@Jun Rao <jun...@gmail.com> Regarding your comment about log compaction. After some deep-diving into this we've decided to propose a new snapshot-based log cleaning mechanism which would be used to replace the current compaction mechanism for this meta log. A new KIP will be proposed specifically for this idea.
All, I've updated the KIP wiki a bit updating one config "election.jitter.max.ms" to "election.backoff.max.ms" to make it more clear about the usage: the configured value will be the upper bound of the binary exponential backoff time after a failed election, before starting a new one. Guozhang On Fri, Jun 12, 2020 at 9:26 AM Boyang Chen <reluctanthero...@gmail.com> wrote: > Thanks for the suggestions Guozhang. > > On Thu, Jun 11, 2020 at 2:51 PM Guozhang Wang <wangg...@gmail.com> wrote: > > > Hello Boyang, > > > > Thanks for the updated information. A few questions here: > > > > 1) Should the quorum-file also update to support multi-raft? > > > > I'm neutral about this, as we don't know yet how the multi-raft modules > would behave. If > we have different threads operating different raft groups, consolidating > the `checkpoint` files seems > not reasonable. We could always add `multi-quorum-file` later if possible. > > 2) In the previous proposal, there's fields in the FetchQuorumRecords like > > latestDirtyOffset, is that dropped intentionally? > > > > I dropped the latestDirtyOffset since it is associated with the log > compaction discussion. This is beyond this KIP scope and we could > potentially get a separate KIP to talk about it. > > > > 3) I think we also need to elaborate a bit more details regarding when to > > send metadata request and discover-brokers; currently we only discussed > > during bootstrap how these requests would be sent. I think the following > > scenarios would also need these requests > > > > 3.a) As long as a broker does not know the current quorum (including the > > leader and the voters), it should continue periodically ask other brokers > > via "metadata. > > 3.b) As long as a broker does not know all the current quorum voter's > > connections, it should continue periodically ask other brokers via > > "discover-brokers". > > 3.c) When the leader's fetch timeout elapsed, it should send metadata > > request. > > > > Make sense, will add to the KIP. > > > > > Guozhang > > > > > > On Wed, Jun 10, 2020 at 5:20 PM Boyang Chen <reluctanthero...@gmail.com> > > wrote: > > > > > Hey all, > > > > > > follow-up on the previous email, we made some more updates: > > > > > > 1. The Alter/DescribeQuorum RPCs are also re-structured to use > > multi-raft. > > > > > > 2. We add observer status into the DescribeQuorumResponse as we see it > > is a > > > low hanging fruit which is very useful for user debugging and > > reassignment. > > > > > > 3. The FindQuorum RPC is replaced with DiscoverBrokers RPC, which is > > purely > > > in charge of discovering broker connections in a gossip manner. The > > quorum > > > leader discovery is piggy-back on the Metadata RPC for the topic > > partition > > > leader, which in our case is the single metadata partition for the > > version > > > one. > > > > > > Let me know if you have any questions. > > > > > > Boyang > > > > > > On Tue, Jun 9, 2020 at 11:01 PM Boyang Chen < > reluctanthero...@gmail.com> > > > wrote: > > > > > > > Hey all, > > > > > > > > Thanks for the great discussions so far. I'm posting some KIP updates > > > from > > > > our working group discussion: > > > > > > > > 1. We will be changing the core RPCs from single-raft API to > > multi-raft. > > > > This means all protocols will be "batch" in the first version, but > the > > > KIP > > > > itself only illustrates the design for a single metadata topic > > partition. > > > > The reason is to "keep the door open" for future extensions of this > > piece > > > > of module such as a sharded controller or general quorum based topic > > > > replication, beyond the current Kafka replication protocol. > > > > > > > > 2. We will piggy-back on the current Kafka Fetch API instead of > > inventing > > > > a new FetchQuorumRecords RPC. The motivation is about the same