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From Stanislav Kozlovski <stanis...@confluent.io>
Subject Re: [DISCUSS] KIP-491: Preferred Leader Deprioritized List (Temporary Blacklist)
Date Tue, 17 Sep 2019 14:56:23 GMT
Hey Harsha,

> If we want to go with making this an option and providing a tool which
abstracts moving the broker to end preferred leader list , it needs to do
it for all the partitions that broker is leader for. As said in the above
comment a broker i.e leader for 1000 partitions we have to this for all the
partitions.  Instead of having a blacklist will help simplify this process
and we can provide monitoring/alerts on such list.

Sorry, I thought that part of the reasoning for not using reassignment was
to optimize the process.

> Do you mind shedding some light what issue you are talking to propose a
KIP for?


The issue I was talking about is the one I quoted in my previous reply. I
understand that you want to have a way of running a "shallow" replica of
sorts - one that is lacking the historical data but has (and continues to
replicate) the latest data. That is the goal of setting the last offsets
for all partitions in replication-offset-checkpoint, right?

Thanks,
Stanislav

On Mon, Sep 16, 2019 at 3:39 PM Satish Duggana <satish.duggana@gmail.com>
wrote:

> Hi George,
> Thanks for explaining the usecase for topic level preferred leader
> blacklist. As I mentioned earlier, I am fine with broker level config
> for now.
>
> ~Satish.
>
>
> On Sat, Sep 7, 2019 at 12:29 AM George Li
> <sql_consulting@yahoo.com.invalid> wrote:
> >
> >  Hi,
> >
> > Just want to ping and bubble up the discussion of KIP-491.
> >
> > On a large scale of Kafka clusters with thousands of brokers in many
> clusters.  Frequent hardware failures are common, although the
> reassignments to change the preferred leaders is a workaround, it incurs
> unnecessary additional work than the proposed preferred leader blacklist in
> KIP-491, and hard to scale.
> >
> > I am wondering whether others using Kafka in a big scale running into
> same problem.
> >
> >
> > Satish,
> >
> > Regarding your previous question about whether there is use-case for
> TopicLevel preferred leader "blacklist",  I thought about one use-case:  to
> improve rebalance/reassignment, the large partition will usually cause
> performance/stability issues, planning to change the say the New Replica
> will start with Leader's latest offset(this way the replica is almost
> instantly in the ISR and reassignment completed), and put this partition's
> NewReplica into Preferred Leader "Blacklist" at the Topic Level config for
> that partition. After sometime(retention time), this new replica has caught
> up and ready to serve traffic, update/remove the TopicConfig for this
> partition's preferred leader blacklist.
> >
> > I will update the KIP-491 later for this use case of Topic Level config
> for Preferred Leader Blacklist.
> >
> >
> > Thanks,
> > George
> >
> >     On Wednesday, August 7, 2019, 07:43:55 PM PDT, George Li <
> sql_consulting@yahoo.com> wrote:
> >
> >   Hi Colin,
> >
> > > In your example, I think we're comparing apples and oranges.  You
> started by outlining a scenario where "an empty broker... comes up...
> [without] any > leadership[s]."  But then you criticize using reassignment
> to switch the order of preferred replicas because it "would not actually
> switch the leader > automatically."  If the empty broker doesn't have any
> leaderships, there is nothing to be switched, right?
> >
> > Let me explained in details of this particular use case example for
> comparing apples to apples.
> >
> > Let's say a healthy broker hosting 3000 partitions, and of which 1000
> are the preferred leaders (leader count is 1000). There is a hardware
> failure (disk/memory, etc.), and kafka process crashed. We swap this host
> with another host but keep the same broker.id, when this new broker
> coming up, it has no historical data, and we manage to have the current
> last offsets of all partitions set in the replication-offset-checkpoint (if
> we don't set them, it could cause crazy ReplicaFetcher pulling of
> historical data from other brokers and cause cluster high latency and other
> instabilities), so when Kafka is brought up, it is quickly catching up as
> followers in the ISR.  Note, we have auto.leader.rebalance.enable
> disabled, so it's not serving any traffic as leaders (leader count = 0),
> even there are 1000 partitions that this broker is the Preferred Leader.
