Colin, have you considered leveraging Apache Ratis (incubating)? Ryanne
On Wed, Aug 21, 2019, 1:28 PM Colin McCabe <cmcc...@apache.org> wrote: > On Wed, Aug 21, 2019, at 06:38, Eno Thereska wrote: > > Hi Colin, > > > > Nice KIP! For such a big change it would be good to add a pointer or > > two to related work that provides some sort of soft proof that the > > approach taken makes sense. Also such work often builds on other work > > and it might be useful to trace its roots. May I recommend adding a > > pointer to "Tango: Distributed Data Structures over a Shared Log" > > (http://www.cs.cornell.edu/~taozou/sosp13/tangososp.pdf)? There are > > other papers that are related (e.g., a more recent one one "The > > FuzzyLog: A Partially Ordered Shared Log" > > (https://www.usenix.org/system/files/osdi18-lockerman.pdf)). > > > > Both papers would add to the strength of your motivation. > > > > Hi Eno, > > Good point. I added a "references" section on the end and added the Tango > paper. I am not sure we need the FuzzyLog one, though. > > I also added a link to the Raft paper and one of the papers on HDFS, since > I feel like these are very relevant here. > > best, > Colin > > > Cheers > > Eno > > > > On Wed, Aug 21, 2019 at 12:22 PM Ron Dagostino <rndg...@gmail.com> > wrote: > > > > > > Hi Colin. I like the concept of a "bridge release" for migrating off > of > > > Zookeeper, but I worry that it may become a bottleneck if people > hesitate > > > to replace Zookeeper -- they would be unable to adopt newer versions of > > > Kafka until taking (what feels to them like) a giant leap. As an > example, > > > assuming version 4.0.x of Kafka is the supported bridge release, I > would > > > not be surprised if uptake of the 4.x release and the time-based > releases > > > that follow it end up being much slower due to the perceived barrier. > > > > > > Any perceived barrier could be lowered if the 4.0.x release could > > > optionally continue to use Zookeeper -- then the cutover would be two > > > incremental steps (move to 4.0.x, then replace Zookeeper while staying > on > > > 4.0.x) as opposed to a single big-bang (upgrade to 4.0.x and replace > > > Zookeeper in one fell swoop). > > > > > > Regardless of whether what I wrote above has merit or not, I think the > KIP > > > should be more explicit about what the upgrade constraints actually > are. > > > Can the bridge release be adopted with Zookeeper remaining in place and > > > then cutting over as a second, follow-on step, or must the Controller > > > Quorum nodes be started first and the bridge release cannot be used > with > > > Zookeeper at all? If the bridge release cannot be used with Zookeeper > at > > > all, then no version at or beyond the bridge release is available > > > unless/until abandoning Zookeeper; if the bridge release can be used > with > > > Zookeeper, then is it the only version that can be used with > Zookeeper, or > > > can Zookeeper be kept for additional releases if desired? > > > > > > Ron > > > > > > On Tue, Aug 20, 2019 at 10:19 AM Ron Dagostino <rndg...@gmail.com> > wrote: > > > > > > > Hi Colin. The diagram up at the top confused me -- specifically, the > > > > lines connecting the controller/active-controller to the brokers. I > had > > > > assumed the arrows on those lines represented the direction of data > flow, > > > > but that is not the case; the arrows actually identify the target of > the > > > > action, and the non-arrowed end indicates the initiator of the > action. For > > > > example, the lines point from the controller to the brokers in the > "today" > > > > section on the left to show that the controller pushes to the > brokers; the > > > > lines point from the brokers to the active-controller in the > "tomorrow" > > > > section on the right to show that the brokers pull from the > > > > active-controller. As I said, this confused me because my gut > instinct was > > > > to interpret the arrow as indicating the direction of data flow, and > when I > > > > look at the "tomorrow" picture on the right I initially thought > information > > > > was moving from the brokers to the