Hi Guozhang,
Yes, I think they are related.
It seems odd to me that there should be any truncation at all since that is
always an opportunity for data loss. It seems like we would want to avoid
that at all costs, assuming we uphold the invariant that messages committed
to an offset on any replica will always be identical. There seems to be two
choices to make sure we don't lose data during leader election:
1- after leader election, requests going to the new leader should block
until the leader is caught up to the highest LEO in the set of replicas (or
until the HW reaches the highest LEO among replicas). I think this is less
"normal" since it would mean that the new leader has to fetch messages from
a non-leader. Maybe you can have the notion of a write-leader and a
read-leader. After leader election, the write-leader becomes whatever the
preferred leader today is, and the read-leader becomes the replica with the
highest LEO. If the write-leader is not also the read-leader, then writes
are blocked. All replicas and consumers fetch from the read-leader, and
when the write-leader is caught up to the read-leader, the write-leader
also becomes the read-leader and writes are unblocked.
2- After leader election, block actual leadership switching and block
responding to new requests until the HW reaches the LEO on the old leader,
then switch leadership, failing the blocked requests (or signaling a retry
for a new leader), which should then retry the writes on the new leader,
and things proceed as normal.
Would either of these work?
Thanks,
Jad.
On Fri, Jul 25, 2014 at 3:37 PM, Guozhang Wang <wangg...@gmail.com> wrote:
> Hi Jad,
>
> Yes. In this case I think you are actually hitting KAFKA-1211. The summary
> of the issue is that, it takes one more fetch request round trip for the
> follower replica to advance the HW after the leader has advanced HW. So for
> your case, the whole process is like this:
>
> 1. leader LEO at 10, follower LEO at 8. Both leader and follower knows the
> LEO is at 8.
> 2. Follower fetch data on Leader starting at 8, leader records its LEO as
> 8.
> 3. Follower gets 9 and 10 and append to its local log.
> 4. Follower fetch data on Leader starting at 10, leader records its LEO as
> 10; now leader knows follower has caught up, it advances its HW to 10 and
> adds the follower to ISR (but follower does not know that yet! It still
> think the HW is 8).
> 5. Leader's fetch response gets back to follower, and now the follower
> knows that HW has been updated to 10.
>
> And let's say there is a leader election between step 4) and 5), for your
> case it is due to preferred leader election, but it could also be that
> current leader fails, etc. Then on becoming the new leader the follower
> will truncate its data to 8, which is the HW it knows. Hence the data loss.
>
> The proposed solution in KAFKA-1211 will tackle this issue.
>
> Guozhang
>
>
> On Fri, Jul 25, 2014 at 2:48 PM, Jad Naous <jad.na...@appdynamics.com>
> wrote:
>
> > Hi Guozhang,
> >
> > Yes, broker 1 is in the ISR (in fact, I wait for broker 1 to be in the
> ISR
> > before triggering election). However, I think there is something still
> > amiss. I still see data loss. Here are some relevant log lines from
> broker
> > 0 (different test run). The log on broker 0 is getting truncated, losing
> > some messages.
