to think through UpdateMetadataRequest
as well since currently, we expect there is an UpdateMetadataRequest before the first LeaderAndIsrRequest
on broker startup.

> Race condition between highwatermark-checkpoint thread and handleLeaderAndIsrRequest
at broker start-up
> -------------------------------------------------------------------------------------------------------
>
>                 Key: KAFKA-3693
>                 URL: https://issues.apache.org/jira/browse/KAFKA-3693
>             Project: Kafka
>          Issue Type: Bug
>          Components: core
>    Affects Versions: 0.9.0.1
>            Reporter: Maysam Yabandeh
>
> Upon broker start-up, a race between highwatermark-checkpoint thread to write replication-offset-checkpoint
file and handleLeaderAndIsrRequest thread reading from it causes the highwatermark for some
partitions to be reset to 0. In the good case, this results the replica to truncate its entire
log to 0 and hence initiates fetching of terabytes of data from the lead broker, which sometimes
leads to hours of downtime. We observed the bad cases that the reset offset can propagate
to recovery-point-offset-checkpoint file, making a lead broker to truncate the file. This
seems to have the potential to lead to data loss if the truncation happens at both follower
and leader brokers.
> This is the particular faulty scenario manifested in our tests:
> # The broker restarts and receive LeaderAndIsr from the controller
> # LeaderAndIsr message however does not contain all the partitions (probably because
other brokers were churning at the same time)
> # becomeLeaderOrFollower calls getOrCreatePartition and updates the allPartitions with
the partitions included in the LeaderAndIsr message {code}
>   def getOrCreatePartition(topic: String, partitionId: Int): Partition = {
>     var partition = allPartitions.get((topic, partitionId))
>     if (partition == null) {
>       allPartitions.putIfNotExists((topic, partitionId), new Partition(topic, partitionId,
time, this))
> {code}
> # replication-offset-checkpoint jumps in taking a snapshot of (the partial) allReplicas'
high watermark into replication-offset-checkpoint file {code}  def checkpointHighWatermarks()
{
>     val replicas = allPartitions.values.map(_.getReplica(config.brokerId)).collect{case
Some(replica) => replica}{code} hence rewriting the previous highwatermarks.
> # Later becomeLeaderOrFollower calls makeLeaders and makeFollowers which read the (now
partial) file through Partition::getOrCreateReplica {code}
>           val checkpoint = replicaManager.highWatermarkCheckpoints(log.dir.getParentFile.getAbsolutePath)
>           val offsetMap = checkpoint.read
>           if (!offsetMap.contains(TopicAndPartition(topic, partitionId)))
>             info("No checkpointed highwatermark is found for partition [%s,%d]".format(topic,
partitionId))
> {code}
> We are not entirely sure whether the initial LeaderAndIsr message including a subset
of partitions is critical in making this race condition manifest or not. But it is an important
detail since it clarifies that a solution based on not letting the highwatermark-checkpoint
thread jumping in the middle of processing a LeaderAndIsr message would not suffice.
> The solution we are thinking of is to force initializing allPartitions by the partitions
listed in the replication-offset-checkpoint (and perhaps recovery-point-offset-checkpoint
file too) when a server starts.
> Thoughts?



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