> >
> > We need to make this broker not serving traffic for a few hours or days
> depending on the SLA of the topic retention requirement until after it's
> having enough historical data.
> >
> >
> > * The traditional way using the reassignments to move this broker in
> that 1000 partitions where it's the preferred leader to the end of
> assignment, this is O(N) operation. and from my experience, we can't submit
> all 1000 at the same time, otherwise cause higher latencies even the
> reassignment in this case can complete almost instantly.  After  a few
> hours/days whatever, this broker is ready to serve traffic,  we have to run
> reassignments again to restore that 1000 partitions preferred leaders for
> this broker: O(N) operation.  then run preferred leader election O(N)
> again.  So total 3 x O(N) operations.  The point is since the new empty
> broker is expected to be the same as the old one in terms of hosting
> partition/leaders, it would seem unnecessary to do reassignments (ordering
> of replica) during the broker catching up time.
> >
> >
> >
> > * The new feature Preferred Leader "Blacklist":  just need to put a
> dynamic config to indicate that this broker should be considered leader
> (preferred leader election or broker failover or unclean leader election)
> to the lowest priority. NO need to run any reassignments. After a few
> hours/days, when this broker is ready, remove the dynamic config, and run
> preferred leader election and this broker will serve traffic for that 1000
> original partitions it was the preferred leader. So total  1 x O(N)
> operation.
> >
> >
> > If auto.leader.rebalance.enable  is enabled,  the Preferred Leader
> "Blacklist" can be put it before Kafka is started to prevent this broker
> serving traffic.  In the traditional way of running reassignments, once the
> broker is up, with auto.leader.rebalance.enable  , if leadership starts
> going to this new empty broker, it might have to do preferred leader
> election after reassignments to remove its leaderships. e.g. (1,2,3) =>
> (2,3,1) reassignment only change the ordering, 1 remains as the current
> leader, and needs prefer leader election to change to 2 after reassignment.
> so potentially one more O(N) operation.
> >
> > I hope the above example can show how easy to "blacklist" a broker
> serving leadership.  For someone managing Production Kafka cluster, it's
> important to react fast to certain alerts and mitigate/resolve some issues.
> As I listed the other use cases in KIP-291, I think this feature can make
> the Kafka product more easier to manage/operate.
> >
> > > In general, using an external rebalancing tool like Cruise Control is
> a good idea to keep things balanced without having deal with manual
> rebalancing.  > We expect more and more people who have a complex or large
> cluster will start using tools like this.
> > >
> > > However, if you choose to do manual rebalancing, it shouldn't be that
> bad.  You would save the existing partition ordering before making your
> changes, then> make your changes (perhaps by running a simple command line
> tool that switches the order of the replicas).  Then, once you felt like
> the broker was ready to> serve traffic, you could just re-apply the old
> ordering which you had saved.
> >
> >
> > We do have our own rebalancing tool which has its own criteria like Rack
> diversity,  disk usage,  spread partitions/leaders across all brokers in
> the cluster per topic, leadership Bytes/BytesIn served per broker, etc.  We
> can run reassignments. The point is whether it's really necessary, and if
> there is more effective, easier, safer way to do it.
> >
> > take another use case example of taking leadership out of busy
> Controller to give it more power to serve metadata requests and other work.
> The controller can failover, with the preferred leader "blacklist",  it
> does not have to run reassignments again when controller failover, just
> change the blacklisted broker_id.
> >
> >
> > > I was thinking about a PlacementPolicy filling the role of preventing
> people from creating single-replica partitions on a node that we didn't
> want to > ever be the leader.  I thought that it could also prevent people
> from designating those nodes as preferred leaders during topic creation, or
> Kafka from doing> itduring random topic creation.  I was assuming that the
> PlacementPolicy would determine which nodes were which through static
> configuration keys.  I agree> static configuration keys are somewhat less
> flexible than dynamic configuration.
> >
> >
> > I think single-replica partition might not be a good example.  There
> should not be any single-replica partition at all. If yes. it's probably
> because of trying to save disk space with less replicas.  I think at least
> minimum 2. The user purposely creating single-replica partition will take
> full responsibilities of data loss and unavailability when a broker fails
> or under maintenance.