> >
> > [2014-07-25 10:40:02,134] [DEBUG] [kafka-request-handler-5]
> > [kafka.cluster.Partition] Partition [EventServiceUpsertTopic,19] on
> broker
> > 0: Old hw for partition [EventServiceUpsertTopic,19] is 8082. New hw is
> > 8082. All leo's are 8111,8082
> >
> > [2014-07-25 10:40:02,134] [DEBUG] [kafka-request-handler-5]
> > [kafka.server.KafkaApis] [KafkaApi-0] Produce to local log in 1 ms
> >
> > [2014-07-25 10:40:02,134] [DEBUG] [main-SendThread(localhost:49893)]
> > [org.apache.zookeeper.ClientCnxn] Reading reply
> > sessionid:0x476e9a9e9a0001, packet:: clientPath:null serverPath:null
> > finished:false header:: 729,5 replyHeader:: 729,4294968217,0 request::
> >
> >
> '/brokers/topics/EventServiceUpsertTopic/partitions/5/state,#7b22636f6e74726f6c6c65725f65706f6368223a312c226c6561646572223a312c2276657273696f6e223a312c226c65616465725f65706f6368223a332c22697372223a5b302c315d7d,3
> > response::
> >
> >
> s{4294967416,4294968217,1406309966419,1406310002132,4,0,0,0,74,0,4294967416}
> >
> > [2014-07-25 10:40:02,134] [DEBUG] [kafka-processor-49917-2]
> > [kafka.request.logger] Completed request:Name: ProducerRequest; Version:
> > 0; CorrelationId: 16248; ClientId: ; RequiredAcks: -1; AckTimeoutMs:
> 10000
> > ms from client /127.0.0.1:50168
> >
> >
> ;totalTime:1,requestQueueTime:0,localTime:1,remoteTime:0,responseQueueTime:0,sendTime:0
> >
> > [2014-07-25 10:40:02,134] [DEBUG]
> > [ZkClient-EventThread-13-localhost:49893,localhost:49896,localhost:49899]
> > [kafka.utils.ZkUtils$] Conditional update of path
> > /brokers/topics/EventServiceUpsertTopic/partitions/5/state with value
> >
> {"controller_epoch":1,"leader":1,"version":1,"leader_epoch":3,"isr":[0,1]}
> > and expected version 3 succeeded, returning the new version: 4
> >
> > [2014-07-25 10:40:02,136] [DEBUG]
> > [ZkClient-EventThread-13-localhost:49893,localhost:49896,localhost:49899]
> > [k.controller.PartitionStateMachine] [Partition state machine on
> > Controller 0]: After leader election, leader cache is updated to
> > Map([EventServiceUpsertTopic,18] ->
> > (Leader:0,ISR:0,LeaderEpoch:0,ControllerEpoch:1),
> > [EventServiceUpsertTopic,4] ->
> > (Leader:0,ISR:0,LeaderEpoch:0,ControllerEpoch:1),
> > [EventServiceUpsertTopic,7] ->
> > (Leader:1,ISR:0,1,LeaderEpoch:3,ControllerEpoch:1),
> > [EventServiceUpsertTopic,17] ->
> > (Leader:1,ISR:0,1,LeaderEpoch:3,ControllerEpoch:1),
> > [EventServiceUpsertTopic,16] ->
> > (Leader:0,ISR:0,LeaderEpoch:0,ControllerEpoch:1),
> > [EventServiceUpsertTopic,5] ->
> > (Leader:1,ISR:0,1,LeaderEpoch:3,ControllerEpoch:1),
> > [EventServiceUpsertTopic,10] ->
> > (Leader:0,ISR:0,LeaderEpoch:0,ControllerEpoch:1),
> > [EventServiceUpsertTopic,1] ->
> > (Leader:1,ISR:0,1,LeaderEpoch:3,ControllerEpoch:1),
> > [EventServiceUpsertTopic,13] ->
> > (Leader:1,ISR:0,1,LeaderEpoch:3,ControllerEpoch:1),
> > [EventServiceUpsertTopic,9] ->
> > (Leader:1,ISR:0,1,LeaderEpoch:3,ControllerEpoch:1),
> > [EventServiceUpsertTopic,8] ->
> > (Leader:0,ISR:0,LeaderEpoch:0,ControllerEpoch:1),
> > [EventServiceUpsertTopic,11] ->
> > (Leader:1,ISR:0,1,LeaderEpoch:3,ControllerEpoch:1),
> > [EventServiceUpsertTopic,6] ->
> > (Leader:0,ISR:0,LeaderEpoch:0,ControllerEpoch:1),
> > [EventServiceUpsertTopic,12] ->
> > (Leader:0,ISR:0,LeaderEpoch:0,ControllerEpoch:1),
> > [EventServiceUpsertTopic,2] ->
> > (Leader:0,ISR:0,LeaderEpoch:0,ControllerEpoch:1),
> > [EventServiceUpsertTopic,3] ->
> > (Leader:1,ISR:0,1,LeaderEpoch:3,ControllerEpoch:1),
> > [EventServiceUpsertTopic,14] ->
> > (Leader:0,ISR:0,LeaderEpoch:0,ControllerEpoch:1), [