active-controller. Did you > consider > > > > drawing that picture with the arrows reversed in the "tomorrow" side > so > > > > that the arrows represent the direction of data flow, and then add > the > > > > labels "push" on the "today" side and "pull" on the "tomorrow" side > to > > > > indicate who initiates the data flow? It occurs to me that this > picture > > > > may end up being widely distributed, so it might be in everyone's > interest > > > > to proactively avoid any possible confusion by being more explicit. > > > > > > > > Minor corrections? > > > > <<<In the current world, a broker which can contact ZooKeeper but > which > > > > is partitioned from the active controller > > > > >>>In the current world, a broker which can contact ZooKeeper but > which > > > > is partitioned from the controller > > > > > > > > <<<Eventually, the controller will ask the broker to finally go > offline > > > > >>>Eventually, the active controller will ask the broker to finally > go > > > > offline > > > > > > > > <<<New versions of the clients should send these operations directly > to > > > > the controller > > > > >>>New versions of the clients should send these operations directly > to > > > > the active controller > > > > > > > > <<<In the post-ZK world, the leader will make an RPC to the > controller > > > > instead > > > > >>>In the post-ZK world, the leader will make an RPC to the active > > > > controller instead > > > > > > > > <<<For example, the brokers may need to forward their requests to the > > > > controller. > > > > >>>For example, the brokers may need to forward their requests to the > > > > active controller. > > > > > > > > <<<The new controller will monitor ZooKeeper for legacy broker node > > > > registrations > > > > >>>The new (active) controller will monitor ZooKeeper for legacy > broker > > > > node registrations > > > > > > > > Ron > > > > > > > > On Mon, Aug 19, 2019 at 6:53 PM Colin McCabe <cmcc...@apache.org> > wrote: > > > > > > > >> Hi all, > > > >> > > > >> The KIP has been out for a while, so I'm thinking about calling a > vote > > > >> some time this week. > > > >> > > > >> best, > > > >> Colin > > > >> > > > >> On Mon, Aug 19, 2019, at 15:52, Colin McCabe wrote: > > > >> > On Mon, Aug 19, 2019, at 12:52, David Arthur wrote: > > > >> > > Thanks for the KIP, Colin. This looks great! > > > >> > > > > > >> > > I really like the idea of separating the Controller and Broker > JVMs. > > > >> > > > > > >> > > As you alluded to above, it might be nice to have a separate > > > >> > > broker-registration API to avoid overloading the metadata fetch > API. > > > >> > > > > > >> > > > > >> > Hi David, > > > >> > > > > >> > Thanks for taking a look. > > > >> > > > > >> > I removed the sentence about MetadataFetch also serving as the > broker > > > >> > registration API. I think I agree that we will probably want a > > > >> > separate RPC to fill this role. We will have a follow-on KIP > that will > > > >> > go into more detail about metadata propagation and registration > in the > > > >> > post-ZK world. That KIP will also have a full description of the > > > >> > registration RPC, etc. For now, I think the important part for > KIP-500 > > > >> > is that the broker registers with the controller quorum. On > > > >> > registration, the controller quorum assigns it a new broker epoch, > > > >> > which can distinguish successive broker incarnations. > > > >> > > > > >> > > > > > >> > > When a broker gets a metadata delta, will it be a sequence of > deltas > > > >> since > > > >> > > the last update or a cumulative delta since the last update? > > > >> > > > > > >> > > > > >> > It will be a sequence of deltas. Basically, the broker will be > reading > > > >> > from the metadata log. > > > >> > > > > >> > > > > > >> > > Will we include any kind of integrity check on the deltas to > ensure > > > >> the brokers > > > >> > > have applied them correctly? Perhaps this will be addressed in > one of > > > >> the > > > >> > > follow-on KIPs. > > > >> > > > > > >> > > > > >> > In general, we will have checksums on the metadata that we > fetch. This > > > >> > is similar to how we have checksums on regular data. Or if the > > > >> > question is about catching logic errors in the metadata handling > code, > > > >> > that sounds more like something that should be caught by test > cases. > > > >> > > > > >> > best, > > > >> > Colin > > > >> > > > > >> > > > > >> > > Thanks! > > > >> > > > > > >> > > On Fri, Aug 9, 2019 at 1:17 PM Colin McCabe <cmcc...