> >
> >
> > I think it would be better to use dynamic instead of static config.  I
> also think it would be better to have topic creation Policy enforced in
> Kafka server OR an external service. We have an external/central service
> managing topic creation/partition expansion which takes into account of
> rack-diversity, replication factor (2, 3 or 4 depending on cluster/topic
> type), Policy replicating the topic between kafka clusters, etc.
> >
> >
> >
> > Thanks,
> > George
> >
> >
> >     On Wednesday, August 7, 2019, 05:41:28 PM PDT, Colin McCabe <
> cmccabe@apache.org> wrote:
> >
> >  On Wed, Aug 7, 2019, at 12:48, George Li wrote:
> > >  Hi Colin,
> > >
> > > Thanks for your feedbacks.  Comments below:
> > > > Even if you have a way of blacklisting an entire broker all at once,
> you still would need to run a leader election > for each partition where
> you want to move the leader off of the blacklisted broker.  So the
> operation is still O(N) in > that sense-- you have to do something per
> partition.
> > >
> > > For a failed broker and swapped with an empty broker, when it comes up,
> > > it will not have any leadership, and we would like it to remain not
> > > having leaderships for a couple of hours or days. So there is no
> > > preferred leader election needed which incurs O(N) operation in this
> > > case.  Putting the preferred leader blacklist would safe guard this
> > > broker serving traffic during that time. otherwise, if another broker
> > > fails(if this broker is the 1st, 2nd in the assignment), or someone
> > > runs preferred leader election, this new "empty" broker can still get
> > > leaderships.
> > >
> > > Also running reassignment to change the ordering of preferred leader
> > > would not actually switch the leader automatically.  e.g.  (1,2,3) =>
> > > (2,3,1). unless preferred leader election is run to switch current
> > > leader from 1 to 2.  So the operation is at least 2 x O(N).  and then
> > > after the broker is back to normal, another 2 x O(N) to rollback.
> >
> > Hi George,
> >
> > Hmm.  I guess I'm still on the fence about this feature.
> >
> > In your example, I think we're comparing apples and oranges.  You
> started by outlining a scenario where "an empty broker... comes up...
> [without] any leadership[s]."  But then you criticize using reassignment to
> switch the order of preferred replicas because it "would not actually
> switch the leader automatically."  If the empty broker doesn't have any
> leaderships, there is nothing to be switched, right?
> >
> > >
> > >
> > > > In general, reassignment will get a lot easier and quicker once
> KIP-455 is implemented.  > Reassignments that just change the order of
> preferred replicas for a specific partition should complete pretty much
> instantly.
> > > >> I think it's simpler and easier just to have one source of truth
> for what the preferred replica is for a partition, rather than two.  So
> for> me, the fact that the replica assignment ordering isn't changed is
> actually a big disadvantage of this KIP.  If you are a new user (or just>
> an existing user that didn't read all of the documentation) and you just
> look at the replica assignment, you might be confused by why> a particular
> broker wasn't getting any leaderships, even  though it appeared like it
> should.  More mechanisms mean more complexity> for users and developers
> most of the time.
> > >
> > >
> > > I would like stress the point that running reassignment to change the
> > > ordering of the replica (putting a broker to the end of partition
> > > assignment) is unnecessary, because after some time the broker is
> > > caught up, it can start serving traffic and then need to run
> > > reassignments again to "rollback" to previous states. As I mentioned in
> > > KIP-491, this is just tedious work.
> >
> > In general, using an external rebalancing tool like Cruise Control is a
> good idea to keep things balanced without having deal with manual
> rebalancing.  We expect more and more people who have a complex or large
> cluster will start using tools like this.
> >
> > However, if you choose to do manual rebalancing, it shouldn't be that
> bad.  You would save the existing partition ordering before making your
> changes, then make your changes (perhaps by running a simple command line
> tool that switches the order of the replicas).  Then, once you felt like
> the broker was ready to serve traffic, you could just re-apply the old
> ordering which you had saved.
> >
> > >
> > > I agree this might introduce some complexities for users/developers.