as #1 > as > > > > well as making the integration work easier, instead of letting two > > > similar > > > > RPCs diverge. > > > > > > > > 3. In the EndQuorumEpoch protocol, instead of only sending the > request > > to > > > > the most caught-up voter, we shall broadcast the information to all > > > voters, > > > > with a sorted voter list in descending order of their corresponding > > > > replicated offset. In this way, the top voter will become a candidate > > > > immediately, while the other voters shall wait for an exponential > > > back-off > > > > to trigger elections, which helps ensure the top voter gets elected, > > and > > > > the election eventually happens when the top voter is not responsive. > > > > > > > > Please see the updated KIP and post any questions or concerns on the > > > > mailing thread. > > > > > > > > Boyang > > > > > > > > On Fri, May 8, 2020 at 5:26 PM Jun Rao <j...@confluent.io> wrote: > > > > > > > >> Hi, Guozhang and Jason, > > > >> > > > >> Thanks for the reply. A couple of more replies. > > > >> > > > >> 102. Still not sure about this. How is the tombstone issue addressed > > in > > > >> the > > > >> non-voter and the observer. They can die at any point and restart > at > > an > > > >> arbitrary later time, and the advancing of the firstDirty offset and > > the > > > >> removal of the tombstone can happen independently. > > > >> > > > >> 106. I agree that it would be less confusing if we used "epoch" > > instead > > > of > > > >> "leader epoch" consistently. > > > >> > > > >> Jun > > > >> > > > >> On Thu, May 7, 2020 at 4:04 PM Guozhang Wang <wangg...@gmail.com> > > > wrote: > > > >> > > > >> > Thanks Jun. Further replies are in-lined. > > > >> > > > > >> > On Mon, May 4, 2020 at 11:58 AM Jun Rao <j...@confluent.io> wrote: > > > >> > > > > >> > > Hi, Guozhang, > > > >> > > > > > >> > > Thanks for the reply. A few more replies inlined below. > > > >> > > > > > >> > > On Sun, May 3, 2020 at 6:33 PM Guozhang Wang < > wangg...@gmail.com> > > > >> wrote: > > > >> > > > > > >> > > > Hello Jun, > > > >> > > > > > > >> > > > Thanks for your comments! I'm replying inline below: > > > >> > > > > > > >> > > > On Fri, May 1, 2020 at 12:36 PM Jun Rao <j...@confluent.io> > > wrote: > > > >> > > > > > > >> > > > > 101. Bootstrapping related issues. > > > >> > > > > 101.1 Currently, we support auto broker id generation. Is > this > > > >> > > supported > > > >> > > > > for bootstrap brokers? > > > >> > > > > > > > >> > > > > > > >> > > > The vote ids would just be the broker ids. "bootstrap.servers" > > > >> would be > > > >> > > > similar to what client configs have today, where > "quorum.voters" > > > >> would > > > >> > be > > > >> > > > pre-defined config values. > > > >> > > > > > > >> > > > > > > >> > > My question was on the auto generated broker id. Currently, the > > > broker > > > >> > can > > > >> > > choose to have its broker Id auto generated. The generation is > > done > > > >> > through > > > >> > > ZK to guarantee uniqueness. Without ZK, it's not clear how the > > > broker > > > >> id > > > >> > is > > > >> > > auto generated. "quorum.voters" also can't be set statically if > > > broker > > > >> > ids > > > >> > > are auto generated. > > > >> > > > > > >> > > Jason has explained some ideas that we've discussed so far, the > > > >> reason we > > > >> > intentional did not include them so far is that we feel it is > > out-side > > > >> the > > > >> > scope of KIP-595. Under the umbrella of KIP-500 we should > definitely > > > >> > address them though. > > > >> > > > > >> > On the high-level, our belief is that "joining a quorum" and > > "joining > > > >> (or > > > >> > more specifically, registering brokers in) the cluster" would be > > > >> > de-coupled a bit, where the former should be completed before we > do > > > the > > > >> > latter. More specifically, assuming the quorum is already up and > > > >> running, > > > >> > after the newly started broker found the leader of the quorum it > can > > > >> send a > > > >> > specific RegisterBroker request including its listener / protocol > / > > > etc, > > > >> > and upon handling it the leader