@apache.org > > > > > >> wrote: > > > >> > > > > > >> > > > Hi Mickael, > > > >> > > > > > > >> > > > Thanks for taking a look. > > > >> > > > > > > >> > > > I don't think we want to support that kind of multi-tenancy > at the > > > >> > > > controller level. If the cluster is small enough that we > want to > > > >> pack the > > > >> > > > controller(s) with something else, we could run them > alongside the > > > >> brokers, > > > >> > > > or possibly inside three of the broker JVMs. > > > >> > > > > > > >> > > > best, > > > >> > > > Colin > > > >> > > > > > > >> > > > > > > >> > > > On Wed, Aug 7, 2019, at 10:37, Mickael Maison wrote: > > > >> > > > > Thank Colin for kickstarting this initiative. > > > >> > > > > > > > >> > > > > Just one question. > > > >> > > > > - A nice feature of Zookeeper is the ability to use chroots > and > > > >> have > > > >> > > > > several Kafka clusters use the same Zookeeper ensemble. Is > this > > > >> > > > > something we should keep? > > > >> > > > > > > > >> > > > > Thanks > > > >> > > > > > > > >> > > > > On Mon, Aug 5, 2019 at 7:44 PM Colin McCabe < > cmcc...@apache.org> > > > >> wrote: > > > >> > > > > > > > > >> > > > > > On Mon, Aug 5, 2019, at 10:02, Tom Bentley wrote: > > > >> > > > > > > Hi Colin, > > > >> > > > > > > > > > >> > > > > > > Thanks for the KIP. > > > >> > > > > > > > > > >> > > > > > > Currently ZooKeeper provides a convenient notification > > > >> mechanism for > > > >> > > > > > > knowing that broker and topic configuration has > changed. While > > > >> > > > KIP-500 does > > > >> > > > > > > suggest that incremental metadata update is expected to > come > > > >> to > > > >> > > > clients > > > >> > > > > > > eventually, that would seem to imply that for some > number of > > > >> > > > releases there > > > >> > > > > > > would be no equivalent mechanism for knowing about > config > > > >> changes. > > > >> > > > Is there > > > >> > > > > > > any thinking at this point about how a similar > notification > > > >> might be > > > >> > > > > > > provided in the future? > > > >> > > > > > > > > >> > > > > > We could eventually have some inotify-like mechanism where > > > >> clients > > > >> > > > could register interest in various types of events and got > notified > > > >> when > > > >> > > > they happened. Reading the metadata log is conceptually > simple. > > > >> The main > > > >> > > > complexity would be in setting up an API that made sense and > that > > > >> didn't > > > >> > > > unduly constrain future implementations. We'd have to think > > > >> carefully > > > >> > > > about what the real use-cases for this were, though. > > > >> > > > > > > > > >> > > > > > best, > > > >> > > > > > Colin > > > >> > > > > > > > > >> > > > > > > > > > >> > > > > > > Thanks, > > > >> > > > > > > > > > >> > > > > > > Tom > > > >> > > > > > > > > > >> > > > > > > On Mon, Aug 5, 2019 at 3:49 PM Viktor Somogyi-Vass < > > > >> > > > viktorsomo...@gmail.com> > > > >> > > > > > > wrote: > > > >> > > > > > > > > > >> > > > > > > > Hey Colin, > > > >> > > > > > > > > > > >> > > > > > > > I think this is a long-awaited KIP, thanks for > driving it. > > > >> I'm > > > >> > > > excited to > > > >> > > > > > > > see this in Kafka once. I collected my questions (and > I > > > >> accept the > > > >> > > > "TBD" > > > >> > > > > > > > answer as they might be a bit deep for this high > level :) ). > > > >> > > > > > > > 1.) Are there any specific reasons for the Controller > just > > > >> > > > periodically > > > >> > > > > > > > persisting its state on disk periodically instead of > > > >> > > > asynchronously with > > > >> > > > > > > > every update? Wouldn't less frequent saves increase > the > > > >> chance for > > > >> > > > missing > > > >> > > > > > > > a state change if the controller crashes between two > saves? > > > >> > > > > > > > 2.) Why can't we allow brokers to fetch metadata from > the > > > >> follower > > > >> > > > > > > > controllers? I assume that followers would have > up-to-date > > > >> > > > information > > > >> > > > > > > > therefore brokers could fetch from there in theory. > > > >> > > > > > > > > > > >> > > > > > > > Thanks, > > > >> > > > > > > > Viktor > > > >> > > > > > > > > > > >> > > > > > > > On Sun, Aug 4, 2019 at 6:58 AM Boyang Chen < > > > >> > > > reluctanthero...