> > > But if this feature is good, and well documented, it is good for the
> > > kafka product/community.  Just like KIP-460 enabling unclean leader
> > > election to override TopicLevel/Broker Level config of
> > > `unclean.leader.election.enable`
> > >
> > > > I agree that it would be nice if we could treat some brokers
> differently for the purposes of placing replicas, selecting leaders, etc. >
> Right now, we don't have any way of implementing that without forking the
> broker.  I would support a new PlacementPolicy class that> would close this
> gap.  But I don't think this KIP is flexible enough to fill this role.  For
> example, it can't prevent users from creating> new single-replica topics
> that get put on the "bad" replica.  Perhaps we should reopen the
> discussion> about
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-201%3A+Rationalising+Policy+interfaces
> > >
> > > Creating topic with single-replica is beyond what KIP-491 is trying to
> > > achieve.  The user needs to take responsibility of doing that. I do see
> > > some Samza clients notoriously creating single-replica topics and that
> > > got flagged by alerts, because a single broker down/maintenance will
> > > cause offline partitions. For KIP-491 preferred leader "blacklist",
> > > the single-replica will still serve as leaders, because there is no
> > > other alternative replica to be chosen as leader.
> > >
> > > Even with a new PlacementPolicy for topic creation/partition expansion,
> > > it still needs the blacklist info (e.g. a zk path node, or broker
> > > level/topic level config) to "blacklist" the broker to be preferred
> > > leader? Would it be the same as KIP-491 is introducing?
> >
> > I was thinking about a PlacementPolicy filling the role of preventing
> people from creating single-replica partitions on a node that we didn't
> want to ever be the leader.  I thought that it could also prevent people
> from designating those nodes as preferred leaders during topic creation, or
> Kafka from doing itduring random topic creation.  I was assuming that the
> PlacementPolicy would determine which nodes were which through static
> configuration keys.  I agree static configuration keys are somewhat less
> flexible than dynamic configuration.
> >
> > best,
> > Colin
> >
> >
> > >
> > >
> > > Thanks,
> > > George
> > >
> > >    On Wednesday, August 7, 2019, 11:01:51 AM PDT, Colin McCabe
> > > <cmccabe@apache.org> wrote:
> > >
> > >  On Fri, Aug 2, 2019, at 20:02, George Li wrote:
> > > >  Hi Colin,
> > > > Thanks for looking into this KIP.  Sorry for the late response. been
> busy.
> > > >
> > > > If a cluster has MAMY topic partitions, moving this "blacklist"
> broker
> > > > to the end of replica list is still a rather "big" operation,
> involving
> > > > submitting reassignments.  The KIP-491 way of blacklist is much
> > > > simpler/easier and can undo easily without changing the replica
> > > > assignment ordering.
> > >
> > > Hi George,
> > >
> > > Even if you have a way of blacklisting an entire broker all at once,
> > > you still would need to run a leader election for each partition where
> > > you want to move the leader off of the blacklisted broker.  So the
> > > operation is still O(N) in that sense-- you have to do something per
> > > partition.
> > >
> > > In general, reassignment will get a lot easier and quicker once KIP-455
> > > is implemented.  Reassignments that just change the order of preferred
> > > replicas for a specific partition should complete pretty much
> instantly.
> > >
> > > I think it's simpler and easier just to have one source of truth for
> > > what the preferred replica is for a partition, rather than two.  So for
> > > me, the fact that the replica assignment ordering isn't changed is
> > > actually a big disadvantage of this KIP.  If you are a new user (or
> > > just an existing user that didn't read all of the documentation) and
> > > you just look at the replica assignment, you might be confused by why a
> > > particular broker wasn't getting any leaderships, even  though it
> > > appeared like it should.  More mechanisms mean more complexity for
> > > users and developers most of the time.
> > >
> > > > Major use case for me, a failed broker got swapped with new hardware,
> > > > and starts up as empty (with latest offset of all partitions), the
> SLA
> > > > of retention is 1 day, so before this broker is up to be in-sync for
> 1
> > > > day, we would like to blacklist this broker from serving traffic.
> after
> > > > 1 day, the blacklist is removed and run preferred leader election.
> > > > This way, no need to run reassignments before/after.  This is the
> > > > "temporary" use-case.
> > >
> > > What if we just add an option to the reassignment tool to generate a
> > > plan to move all the leaders off of a specific broker?  The tool could
> > > also run a leader election as well.  That would be a simple way of
> > > doing this without adding new mechanisms or broker-side configurations,
> > > etc.