can send back the uniquely > generated > > > >> broker > > > >> > id to the new broker, while also executing the "startNewBroker" > > > >> callback as > > > >> > the controller. > > > >> > > > > >> > > > > >> > > > > > >> > > > > 102. Log compaction. One weak spot of log compaction is for > > the > > > >> > > consumer > > > >> > > > to > > > >> > > > > deal with deletes. When a key is deleted, it's retained as a > > > >> > tombstone > > > >> > > > > first and then physically removed. If a client misses the > > > >> tombstone > > > >> > > > > (because it's physically removed), it may not be able to > > update > > > >> its > > > >> > > > > metadata properly. The way we solve this in Kafka is based > on > > a > > > >> > > > > configuration (log.cleaner.delete.retention.ms) and we > > expect a > > > >> > > consumer > > > >> > > > > having seen an old key to finish reading the deletion > > tombstone > > > >> > within > > > >> > > > that > > > >> > > > > time. There is no strong guarantee for that since a broker > > could > > > >> be > > > >> > > down > > > >> > > > > for a long time. It would be better if we can have a more > > > reliable > > > >> > way > > > >> > > of > > > >> > > > > dealing with deletes. > > > >> > > > > > > > >> > > > > > > >> > > > We propose to capture this in the "FirstDirtyOffset" field of > > the > > > >> > quorum > > > >> > > > record fetch response: the offset is the maximum offset that > log > > > >> > > compaction > > > >> > > > has reached up to. If the follower has fetched beyond this > > offset > > > it > > > >> > > means > > > >> > > > itself is safe hence it has seen all records up to that > offset. > > On > > > >> > > getting > > > >> > > > the response, the follower can then decide if its end offset > > > >> actually > > > >> > > below > > > >> > > > that dirty offset (and hence may miss some tombstones). If > > that's > > > >> the > > > >> > > case: > > > >> > > > > > > >> > > > 1) Naively, it could re-bootstrap metadata log from the very > > > >> beginning > > > >> > to > > > >> > > > catch up. > > > >> > > > 2) During that time, it would refrain itself from answering > > > >> > > MetadataRequest > > > >> > > > from any clients. > > > >> > > > > > > >> > > > > > > >> > > I am not sure that the "FirstDirtyOffset" field fully addresses > > the > > > >> > issue. > > > >> > > Currently, the deletion tombstone is not removed immediately > > after a > > > >> > round > > > >> > > of cleaning. It's removed after a delay in a subsequent round of > > > >> > cleaning. > > > >> > > Consider an example where a key insertion is at offset 200 and a > > > >> deletion > > > >> > > tombstone of the key is at 400. Initially, FirstDirtyOffset is > at > > > >> 300. A > > > >> > > follower/observer fetches from offset 0 and fetches the key at > > > offset > > > >> > 200. > > > >> > > A few rounds of cleaning happen. FirstDirtyOffset is at 500 and > > the > > > >> > > tombstone at 400 is physically removed. The follower/observer > > > >> continues > > > >> > the > > > >> > > fetch, but misses offset 400. It catches all the way to > > > >> FirstDirtyOffset > > > >> > > and declares its metadata as ready. However, its metadata could > be > > > >> stale > > > >> > > since it actually misses the deletion of the key. > > > >> > > > > > >> > > Yeah good question, I should have put more details in my > > explanation > > > >> :) > > > >> > > > > >> > The idea is that we will adjust the log compaction for this raft > > based > > > >> > metadata log: before more details to be explained, since we have > two > > > >> types > > > >> > of "watermarks" here, whereas in Kafka the watermark indicates > where > > > >> every > > > >> > replica have replicated up to and in Raft the watermark indicates > > > where > > > >> the > > > >> > majority of replicas (here only indicating voters of the quorum, > not > > > >> > counting observers) have replicated up to, let's call them Kafka > > > >> watermark > > > >> > and Raft watermark. For this special log, we would maintain both > > > >> > watermarks. > > > >> > > > > >> > When log compacting on the leader, we would only compact up to the > > > Kafka > > > >> > watermark, i.e. if there is at least one voter who have not > > replicated > > > >> an > > > >> > entry, it would not be compacted. The "dirty-offset" is the offset > > > that > > > >> > we've compacted up to and is communicated to other voters, and the > > > other > > > >> > voters would also compact up to this value --- i.e. the difference > > > here > > > >> is > > > >> > that instead of letting each replica doing log compaction > > > independently, > > > >> > we'll have the leader to decide upon which offset to compact to, > and > > > >> > propagate this value to others to follow, in a more coordinated > > > manner. > > > >> > Also note when there are new voters joining the quorum who has not > > > >> > replicated up to the dirty-offset, of because of other issues they > > > >> > truncated their logs to below the dirty-offset, they'd have to > > > >> re-bootstrap > > > >> > from the beginning, and during this period of time the leader > > learned > > > >> about > > > >> > this lagging voter would not advance the watermark (also it would > > not > > > >> > decrement it), and hence not compacting either, until the voter(s) > > has > > > >> > caught up to that dirty-offset. > > > >> > > > > >> > So back to your example above, before the bootstrap voter gets to > > 300 > > > no > > > >> > log compaction would happen on the leader; and until later when > the > > > >> voter > > > >> > have got to beyond 400 and hence replicated that tombstone, the > log > > > >> > compaction would possibly get to that tombstone and remove it. Say > > > >> later it > > > >> > the leader's log compaction reaches 500, it can send this back to > > the > > > >> voter > > > >> > who can then also compact locally up to 500. > > > >> > > > > >> > > > > >> > > > > 105. Quorum State: In addition to VotedId, do we need the > > epoch > > > >> > > > > corresponding to VotedId? Over time, the same broker Id > could > > be > > > >> > voted > > > >> > > in > > > >> > > > > different generations with different epoch. > > > >> > > > > > > > >> > > > > > > > >> > > > Hmm, this is a good point. Originally I think the > "LeaderEpoch" > > > >> field > > > >> > in > > > >> > > > that file is corresponding to the "latest known leader epoch", > > not > > > >> the > > > >> > > > "current leader epoch". For example, if the current epoch is > N, > > > and > > > >> > then > > > >> > > a > > > >> > > > vote-request with epoch N+1 is received and the voter granted > > the > > > >> vote > > > >> > > for > > > >> > > > it, then it means for this voter it knows the "latest epoch" > is > > N > > > + > > > >> 1 > > > >> > > > although it is unknown if that sending candidate will indeed > > > become > > > >> the > > > >> > > new > > > >> > > > leader (which would only be notified via begin-quorum > request). > > > >> > However, > > > >> > > > when persisting the quorum state, we would encode leader-epoch > > to > > > >> N+1, > > > >> > > > while the leaderId to be the older leader. > > > >> > > > > > > >> > > > But now thinking about this a bit more, I feel we should use > two > > > >> > separate > > > >> > > > epochs, one for the "lates known" and one for the "current" to > > > pair > > > >> > with > > > >> > > > the leaderId. I will update the wiki page. > > > >> > > > > > > >> > > > > > > >> > > Hmm, it's kind of weird to bump up the leader epoch before the > new > > > >> leader > > > >> > > is actually elected, right. > > > >> > > > > > >> > > > > > >> > > > > 106. "OFFSET_OUT_OF_RANGE: Used in the FetchQuorumRecords > API > > to > > > >> > > indicate > > > >> > > > > that the follower has fetched from an invalid offset and > > should > > > >> > > truncate > > > >> > > > to > > > >> > > > > the offset/epoch indicated in the response." Observers can't > > > >> truncate > > > >> > > > their > > > >> > > > > logs. What should they do with OFFSET_OUT_OF_RANGE? > > > >> > > > > > > > >> > > > > > > > >> > > > I'm not sure if I understand your question? Observers should > > still > > > >> be > > > >> > > able > > > >> > > > to truncate their logs as well. > > > >> > > > > > > >> > > > > > > >> > > Hmm, I