One possibility would be writing a new command-line tool that would deprioritize a given replica using the new KIP-455 API. Then it could write out a JSON files containing the old priorities, which could be restored when (or if) we needed to do so. This seems like it might be simpler and easier to maintain than a separate set of metadata about blacklists.
best, Colin On Fri, Sep 6, 2019, at 11:58, George Li 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_consult...@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 > <cmcc...@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 > > <cmcc...@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 > > > <cmcc...@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.dugg...@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 > > > > <stanis...@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_consult...@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_consult...@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) < > > > > > > j...@apache.org>To: "sql_consult...@yahoo.com" > > > > > > <sql_consult...@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)