> > >
> > > >
> > > > There are use-cases that this Preferred Leader "blacklist" can be
> > > > somewhat permanent, as I explained in the AWS data center instances
> Vs.
> > > > on-premises data center bare metal machines (heterogenous hardware),
> > > > that the AWS broker_ids will be blacklisted.  So new topics created,
> > > > or existing topic expansion would not make them serve traffic even
> they
> > > > could be the preferred leader.
> > >
> > > I agree that it would be nice if we could treat some brokers
> > > differently for the purposes of placing replicas, selecting leaders,
> > > etc.  Right now, we don't have any way of implementing that without
> > > forking the broker.  I would support a new PlacementPolicy class that
> > > would close this gap.  But I don't think this KIP is flexible enough to
> > > fill this role.  For example, it can't prevent users from creating new
> > > single-replica topics that get put on the "bad" replica.  Perhaps we
> > > should reopen the discussion about
> > >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-201%3A+Rationalising+Policy+interfaces
> > >
> > > regards,
> > > Colin
> > >
> > > >
> > > > Please let me know there are more question.
> > > >
> > > >
> > > > Thanks,
> > > > George
> > > >
> > > >    On Thursday, July 25, 2019, 08:38:28 AM PDT, Colin McCabe
> > > > <cmccabe@apache.org> wrote:
> > > >
> > > >  We still want to give the "blacklisted" broker the leadership if
> > > > nobody else is available.  Therefore, isn't putting a broker on the
> > > > blacklist pretty much the same as moving it to the last entry in the
> > > > replicas list and then triggering a preferred leader election?
> > > >
> > > > If we want this to be undone after a certain amount of time, or under
> > > > certain conditions, that seems like something that would be more
> > > > effectively done by an external system, rather than putting all these
> > > > policies into Kafka.
> > > >
> > > > best,
> > > > Colin
> > > >
> > > >
> > > > On Fri, Jul 19, 2019, at 18:23, George Li wrote:
> > > > >  Hi Satish,
> > > > > Thanks for the reviews and feedbacks.
> > > > >
> > > > > > > The following is the requirements this KIP is trying to
> accomplish:
> > > > > > This can be moved to the"Proposed changes" section.
> > > > >
> > > > > Updated the KIP-491.
> > > > >
> > > > > > >>The logic to determine the priority/order of which broker
> should be
> > > > > > preferred leader should be modified.  The broker in the
> preferred leader
> > > > > > blacklist should be moved to the end (lowest priority) when
> > > > > > determining leadership.
> > > > > >
> > > > > > I believe there is no change required in the ordering of the
> preferred
> > > > > > replica list. Brokers in the preferred leader blacklist are
> skipped
> > > > > > until other brokers int he list are unavailable.
> > > > >
> > > > > Yes. partition assignment remained the same, replica & ordering.
> The
> > > > > blacklist logic can be optimized during implementation.
> > > > >
> > > > > > >>The blacklist can be at the broker level. However, there
might
> be use cases
> > > > > > where a specific topic should blacklist particular brokers,
which
> > > > > > would be at the
> > > > > > Topic level Config. For this use cases of this KIP, it seems
> that broker level
> > > > > > blacklist would suffice.  Topic level preferred leader blacklist
> might
> > > > > > be future enhancement work.
> > > > > >
> > > > > > I agree that the broker level preferred leader blacklist would
be
> > > > > > sufficient. Do you have any use cases which require topic level
> > > > > > preferred blacklist?
> > > > >
> > > > >
> > > > >
> > > > > I don't have any concrete use cases for Topic level preferred
> leader
> > > > > blacklist.  One scenarios I can think of is when a broker has high
> CPU
> > > > > usage, trying to identify the big topics (High MsgIn, High BytesIn,
> > > > > etc), then try to move the leaders away from this broker,  before
> doing
> > > > > an actual reassignment to change its preferred leader,  try to put
> this
> > > > > preferred_leader_blacklist in the Topic Level config, and run
> preferred
> > > > > leader election, and see whether CPU decreases for this broker, 
if
> > > > > yes, then do the reassignments to change the preferred leaders to
> be
> > > > > "permanent" (the topic may have many partitions like 256 that has
> quite
> > > > > a few of them having this broker as preferred leader).  So this
> Topic
> > > > > Level config is an easy way of doing trial and check the result.