@gmail.com> > > > >> > > > > > > > wrote: > > > >> > > > > > > > > > > >> > > > > > > > > Thanks for explaining Ismael! Breaking down into > > > >> follow-up KIPs > > > >> > > > sounds > > > >> > > > > > > > like > > > >> > > > > > > > > a good idea. > > > >> > > > > > > > > > > > >> > > > > > > > > On Sat, Aug 3, 2019 at 10:14 AM Ismael Juma < > > > >> ism...@juma.me.uk> > > > >> > > > wrote: > > > >> > > > > > > > > > > > >> > > > > > > > > > Hi Boyang, > > > >> > > > > > > > > > > > > >> > > > > > > > > > Yes, there will be several KIPs that will discuss > the > > > >> items you > > > >> > > > > > > > describe > > > >> > > > > > > > > in > > > >> > > > > > > > > > detail. Colin, it may be helpful to make this > clear in > > > >> the KIP > > > >> > > > 500 > > > >> > > > > > > > > > description. > > > >> > > > > > > > > > > > > >> > > > > > > > > > Ismael > > > >> > > > > > > > > > > > > >> > > > > > > > > > On Sat, Aug 3, 2019 at 9:32 AM Boyang Chen < > > > >> > > > reluctanthero...@gmail.com > > > >> > > > > > > > > > > > >> > > > > > > > > > wrote: > > > >> > > > > > > > > > > > > >> > > > > > > > > > > Thanks Colin for initiating this important > effort! > > > >> > > > > > > > > > > > > > >> > > > > > > > > > > One question I have is whether we have a session > > > >> discussing > > > >> > > > the > > > >> > > > > > > > > > controller > > > >> > > > > > > > > > > failover in the new architecture? I know we are > using > > > >> Raft > > > >> > > > protocol > > > >> > > > > > > > to > > > >> > > > > > > > > > > failover, yet it's still valuable to discuss the > > > >> steps new > > > >> > > > cluster is > > > >> > > > > > > > > > going > > > >> > > > > > > > > > > to take to reach the stable stage again, so > that we > > > >> could > > > >> > > > easily > > > >> > > > > > > > > measure > > > >> > > > > > > > > > > the availability of the metadata servers. > > > >> > > > > > > > > > > > > > >> > > > > > > > > > > Another suggestion I have is to write a > step-by-step > > > >> design > > > >> > > > doc like > > > >> > > > > > > > > what > > > >> > > > > > > > > > > we did in KIP-98 > > > >> > > > > > > > > > > < > > > >> > > > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > >> > https://cwiki.apache.org/confluence/display/KAFKA/KIP-98+-+Exactly+Once+Delivery+and+Transactional+Messaging > > > >> > > > > > > > > > > >, > > > >> > > > > > > > > > > including the new request protocols and how > they are > > > >> > > > interacting in > > > >> > > > > > > > the > > > >> > > > > > > > > > new > > > >> > > > > > > > > > > cluster. For a complicated change like this, an > > > >> > > > implementation design > > > >> > > > > > > > > doc > > > >> > > > > > > > > > > help a lot in the review process, otherwise most > > > >> discussions > > > >> > > > we have > > > >> > > > > > > > > will > > > >> > > > > > > > > > > focus on high level and lose important details > as we > > > >> > > > discover them in > > > >> > > > > > > > > the > > > >> > > > > > > > > > > post-agreement phase. > > > >> > > > > > > > > > > > > > >> > > > > > > > > > > Boyang > > > >> > > > > > > > > > > > > > >> > > > > > > > > > > On Fri, Aug 2, 2019 at 5:17 PM Colin McCabe < > > > >> > > > cmcc...