> > EventServiceUpsertTopic,19] ->
> > (Leader:1,ISR:0,1,LeaderEpoch:3,ControllerEpoch:1),
> > [EventServiceUpsertTopic,0] ->
> > (Leader:0,ISR:0,LeaderEpoch:0,ControllerEpoch:1),
> > [EventServiceUpsertTopic,15] ->
> > (Leader:1,ISR:0,1,LeaderEpoch:3,ControllerEpoch:1))
> >
> > [2014-07-25 10:40:02,136] [DEBUG] [kafka-request-handler-1]
> > [kafka.cluster.Partition] Partition [EventServiceUpsertTopic,19] on
> broker
> > 0: Old hw for partition [EventServiceUpsertTopic,19] is 8082. New hw is
> > 8082. All leo's are 8112,8082
> >
> > [2014-07-25 10:40:02,136] [DEBUG] [kafka-request-handler-1]
> > [kafka.server.KafkaApis] [KafkaApi-0] Produce to local log in 0 ms
> >
> > [2014-07-25 10:40:02,137] [DEBUG] [kafka-processor-49917-2]
> > [kafka.request.logger] Completed request:Name: ProducerRequest; Version:
> > 0; CorrelationId: 16250; ClientId: ; RequiredAcks: -1; AckTimeoutMs:
> 10000
> > ms from client /127.0.0.1:50168
> >
> >
> ;totalTime:1,requestQueueTime:0,localTime:1,remoteTime:0,responseQueueTime:0,sendTime:0
> >
> > [2014-07-25 10:40:02,140] [INFO ]
> > [ZkClient-EventThread-13-localhost:49893,localhost:49896,localhost:49899]
> > [kafka.controller.KafkaController] [Controller 0]: Partition [
> > EventServiceUpsertTopic,19] completed preferred replica leader election.
> > New leader is 1
> >
> > [2014-07-25 10:40:02,140] [INFO ]
> > [ZkClient-EventThread-13-localhost:49893,localhost:49896,localhost:49899]
> > [kafka.controller.KafkaController] [Controller 0]: Partition
> > [EventServiceUpsertTopic,5] completed preferred replica leader election.
> > New leader is 1
> >
> > [2014-07-25 10:40:02,142] [DEBUG] [main-SendThread(localhost:49893)]
> > [org.apache.zookeeper.ClientCnxn] Got notification
> > sessionid:0x476e9a9e9a0001
> >
> > [2014-07-25 10:40:02,142] [DEBUG] [main-SendThread(localhost:49893)]
> > [org.apache.zookeeper.ClientCnxn] Got WatchedEvent state:SyncConnected
> > type:NodeDeleted path:/admin/preferred_replica_election for sessionid
> > 0x476e9a9e9a0001
> >
> > [2014-07-25 10:40:02,143] [DEBUG] [main-SendThread(localhost:49893)]
> > [org.apache.zookeeper.ClientCnxn] Reading reply
> > sessionid:0x476e9a9e9a0001, packet:: clientPath:null serverPath:null
> > finished:false header:: 730,2 replyHeader:: 730,4294968218,0 request::
> > '/admin/preferred_replica_election,-1 response:: null
> >
> > [2014-07-25 10:40:02,145] [INFO ] [kafka-request-handler-0]
> > [kafka.server.ReplicaFetcherManager] [ReplicaFetcherManager on broker 0]
> > Removed fetcher for partitions
> >
> >
> [EventServiceUpsertTopic,13],[EventServiceUpsertTopic,11],[EventServiceUpsertTopic,17],[EventServiceUpsertTopic,7],[EventServiceUpsertTopic,9],[EventServiceUpsertTopic,1],[EventServiceUpsertTopic,15],[
> > EventServiceUpsertTopic,19
> > ],[EventServiceUpsertTopic,3],[EventServiceUpsertTopic,5]
> >
> > [2014-07-25 10:40:02,153] [INFO ] [kafka-request-handler-0]
> > [kafka.log.Log] Truncating log EventServiceUpsertTopic-19 to offset
> 8082.
> >
> > [2014-07-25 10:40:02,159] [INFO ] [kafka-request-handler-0]
> > [kafka.log.Log] Truncating log EventServiceUpsertTopic-5 to offset 0.
> >
> >
> > I have removed some irrelevant lines. As you can see, the log for
> > EventServiceUpsertTopic-19 is being truncated, losing messages that have
> > not yet been replicated to broker 1. I'm not sure exactly what the issue
> > is. Maybe new requests should be completely blocked after leader election
> > until the new leader catches up to the messages that have been acked and
> it
> > has not yet received.
> >
> > Thanks,
> >
> > Jad.