thought only the quorum nodes have local logs and > observers > > > >> don't? > > > >> > > > > > >> > > > 107. "The leader will continue sending BeginQuorumEpoch to > each > > > >> known > > > >> > > > voter > > > >> > > > > until it has received its endorsement." If a voter is down > > for a > > > >> long > > > >> > > > time, > > > >> > > > > sending BeginQuorumEpoch seems to add unnecessary overhead. > > > >> > Similarly, > > > >> > > > if a > > > >> > > > > follower stops sending FetchQuorumRecords, does the leader > > keep > > > >> > sending > > > >> > > > > BeginQuorumEpoch? > > > >> > > > > > > > >> > > > > > > >> > > > Regarding BeginQuorumEpoch: that is a good point. The > > > >> > begin-quorum-epoch > > > >> > > > request is for voters to quickly get the new leader > information; > > > >> > however > > > >> > > > even if they do not get them they can still eventually learn > > about > > > >> that > > > >> > > > from others via gossiping FindQuorum. I think we can adjust > the > > > >> logic > > > >> > to > > > >> > > > e.g. exponential back-off or with a limited num.retries. > > > >> > > > > > > >> > > > Regarding FetchQuorumRecords: if the follower sends > > > >> FetchQuorumRecords > > > >> > > > already, it means that follower already knows that the broker > is > > > the > > > >> > > > leader, and hence we can stop retrying BeginQuorumEpoch; > however > > > it > > > >> is > > > >> > > > possible that after a follower sends FetchQuorumRecords > already, > > > >> > suddenly > > > >> > > > it stops send it (possibly because it learned about a higher > > epoch > > > >> > > leader), > > > >> > > > and hence this broker may be a "zombie" leader and we propose > to > > > use > > > >> > the > > > >> > > > fetch.timeout to let the leader to try to verify if it has > > already > > > >> been > > > >> > > > stale. > > > >> > > > > > > >> > > > > > > >> > > It just seems that we should handle these two cases in a > > consistent > > > >> way? > > > >> > > > > > >> > > Yes I agree, on the leader's side, the FetchQuorumRecords from a > > > >> follower > > > >> > could mean that we no longer needs to send BeginQuorumEpoch > anymore > > > --- > > > >> and > > > >> > it is already part of our current implementations in > > > >> > https://github.com/confluentinc/kafka/commits/kafka-raft > > > >> > > > > >> > > > > >> > > Thanks, > > > >> > > > > > >> > > Jun > > > >> > > > > > >> > > > > > > >> > > > > > > > >> > > > > Jun > > > >> > > > > > > > >> > > > > On Wed, Apr 29, 2020 at 8:56 PM Guozhang Wang < > > > wangg...@gmail.com > > > >> > > > > >> > > > wrote: > > > >> > > > > > > > >> > > > > > Hello Leonard, > > > >> > > > > > > > > >> > > > > > Thanks for your comments, I'm relying in line below: > > > >> > > > > > > > > >> > > > > > On Wed, Apr 29, 2020 at 1:58 AM Wang (Leonard) Ge < > > > >> > w...@confluent.io> > > > >> > > > > > wrote: > > > >> > > > > > > > > >> > > > > > > Hi Kafka developers, > > > >> > > > > > > > > > >> > > > > > > It's great to see this proposal and it took me some time > > to > > > >> > finish > > > >> > > > > > reading > > > >> > > > > > > it. > > > >> > > > > > > > > > >> > > > > > > And I have the following questions about the Proposal: > > > >> > > > > > > > > > >> > > > > > > - How do we plan to test this design to ensure its > > > >> > correctness? > > > >> > > Or > > > >> > > > > > more > > > >> > > > > > > broadly, how do we ensure that our new ‘pull’ based > > model > > > >> is > > > >> > > > > > functional > > > >> > > > > > > and > > > >> > > > > > > correct given that it is different from the original > > RAFT > > > >> > > > > > implementation > > > >> > > > > > > which has formal proof of correctness? > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > We have two planned verifications on the correctness and > > > >> liveness > > > >> > of > > > >> > > > the > > > >> > > > > > design. One is via model verification (TLA+) > > > >> > > > > > https://github.com/guozhangwang/kafka-specification > > > >> > > > > > > > > >> > > > > > Another is via the concurrent simulation tests > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > > > >> > > > > > > >> > > > > > >> > > > > >> > > > > > > https://github.com/confluentinc/kafka/commit/5c0c054597d2d9f458cad0cad846b0671efa2d91 > > > >> > > > > > > > > >> > > > > > - Have we considered any sensible defaults for the > > > >> > configuration, > > > >> > > > i.e. > > > >> > > > > > > all the election timeout, fetch time out, etc.? Or we > > > want > > > >> to > > > >> > > > leave > > > >> > > > > > > this to > > > >> > > > > > > a later stage when we do the performance testing, > etc. > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > This is a good question, the reason we did not set any > > default > > > >> > values > > > >> > > > for > > > >> > > > > > the timeout configurations is that we think it may take > some > > > >> > > > benchmarking > > > >> > > > > > experiments to get these defaults right. Some high-level > > > >> principles > > > >> > > to > > > >> > > > > > consider: 1) the fetch.timeout should be around the same > > scale > > > >> with > > > >> > > zk > > > >> > > > > > session timeout, which is now 18 seconds by default -- in > > > >> practice > > > >> > > > we've > > > >> > > > > > seen unstable networks having more than 10 secs of > transient > > > >> > > > > connectivity, > > > >> > > > > > 2) the election.timeout, however, should be smaller than > the > > > >> fetch > > > >> > > > > timeout > > > >> > > > > > as is also suggested as a practical optimization in > > > literature: > > > >> > > > > > > > https://www.cl.cam.ac.uk/~ms705/pub/papers/2015-osr-raft.pdf > > > >> > > > > > > > > >> > > > > > Some more discussions can be found here: > > > >> > > > > > > > > https://github.com/confluentinc/kafka/pull/301/files#r415420081 > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > > - Have we considered piggybacking `BeginQuorumEpoch` > > with > > > >> the > > > >> > ` > > > >> > > > > > > FetchQuorumRecords`? I might be missing something > > obvious > > > >> but > > > >> > I > > > >> > > am > > > >> > > > > > just > > > >> > > > > > > wondering why don’t we just use the `FindQuorum` and > > > >> > > > > > > `FetchQuorumRecords` > > > >> > > > > > > APIs and remove the `BeginQuorumEpoch` API? > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > Note that Begin/EndQuorumEpoch is sent from leader -> > other > > > >> voter > > > >> > > > > > followers, while FindQuorum / Fetch are sent from follower > > to > > > >> > leader. > > > >> > > > > > Arguably one can eventually realize the new leader and > epoch > > > via > > > >> > > > > gossiping > > > >> > > > > > FindQuorum, but that could in practice require a long > delay. > > > >> > Having a > > > >> > > > > > leader -> other voters request helps the new leader epoch > to > > > be > > > >> > > > > propagated > > > >> > > > > > faster under a pull model. > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > > - And about the `FetchQuorumRecords` response schema, > > in > > > >> the > > > >> > > > > `Records` > > > >> > > > > > > field of the response, is it just one record or all > the > > > >> > records > > > >> > > > > > starting > > > >> > > > > > > from the FetchOffset? It seems a lot more efficient > if > > we > > > >> sent > > > >> > > all > > > >> > > > > the > > > >> > > > > > > records during the bootstrapping of the brokers. > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > Yes the fetching is batched: FetchOffset is just the > > starting > > > >> > offset > > > >> > > of > > > >> > > > > the > > > >> > > > > > batch of records. > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > > - Regarding the disruptive broker issues, does our > pull > > > >> based > > > >> > > > model > > > >> > > > > > > suffer from it? If so, have we considered the > Pre-Vote > > > >> stage? > > > >> > If > > > >> > > > > not, > > > >> > > > > > > why? > > > >> > > > > > > > > > >> > > > > > > > > > >> > > > > > The disruptive broker is stated in the original Raft paper > > > >> which is > > > >> > > the > > > >> > > > > > result of the push model design. Our analysis showed that > > with > > > >> the > > > >> > > pull > > > >> > > > > > model it is no longer an issue. > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > > Thanks a lot for putting this up, and I hope that my > > > questions > > > >> > can > > > >> > > be > > > >> > > > > of > > > >> > > > > > > some value to make this KIP better. > > > >> > > > > > > > > > >> > > > > > > Hope to hear from you soon! > > > >> > > > > > > > > > >> > > > > > > Best wishes, > > > >> > > > > > > Leonard > > > >> > > > > > > > > > >> > > > > > > > > > >> > > > > > > On Wed, Apr 29, 2020 at 1:46 AM Colin McCabe < > > > >> cmcc...