> > > > >
> > > > >
> > > > > > You can add the below workaround as an item in the rejected
> alternatives section
> > > > > > "Reassigning all the topic/partitions which the intended broker
> is a
> > > > > > replica for."
> > > > >
> > > > > Updated the KIP-491.
> > > > >
> > > > >
> > > > >
> > > > > Thanks,
> > > > > George
> > > > >
> > > > >    On Friday, July 19, 2019, 08:20:22 AM PDT, Satish Duggana
> > > > > <satish.duggana@gmail.com> wrote:
> > > > >
> > > > >  Thanks for the KIP. I have put my comments below.
> > > > >
> > > > > This is a nice improvement to avoid cumbersome maintenance.
> > > > >
> > > > > >> The following is the requirements this KIP is trying to
> accomplish:
> > > > >   The ability to add and remove the preferred leader deprioritized
> > > > > list/blacklist. e.g. new ZK path/node or new dynamic config.
> > > > >
> > > > > This can be moved to the"Proposed changes" section.
> > > > >
> > > > > >>The logic to determine the priority/order of which broker
should
> be
> > > > > preferred leader should be modified.  The broker in the preferred
> leader
> > > > > blacklist should be moved to the end (lowest priority) when
> > > > > determining leadership.
> > > > >
> > > > > I believe there is no change required in the ordering of the
> preferred
> > > > > replica list. Brokers in the preferred leader blacklist are skipped
> > > > > until other brokers int he list are unavailable.
> > > > >
> > > > > >>The blacklist can be at the broker level. However, there
might
> be use cases
> > > > > where a specific topic should blacklist particular brokers, which
> > > > > would be at the
> > > > > Topic level Config. For this use cases of this KIP, it seems that
> broker level
> > > > > blacklist would suffice.  Topic level preferred leader blacklist
> might
> > > > > be future enhancement work.
> > > > >
> > > > > I agree that the broker level preferred leader blacklist would be
> > > > > sufficient. Do you have any use cases which require topic level
> > > > > preferred blacklist?
> > > > >
> > > > > You can add the below workaround as an item in the rejected
> alternatives section
> > > > > "Reassigning all the topic/partitions which the intended broker is
> a
> > > > > replica for."
> > > > >
> > > > > Thanks,
> > > > > Satish.
> > > > >
> > > > > On Fri, Jul 19, 2019 at 7:33 AM Stanislav Kozlovski
> > > > > <stanislav@confluent.io> wrote:
> > > > > >
> > > > > > Hey George,
> > > > > >
> > > > > > Thanks for the KIP, it's an interesting idea.
> > > > > >
> > > > > > I was wondering whether we could achieve the same thing via
the
> > > > > > kafka-reassign-partitions tool. As you had also said in the
> JIRA,  it is
> > > > > > true that this is currently very tedious with the tool. My
> thoughts are
> > > > > > that we could improve the tool and give it the notion of a
> "blacklisted
> > > > > > preferred leader".
> > > > > > This would have some benefits like:
> > > > > > - more fine-grained control over the blacklist. we may not want
> to
> > > > > > blacklist all the preferred leaders, as that would make the
> blacklisted
> > > > > > broker a follower of last resort which is not very useful. In
> the cases of
> > > > > > an underpowered AWS machine or a controller, you might overshoot
> and make
> > > > > > the broker very underutilized if you completely make it
> leaderless.
> > > > > > - is not permanent. If we are to have a blacklist leaders config,
> > > > > > rebalancing tools would also need to know about it and
> manipulate/respect
> > > > > > it to achieve a fair balance.