> >
> >
> > On Fri, Jul 25, 2014 at 2:14 PM, Guozhang Wang <wangg...@gmail.com>
> wrote:
> >
> > > Hello Jad,
> > >
> > > I double-checked the source code again, and found that actually the
> > > preferred leader elector does consider whether the selected replica is
> in
> > > ISR or not. This means that by the time the election is triggered,
> > broker 1
> > > is added to ISR by broker 0. Could you check before step 3, is there
> any
> > > log entries on broker 0 adding broker 1 to ISR or updating HW to 975?
> > >
> > >
> > > On Thu, Jul 24, 2014 at 4:21 PM, Jad Naous <jad.na...@appdynamics.com>
> > > wrote:
> > >
> > > > Hi,
> > > >
> > > > I have a test that continuously sends messages to one broker, brings
> up
> > > > another broker, and adds it as a replica for all partitions, with it
> > > being
> > > > the preferred replica for some. I have
> > auto.leader.rebalance.enable=true,
> > > > so replica election gets triggered. Data is being pumped to the old
> > > broker
> > > > all the while. It seems that some data gets lost while switching over
> > to
> > > > the new leader. Is this a bug, or do I have something misconfigured?
> I
> > > also
> > > > have request.required.acks=-1 on the producer.
> > > >
> > > > Here's what I think is happening:
> > > >
> > > > 1. Producer writes message to broker 0, [EventServiceUpsertTopic,13],
> > w/
> > > > broker 0 currently leader, with ISR=(0), so write returns
> successfully,
> > > > even when acks = -1. Correlation id 35836
> > > >
> > > > Producer log:
> > > >
> > > > [2014-07-24 14:44:26,991] [DEBUG] [dw-97 - PATCH
> > > >
> > > >
> > >
> >
> /v1/events/type_for_test_bringupNewBroker_shouldRebalance_shouldNotLoseData/event?_idPath=idField&_mergeFields=field1]
> > > > [kafka.producer.BrokerPartitionInfo] Partition
> > > > [EventServiceUpsertTopic,13] has leader 0
> > > >
> > > > [2014-07-24 14:44:26,993] [DEBUG] [dw-97 - PATCH
> > > >
> > > >
> > >
> >
> /v1/events/type_for_test_bringupNewBroker_shouldRebalance_shouldNotLoseData/event?_idPath=idField&_mergeFields=field1]
> > > > [k.producer.async.DefaultEventHandler] Producer sent messages with
> > > > correlation id 35836 for topics [EventServiceUpsertTopic,13] to
> broker
> > 0
> > > on
> > > > localhost:56821
> > > > 2. Broker 1 is still catching up
> > > >
> > > > Broker 0 Log:
> > > >
> > > > [2014-07-24 14:44:26,992] [DEBUG] [kafka-request-handler-3]
> > > > [kafka.cluster.Partition] Partition [EventServiceUpsertTopic,13] on
> > > broker
> > > > 0: Old hw for partition [EventServiceUpsertTopic,13] is 971. New hw
> is
> > > 971.
> > > > All leo's are 975,971
> > > >
> > > > [2014-07-24 14:44:26,992] [DEBUG] [kafka-request-handler-3]
> > > > [kafka.server.KafkaApis] [KafkaApi-0] Produce to local log in 0 ms
> > > >
> > > > [2014-07-24 14:44:26,992] [DEBUG] [kafka-processor-56821-0]
> > > > [kafka.request.logger] Completed request:Name: ProducerRequest;
> > Version:
> > > > 0; CorrelationId: 35836; ClientId: ; RequiredAcks: -1; AckTimeoutMs:
> > > 10000
> > > > ms from client /127.0.0.1:57086
> > > >
> > > >
> > >
> >
> ;totalTime:0,requestQueueTime:0,localTime:0,remoteTime:0,responseQueueTime:0,sendTime:0
> > > > 3. Leader election is triggered by the scheduler:
> > > >
> > > > Broker 0 Log:
> > > >
> > > > [2014-07-24 14:44:26,991] [INFO ] [kafka-scheduler-0]
> > > > [k.c.PreferredReplicaPartitionLeaderSelector]
> > > > [PreferredReplicaPartitionLeaderSelector]: Current leader 0 for
> > > partition [
> > > > EventServiceUpsertTopic,13] is not the preferred replica. Trigerring
> > > > preferred replica leader election
> > > >
> > > > [2014-07-24 14:44:26,993] [DEBUG] [kafka-scheduler-0]
> > > > [kafka.utils.ZkUtils$] Conditional update of path
> > > > /brokers/topics/EventServiceUpsertTopic/partitions/13/state with
> value
> > > >
> > >
> >
> {"controller_epoch":1,"leader":1,"version":1,"leader_epoch":3,"isr":[0,1]}
> > > > and expected version 3 succeeded, returning the new version: 4
> > > >
> > > > [2014-07-24 14:44:26,994] [DEBUG] [kafka-scheduler-0]
> > > > [k.controller.PartitionStateMachine] [Partition state machine on
> > > > Controller 0]: After leader election, leader cache is updated to
> > > >
> > >
> >
> Map(<Snipped>(Leader:1,ISR:0,1,LeaderEpoch:3,ControllerEpoch:1),<EndSnip>)
> > > >
> > > > [2014-07-24 14:44:26,994] [INFO ] [kafka-scheduler-0]
> > > > [kafka.controller.KafkaController] [Controller 0]: Partition [
> > > > EventServiceUpsertTopic,13] completed preferred replica leader
> > election.