@apache.org> > > > >> > > > > > > > > wrote: > > > >> > > > > > > > > > > > > > >> > > > > > > > > > > > On Fri, Aug 2, 2019, at 16:33, Jose Armando > Garcia > > > >> Sancio > > > >> > > > wrote: > > > >> > > > > > > > > > > > > Thanks Colin for the detail KIP. I have a > few > > > >> comments > > > >> > > > and > > > >> > > > > > > > > questions. > > > >> > > > > > > > > > > > > > > > >> > > > > > > > > > > > > In the KIP's Motivation and Overview you > > > >> mentioned the > > > >> > > > > > > > LeaderAndIsr > > > >> > > > > > > > > > and > > > >> > > > > > > > > > > > > UpdateMetadata RPC. For example, "updates > which > > > >> the > > > >> > > > controller > > > >> > > > > > > > > > pushes, > > > >> > > > > > > > > > > > such > > > >> > > > > > > > > > > > > as LeaderAndIsr and UpdateMetadata > messages". Is > > > >> your > > > >> > > > thinking > > > >> > > > > > > > that > > > >> > > > > > > > > > we > > > >> > > > > > > > > > > > will > > > >> > > > > > > > > > > > > use MetadataFetch as a replacement to just > > > >> > > > UpdateMetadata only > > > >> > > > > > > > and > > > >> > > > > > > > > > add > > > >> > > > > > > > > > > > > topic configuration in this state? > > > >> > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > Hi Jose, > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > Thanks for taking a look. > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > The goal is for MetadataFetchRequest to > replace both > > > >> > > > > > > > > > LeaderAndIsrRequest > > > >> > > > > > > > > > > > and UpdateMetadataRequest. Topic > configurations > > > >> would be > > > >> > > > fetched > > > >> > > > > > > > > along > > > >> > > > > > > > > > > > with the other metadata. > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > In the section "Broker Metadata > Management", you > > > >> mention > > > >> > > > "Just > > > >> > > > > > > > like > > > >> > > > > > > > > > > with > > > >> > > > > > > > > > > > a > > > >> > > > > > > > > > > > > fetch request, the broker will track the > offset > > > >> of the > > > >> > > > last > > > >> > > > > > > > updates > > > >> > > > > > > > > > it > > > >> > > > > > > > > > > > > fetched". To keep the log consistent Raft > > > >> requires that > > > >> > > > the > > > >> > > > > > > > > followers > > > >> > > > > > > > > > > > keep > > > >> > > > > > > > > > > > > all of the log entries (term/epoch and > offset) > > > >> that are > > > >> > > > after the > > > >> > > > > > > > > > > > > highwatermark. Any log entry before the > > > >> highwatermark > > > >> > > > can be > > > >> > > > > > > > > > > > > compacted/snapshot. Do we expect the > > > >> MetadataFetch API > > > >> > > > to only > > > >> > > > > > > > > return > > > >> > > > > > > > > > > log > > > >> > > > > > > > > > > > > entries up to the highwatermark? Unlike > the Raft > > > >> > > > replication API > > > >> > > > > > > > > > which > > > >> > > > > > > > > > > > > will replicate/fetch log entries after the > > > >> highwatermark > > > >> > > > for > > > >> > > > > > > > > > consensus? > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > Good question. Clearly, we shouldn't expose > > > >> metadata > > > >> > > > updates to > > > >> > > > > > > > the > > > >> > > > > > > > > > > > brokers until they've been stored on a > majority of > > > >> the > > > >> > > > Raft nodes. > > > >> > > > > > > > > The > > > >> > > > > > > > > > > > most obvious way to do that, like you > mentioned, is > > > >> to > > > >> > > > have the > > > >> > > > > > > > > brokers > > > >> > > > > > > > > > > > only fetch up to the HWM, but not beyond. > There > > > >> might be > > > >> > > > a more > > > >> > > > > > > > > clever > > > >> > > > > > > > > > > way > > > >> > > > > > > > > > > > to do it by fetching the data, but not having > the > > > >> brokers > > > >> > > > act on it > > > >> > > > > > > > > > until > > > >> > > > > > > > > > > > the HWM advances. I'm not sure if that's > worth it > > > >> or > > > >> > > > not. We'll > > > >> > > > > > > > > > discuss > > > >> > > > > > > > > > > > this more in a separate KIP that just > discusses > > > >> just Raft. > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > > > > > > > > > In section "Broker Metadata Management", you > > > >> mention "the > > > >> > > > > > > > > controller > > > >> > > > > > > > > > > will > > > >> > > > > > > > > > > > > send a full metadata image rather than a > series of > > > >> > > > deltas". This > > > >> > > > > > > > > KIP > > > >> > > > > > > > > > > > > doesn't go into the set of operations that > need > > > >> to be > > > >> > > > supported > > > >> > > > > > > > on > > > >> > > > > > > > > > top > > > >> > > > > > > > > > > of > > > >> > > > > > > > > > > > > Raft but it would be interested if this > "full > > > >> metadata > > > >> > > > image" > > > >> > > > > > > > could > > > >> > > > > > > > > > be > > > >> > > > > > > > > > > > > express also as deltas. For example, > assuming we > > > >> are > > > >> > > > replicating > > > >> > > > > > > > a > > > >> > > > > > > > > > map > > > >> > > > > > > > > > > > this > > > >> > > > > > > > > > > > > "full metadata image" could be a sequence > of "put" > > > >> > > > operations > > > >> > > > > > > > > (znode > > > >> > > > > > > > > > > > create > > > >> > > > > > > > > > > > > to borrow ZK semantics). > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > The full image can definitely be expressed as > a sum > > > >> of > > > >> > > > deltas. At > > > >> > > > > > > > > some > > > >> > > > > > > > > > > > point, the number of deltas will get large > enough > > > >> that > > > >> > > > sending a > > > >> > > > > > > > full > > > >> > > > > > > > > > > image > > > >> > > > > > > > > > > > is better, though. One question that we're > still > > > >> thinking > > > >> > > > about is > > > >> > > > > > > > > how > > > >> > > > > > > > > > > > much of this can be shared with generic Kafka > log > > > >> code, > > > >> > > > and how > > > >> > > > > > > > much > > > >> > > > > > > > > > > should > > > >> > > > > > > > > > > > be different. > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > > > > > > > > > In section "Broker Metadata Management", you > > > >> mention > > > >> > > > "This > > > >> > > > > > > > request > > > >> > > > > > > > > > will > > > >> > > > > > > > > > > > > double as a heartbeat, letting the > controller > > > >> know that > > > >> > > > the > > > >> > > > > > > > broker > > > >> > > > > > > > > is > > > >> > > > > > > > > > > > > alive". In section "Broker State Machine", > you > > > >> mention > > > >> > > > "The > > > >> > > > > > > > > > > MetadataFetch > > > >> > > > > > > > > > > > > API serves as this registration mechanism". > Does > > > >> this > > > >> > > > mean that > > > >> > > > > > > > the > > > >> > > > > > > > > > > > > MetadataFetch Request will optionally > include > > > >> broker > > > >> > > > > > > > configuration > > > >> > > > > > > > > > > > > information? > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > I was originally thinking that the > > > >> MetadataFetchRequest > > > >> > > > should > > > >> > > > > > > > > include > > > >> > > > > > > > > > > > broker configuration information. Thinking > about > > > >> this > > > >> > > > more, maybe > > > >> > > > > > > > we > > > >> > > > > > > > > > > > should just have a special registration RPC > that > > > >> contains > > > >> > > > that > > > >> > > > > > > > > > > information, > > > >> > > > > > > > > > > > to avoid sending it over the wire all the > time. > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > Does this also mean that MetadataFetch > request > > > >> will > > > >> > > > result in > > > >> > > > > > > > > > > > > a "write"/AppendEntries through the Raft > > > >> replication > > > >> > > > protocol > > > >> > > > > > > > > before > > > >> > > > > > > > > > > you > > > >> > > > > > > > > > > > > can send the associated MetadataFetch > Response? > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > I think we should require the broker to be > out of > > > >> the > > > >> > > > Offline state > > > >> > > > > > > > > > > before > > > >> > > > > > > > > > > > allowing it to fetch metadata, yes. So the > separate > > > >> > > > registration > > > >> > > > > > > > RPC > > > >> > > > > > > > > > > > should have completed first. > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > > > > > > > > > In section "Broker State", you mention that > a > > > >> broker can > > > >> > > > > > > > transition > > > >> > > > > > > > > > to > > > >> > > > > > > > > > > > > online after