> > > > > > It seems like both problems are tied to balancing partitions,
> it's just
> > > > > > that KIP-491's use case wants to balance them against other
> factors in a
> > > > > > more nuanced way. It makes sense to have both be done from the
> same place
> > > > > >
> > > > > > To make note of the motivation section:
> > > > > > > Avoid bouncing broker in order to lose its leadership
> > > > > > The recommended way to make a broker lose its leadership is
to
> run a
> > > > > > reassignment on its partitions
> > > > > > > The cross-data center cluster has AWS cloud instances which
> have less
> > > > > > computing power
> > > > > > We recommend running Kafka on homogeneous machines. It would
be
> cool if the
> > > > > > system supported more flexibility in that regard but that is
> more nuanced
> > > > > > and a preferred leader blacklist may not be the best first
> approach to the
> > > > > > issue
> > > > > >
> > > > > > Adding a new config which can fundamentally change the way
> replication is
> > > > > > done is complex, both for the system (the replication code is
> complex
> > > > > > enough) and the user. Users would have another potential config
> that could
> > > > > > backfire on them - e.g if left forgotten.
> > > > > >
> > > > > > Could you think of any downsides to implementing this
> functionality (or a
> > > > > > variation of it) in the kafka-reassign-partitions.sh tool?
> > > > > > One downside I can see is that we would not have it handle new
> partitions
> > > > > > created after the "blacklist operation". As a first iteration
I
> think that
> > > > > > may be acceptable
> > > > > >
> > > > > > Thanks,
> > > > > > Stanislav
> > > > > >
> > > > > > On Fri, Jul 19, 2019 at 3:20 AM George Li <
> sql_consulting@yahoo.com.invalid>
> > > > > > wrote:
> > > > > >
> > > > > > >  Hi,
> > > > > > >
> > > > > > > Pinging the list for the feedbacks of this KIP-491  (
> > > > > > >
> https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=120736982
> > > > > > > )
> > > > > > >
> > > > > > >
> > > > > > > Thanks,
> > > > > > > George
> > > > > > >
> > > > > > >    On Saturday, July 13, 2019, 08:43:25 PM PDT, George
Li <
> > > > > > > sql_consulting@yahoo.com.INVALID> wrote:
> > > > > > >
> > > > > > >  Hi,
> > > > > > >
> > > > > > > I have created KIP-491 (
> > > > > > >
> https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=120736982
> )
> > > > > > > for putting a broker to the preferred leader blacklist
or
> deprioritized
> > > > > > > list so when determining leadership,  it's moved to the
lowest
> priority for
> > > > > > > some of the listed use-cases.
> > > > > > >
> > > > > > > Please provide your comments/feedbacks.
> > > > > > >
> > > > > > > Thanks,
> > > > > > > George
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > >  ----- Forwarded Message ----- From: Jose Armando Garcia
> Sancio (JIRA) <
> > > > > > > jira@apache.org>To: "sql_consulting@yahoo.com" <
> sql_consulting@yahoo.com>Sent:
> > > > > > > Tuesday, July 9, 2019, 01:06:05 PM PDTSubject: [jira]
> [Commented]
> > > > > > > (KAFKA-8638) Preferred Leader Blacklist (deprioritized
list)
> > > > > > >
> > > > > > >    [
> > > > > > >
> https://issues.apache.org/jira/browse/KAFKA-8638?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16881511#comment-16881511
> > > > > > > ]
> > > > > > >
> > > > > > > Jose Armando Garcia Sancio commented on KAFKA-8638:
> > > > > > > ---------------------------------------------------
> > > > > > >
> > > > > > > Thanks for feedback and clear use cases [~sql_consulting].
> > > > > > >
> > > > > > > > Preferred Leader Blacklist (deprioritized list)
> > > > > > > > -----------------------------------------------
> > > > > > > >
> > > > > > > >                Key: KAFKA-8638
> > > > > > > >                URL:
> https://issues.apache.org/jira/browse/KAFKA-8638
> > > > > > > >            Project: Kafka
> > > > > > > >          Issue Type: Improvement
> > > > > > > >          Components: config, controller, core
> > > > > > > >    Affects Versions: 1.1.1, 2.3.0, 2.2.1
> > > > > > > >            Reporter: GEORGE LI
> > > > > > > >            Assignee: GEORGE LI
> > > > > > > >            Priority: Major
> > > > > > > >
> > > > > > > > Currently, the kafka preferred leader election will
pick the
> broker_id
> > > > > > > in the topic/partition replica assignments in a priority
order
> when the
> > > > > > > broker is in ISR. The preferred leader is the broker id
in the
> first
> > > > > > > position of replica. There are use-cases that, even the
first
> broker in the
> > > > > > > replica assignment is in ISR, there is a need for it to
be
> moved to the end
> > > > > > > of ordering (lowest priority) when deciding leadership
during
> preferred
> > > > > > > leader election.