> > > > New leader is 1
> > > > 4. Broker 1 is still behind, but it sets the high water mark to
> 971!!!
> > > >
> > > > Broker 1 Log:
> > > >
> > > > [2014-07-24 14:44:26,999] [INFO ] [kafka-request-handler-6]
> > > > [kafka.server.ReplicaFetcherManager] [ReplicaFetcherManager on
> broker
> > 1]
> > > > Removed fetcher for partitions [EventServiceUpsertTopic,13]
> > > >
> > > > [2014-07-24 14:44:27,000] [DEBUG] [kafka-request-handler-6]
> > > > [kafka.cluster.Partition] Partition [EventServiceUpsertTopic,13] on
> > > broker
> > > > 1: Old hw for partition [EventServiceUpsertTopic,13] is 970. New hw
> is
> > > -1.
> > > > All leo's are -1,971
> > > >
> > > > [2014-07-24 14:44:27,098] [DEBUG] [kafka-request-handler-3]
> > > > [kafka.server.KafkaApis] [KafkaApi-1] Maybe update partition HW due
> to
> > > > fetch request: Name: FetchRequest; Version: 0; CorrelationId: 1;
> > > ClientId:
> > > > ReplicaFetcherThread-0-1; ReplicaId: 0; MaxWait: 500 ms; MinBytes: 1
> > > bytes;
> > > > RequestInfo: [EventServiceUpsertTopic,13] ->
> > > > PartitionFetchInfo(971,1048576), <Snipped>
> > > >
> > > > [2014-07-24 14:44:27,098] [DEBUG] [kafka-request-handler-3]
> > > > [kafka.cluster.Partition] Partition [EventServiceUpsertTopic,13] on
> > > broker
> > > > 1: Recording follower 0 position 971 for partition [
> > > > EventServiceUpsertTopic,13].
> > > >
> > > > [2014-07-24 14:44:27,100] [DEBUG] [kafka-request-handler-3]
> > > > [kafka.cluster.Partition] Partition [EventServiceUpsertTopic,13] on
> > > broker
> > > > 1: Highwatermark for partition [EventServiceUpsertTopic,13] updated
> to
> > > 971
> > > > 5. Consumer is none the wiser. All data that was in offsets 972-975
> > > doesn't
> > > > show up!
> > > >
> > > > I tried this with 2 initial replicas, and adding a 3rd which is
> > supposed
> > > to
> > > > be the leader for some new partitions, and this problem also happens
> > > there.
> > > > The log on the old leader gets truncated to the offset on the new
> > leader.
> > > > What's the solution? Can I make a new broker leader for partitions
> that
> > > are
> > > > currently active without losing data?
> > > >
> > > > Thanks,
> > > > Jad.
> > > >
> > > > --
> > > > *Jad Naous* | Engineering | AppDynamics
> > > > <http://www.appdynamics.com>
> > > >
> > >
> > >
> > >
> > > --
> > > -- Guozhang
> > >
> >
> >
> >
> > --
> > *Jad Naous* | Engineering | AppDynamics
> > <http://www.appdynamics.com>
> >
>
>
>
> --
> -- Guozhang
>
--
*Jad Naous* | Engineering | AppDynamics
<http://www.appdynamics.com>