it is caught with the > metadata. What > > > >> do you > > > >> > > > mean by > > > >> > > > > > > > > > this? > > > >> > > > > > > > > > > > > Metadata is always changing. How does the > broker > > > >> know > > > >> > > > that it is > > > >> > > > > > > > > > caught > > > >> > > > > > > > > > > > up > > > >> > > > > > > > > > > > > since it doesn't participate in the > consensus or > > > >> the > > > >> > > > advancement > > > >> > > > > > > > of > > > >> > > > > > > > > > the > > > >> > > > > > > > > > > > > highwatermark? > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > That's a good point. Being "caught up" is > somewhat > > > >> of a > > > >> > > > fuzzy > > > >> > > > > > > > > concept > > > >> > > > > > > > > > > > here, since the brokers do not participate in > the > > > >> metadata > > > >> > > > > > > > consensus. > > > >> > > > > > > > > > I > > > >> > > > > > > > > > > > think ideally we would want to define it in > terms > > > >> of time > > > >> > > > ("the > > > >> > > > > > > > > broker > > > >> > > > > > > > > > > has > > > >> > > > > > > > > > > > all the updates from the last 2 minutes", for > > > >> example.) > > > >> > > > We should > > > >> > > > > > > > > > spell > > > >> > > > > > > > > > > > this out better in the KIP. > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > > > > > > > > > In section "Start the controller quorum > nodes", > > > >> you > > > >> > > > mention "Once > > > >> > > > > > > > > it > > > >> > > > > > > > > > > has > > > >> > > > > > > > > > > > > taken over the /controller node, the active > > > >> controller > > > >> > > > will > > > >> > > > > > > > proceed > > > >> > > > > > > > > > to > > > >> > > > > > > > > > > > load > > > >> > > > > > > > > > > > > the full state of ZooKeeper. It will write > out > > > >> this > > > >> > > > information > > > >> > > > > > > > to > > > >> > > > > > > > > > the > > > >> > > > > > > > > > > > > quorum's metadata storage. After this > point, the > > > >> > > > metadata quorum > > > >> > > > > > > > > > will > > > >> > > > > > > > > > > be > > > >> > > > > > > > > > > > > the metadata store of record, rather than > the > > > >> data in > > > >> > > > ZooKeeper." > > > >> > > > > > > > > > > During > > > >> > > > > > > > > > > > > this migration do should we expect to have a > > > >> small period > > > >> > > > > > > > > controller > > > >> > > > > > > > > > > > > unavailability while the controller > replicas this > > > >> state > > > >> > > > to all of > > > >> > > > > > > > > the > > > >> > > > > > > > > > > > raft > > > >> > > > > > > > > > > > > nodes in the controller quorum and we > buffer new > > > >> > > > controller API > > > >> > > > > > > > > > > requests? > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > Yes, the controller would be unavailable > during this > > > >> > > > time. I don't > > > >> > > > > > > > > > think > > > >> > > > > > > > > > > > this will be that different from the current > period > > > >> of > > > >> > > > > > > > unavailability > > > >> > > > > > > > > > > when > > > >> > > > > > > > > > > > a new controller starts up and needs to load > the > > > >> full > > > >> > > > state from > > > >> > > > > > > > ZK. > > > >> > > > > > > > > > The > > > >> > > > > > > > > > > > main difference is that in this period, we'd > have > > > >> to write > > > >> > > > to the > > > >> > > > > > > > > > > > controller quorum rather than just to > memory. But > > > >> we > > > >> > > > believe this > > > >> > > > > > > > > > should > > > >> > > > > > > > > > > > be pretty fast. > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > regards, > > > >> > > > > > > > > > > > Colin > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > > >> > > > > > > > > > > > > Thanks! > > > >> > > > > > > > > > > > > -Jose > > > >> > > > > > > > > > > > > > > > >> > > > > > > > > > > > > > > >> > > > > > > > > > > > > > >> > > > > > > > > > > > > >> > > > > > > > > > > > >> > > > > > > > > > > >> > > > > > > > > > >> > > > > > > > >> > > > > > > >> > > > > > >> > > > > > >> > > -- > > > >> > > David Arthur > > > >> > > > > > >> > > > > >> > > > > > > >