> > > > > > > > Let’s use topic/partition replica (1,2,3) as an
example. 1
> is the
> > > > > > > preferred leader.  When preferred leadership is run, it
will
> pick 1 as the
> > > > > > > leader if it's ISR, if 1 is not online and in ISR, then
pick
> 2, if 2 is not
> > > > > > > in ISR, then pick 3 as the leader. There are use cases
that,
> even 1 is in
> > > > > > > ISR, we would like it to be moved to the end of ordering
> (lowest priority)
> > > > > > > when deciding leadership during preferred leader election.
> Below is a list
> > > > > > > of use cases:
> > > > > > > > * (If broker_id 1 is a swapped failed host and brought
up
> with last
> > > > > > > segments or latest offset without historical data (There
is
> another effort
> > > > > > > on this), it's better for it to not serve leadership till
it's
> caught-up.
> > > > > > > > * The cross-data center cluster has AWS instances
which have
> less
> > > > > > > computing power than the on-prem bare metal machines. 
We
> could put the AWS
> > > > > > > broker_ids in Preferred Leader Blacklist, so on-prem brokers
> can be elected
> > > > > > > leaders, without changing the reassignments ordering of
the
> replicas.
> > > > > > > > * If the broker_id 1 is constantly losing leadership
after
> some time:
> > > > > > > "Flapping". we would want to exclude 1 to be a leader unless
> all other
> > > > > > > brokers of this topic/partition are offline.  The “Flapping”
> effect was
> > > > > > > seen in the past when 2 or more brokers were bad, when
they
> lost leadership
> > > > > > > constantly/quickly, the sets of partition replicas they
belong
> to will see
> > > > > > > leadership constantly changing.  The ultimate solution
is to
> swap these bad
> > > > > > > hosts.  But for quick mitigation, we can also put the bad
> hosts in the
> > > > > > > Preferred Leader Blacklist to move the priority of its
being
> elected as
> > > > > > > leaders to the lowest.
> > > > > > > > *  If the controller is busy serving an extra load
of
> metadata requests
> > > > > > > and other tasks. we would like to put the controller's
leaders
> to other
> > > > > > > brokers to lower its CPU load. currently bouncing to lose
> leadership would
> > > > > > > not work for Controller, because after the bounce, the
> controller fails
> > > > > > > over to another broker.
> > > > > > > > * Avoid bouncing broker in order to lose its leadership:
it
> would be
> > > > > > > good if we have a way to specify which broker should be
> excluded from
> > > > > > > serving traffic/leadership (without changing the replica
> assignment
> > > > > > > ordering by reassignments, even though that's quick), and
run
> preferred
> > > > > > > leader election.  A bouncing broker will cause temporary
URP,
> and sometimes
> > > > > > > other issues.  Also a bouncing of broker (e.g. broker_id
1)
> can temporarily
> > > > > > > lose all its leadership, but if another broker (e.g. broker_id
> 2) fails or
> > > > > > > gets bounced, some of its leaderships will likely failover
to
> broker_id 1
> > > > > > > on a replica with 3 brokers.  If broker_id 1 is in the
> blacklist, then in
> > > > > > > such a scenario even broker_id 2 offline,  the 3rd broker
can
> take
> > > > > > > leadership.
> > > > > > > > The current work-around of the above is to change
the
> topic/partition's
> > > > > > > replica reassignments to move the broker_id 1 from the
first
> position to
> > > > > > > the last position and run preferred leader election. e.g.
(1,
> 2, 3) => (2,
> > > > > > > 3, 1). This changes the replica reassignments, and we need
to
> keep track of
> > > > > > > the original one and restore if things change (e.g. controller
> fails over
> > > > > > > to another broker, the swapped empty broker caught up).
That’s
> a rather
> > > > > > > tedious task.
> > > > > > > >
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > --
> > > > > > > This message was sent by Atlassian JIRA
> > > > > > > (v7.6.3#76005)
>


-- 
Best,
Stanislav

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