@apache.org > > > >> > > > > > >> > > > > wrote: > > > >> > > > > > > > > > >> > > > > > > > Hi Jason, > > > >> > > > > > > > > > > >> > > > > > > > It's amazing to see this coming together :) > > > >> > > > > > > > > > > >> > > > > > > > I haven't had a chance to read in detail, but I read > the > > > >> > outline > > > >> > > > and > > > >> > > > > a > > > >> > > > > > > few > > > >> > > > > > > > things jumped out at me. > > > >> > > > > > > > > > > >> > > > > > > > First, for every epoch that is 32 bits rather than > 64, I > > > >> sort > > > >> > of > > > >> > > > > wonder > > > >> > > > > > > if > > > >> > > > > > > > that's a good long-term choice. I keep reading about > > > stuff > > > >> > like > > > >> > > > > this: > > > >> > > > > > > > https://issues.apache.org/jira/browse/ZOOKEEPER-1277 > . > > > >> > > Obviously, > > > >> > > > > > that > > > >> > > > > > > > JIRA is about zxid, which increments much faster than > we > > > >> expect > > > >> > > > these > > > >> > > > > > > > leader epochs to, but it would still be good to see > some > > > >> rough > > > >> > > > > > > calculations > > > >> > > > > > > > about how long 32 bits (or really, 31 bits) will last > us > > > in > > > >> the > > > >> > > > cases > > > >> > > > > > > where > > > >> > > > > > > > we're using it, and what the space savings we're > getting > > > >> really > > > >> > > is. > > > >> > > > > It > > > >> > > > > > > > seems like in most cases the tradeoff may not be worth > > it? > > > >> > > > > > > > > > > >> > > > > > > > Another thing I've been thinking about is how we do > > > >> > > > bootstrapping. I > > > >> > > > > > > > would prefer to be in a world where formatting a new > > Kafka > > > >> node > > > >> > > > was a > > > >> > > > > > > first > > > >> > > > > > > > class operation explicitly initiated by the admin, > > rather > > > >> than > > > >> > > > > > something > > > >> > > > > > > > that happened implicitly when you started up the > broker > > > and > > > >> > > things > > > >> > > > > > > "looked > > > >> > > > > > > > blank." > > > >> > > > > > > > > > > >> > > > > > > > The first problem is that things can "look blank" > > > >> accidentally > > > >> > if > > > >> > > > the > > > >> > > > > > > > storage system is having a bad day. Clearly in the > > > non-Raft > > > >> > > world, > > > >> > > > > > this > > > >> > > > > > > > leads to data loss if the broker that is (re)started > > this > > > >> way > > > >> > was > > > >> > > > the > > > >> > > > > > > > leader for some partitions. > > > >> > > > > > > > > > > >> > > > > > > > The second problem is that we have a bit of a chicken > > and > > > >> egg > > > >> > > > problem > > > >> > > > > > > with > > > >> > > > > > > > certain configuration keys. For example, maybe you > want > > > to > > > >> > > > configure > > > >> > > > > > > some > > > >> > > > > > > > connection security settings in your cluster, but you > > > don't > > > >> > want > > > >> > > > them > > > >> > > > > > to > > > >> > > > > > > > ever be stored in a plaintext config file. (For > > example, > > > >> SCRAM > > > >> > > > > > > passwords, > > > >> > > > > > > > etc.) You could use a broker API to set the > > > configuration, > > > >> but > > > >> > > > that > > > >> > > > > > > brings > > > >> > > > > > > > up the chicken and egg problem. The broker needs to > be > > > >> > > configured > > > >> > > > to > > > >> > > > > > > know > > > >> > > > > > > > how to talk to you, but you need to configure it > before > > > you > > > >> can > > > >> > > > talk > > > >> > > > > to > > > >> > > > > > > > it. Using an external secret manager like Vault is > one > > > way > > > >> to > > > >> > > > solve > > > >> > > > > > > this, > > > >> > > > > > > > but not everyone uses an external secret manager. > > > >> > > > > > > > > > > >> > > > > > > > quorum.voters seems like a similar configuration key. > > In > > > >> the > > > >> > > > current > > > >> > > > > > > KIP, > > > >> > > > > > > > this is only read if there is no other configuration > > > >> specifying > > > >> > > the > > > >> > > > > > > quorum > > > >> > > > > > > > voter set. If we had a kafka.mkfs command, we > wouldn't > > > need > > > >> > this > > > >> > > > key > > > >> > > > > > > > because we could assume that there was always quorum > > > >> > information > > > >> > > > > stored > > > >> > > > > > > > locally. > > > >> > > > > > > > > > > >> > > > > > > > best, > > > >> > > > > > > > Colin > > > >> > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > On Thu, Apr 16, 2020, at 16:44, Jason Gustafson wrote: > > > >> > > > > > > > > Hi All, > > > >> > > > > > > > > > > > >> > > > > > > > > I'd like to start a discussion on KIP-595: > > > >> > > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > > > >> > > > > > > >> > > > > > >> > > > > >> > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-595%3A+A+Raft+Protocol+for+the+Metadata+Quorum > > > >> > > > > > > > . > > > >> > > > > > > > > This proposal specifies a Raft protocol to > ultimately > > > >> replace > > > >> > > > > > Zookeeper > > > >> > > > > > > > > as > > > >> > > > > > > > > documented in KIP-500. Please take a look and share > > your > > > >> > > > thoughts. > > > >> > > > > > > > > > > > >> > > > > > > > > A few minor notes to set the stage a little bit: > > > >> > > > > > > > > > > > >> > > > > > > > > - This KIP does not specify the structure of the > > > messages > > > >> > used > > > >> > > to > > > >> > > > > > > > represent > > > >> > > > > > > > > metadata in Kafka, nor does it specify the internal > > API > > > >> that > > > >> > > will > > > >> > > > > be > > > >> > > > > > > used > > > >> > > > > > > > > by the controller. Expect these to come in later > > > >> proposals. > > > >> > > Here > > > >> > > > we > > > >> > > > > > are > > > >> > > > > > > > > primarily concerned with the replication protocol > and > > > >> basic > > > >> > > > > > operational > > > >> > > > > > > > > mechanics. > > > >> > > > > > > > > - We expect many details to change as we get closer > to > > > >> > > > integration > > > >> > > > > > with > > > >> > > > > > > > > the controller. Any changes we make will be made > > either > > > as > > > >> > > > > amendments > > > >> > > > > > > to > > > >> > > > > > > > > this KIP or, in the case of larger changes, as new > > > >> proposals. > > > >> > > > > > > > > - We have a prototype implementation which I will > put > > > >> online > > > >> > > > within > > > >> > > > > > the > > > >> > > > > > > > > next week which may help in understanding some > > details. > > > It > > > >> > has > > > >> > > > > > > diverged a > > > >> > > > > > > > > little bit from our proposal, so I am taking a > little > > > >> time to > > > >> > > > bring > > > >> > > > > > it > > > >> > > > > > > in > > > >> > > > > > > > > line. I'll post an update to this thread when it is > > > >> available > > > >> > > for > > > >> > > > > > > review. > > > >> > > > > > > > > > > > >> > > > > > > > > Finally, I want to mention that this proposal was > > > drafted > > > >> by > > > >> > > > > myself, > > > >> > > > > > > > Boyang > > > >> > > > > > > > > Chen, and Guozhang Wang. > > > >> > > > > > > > > > > > >> > > > > > > > > Thanks, > > > >> > > > > > > > > Jason > > > >> > > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > >> > > > > > > > > > >> > > > > > > -- > > > >> > > > > > > Leonard Ge > > > >> > > > > > > Software Engineer Intern - Confluent > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > -- > > > >> > > > > > -- Guozhang > > > >> > > > > > > > > >> > > > > > > > >> > > > > > > >> > > > > > > >> > > > -- > > > >> > > > -- Guozhang > > > >> > > > > > > >> > > > > > >> > > > > >> > > > > >> > -- > > > >> > -- Guozhang > > > >> > > > > >> > > > > > > > > > > > > > -- > > -- Guozhang > > > -- -- Guozhang
