Jun, Jad,
I think in this case data loss can still happen, since the replica factor
was previously one, and in handling the produce requests, if the server
decides that all the produced partitions have a replica factor of 1 it will
also directly send back the response instead of putting the request into
purgatory even if currently the number of replicas is 2 (for details look
at ReplicaManager.getReplicationFactorForPartition and search of the usage
of Partition.replicationFactor).
I now agree that this is not related to KAFKA-1211 but a different small
bug. We need to probably file another JIRA for this. But I think after this
one is fixed (which should be much easier than KAFKA-1211), Jad's scenario
should not cause data loss anymore.
Guozhang
On Sun, Jul 27, 2014 at 6:11 PM, Jad Naous <jad.na...@appdynamics.com>
wrote:
> So in summary, is it true to say that currently triggering leader
> reelection is not a safe operation? I have been able to reproduce that
> message loss pretty reliably in tests. If that is the case, isn't that an
> important operation in a large cluster where nodes go up and down?
> On Jul 25, 2014 10:00 PM, "Jun Rao" <jun...@gmail.com> wrote:
>
> > Actually, I don't think KAFKA-1211 will happen with just 2 replicas.
> When a
> > replica becomes a leader, it never truncates its log. Only when a replica
> > becomes follower, it truncates its log to HW. So in this particular case,
> > the new leader will not truncate data to offset 8.
> >
> > Thanks,
> >
> > Jun
> >
> >
> > 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
> > >
> >
>
--
-- Guozhang
