Hi Colin. Thanks for the updates. It's now clear to me that brokers keep their broker epoch for the life of their JVM -- they register once, get their broker epoch in the response, and then never re-register again. Brokers may get fenced, but they keep the same broker epoch for the life of their JVM. The incarnation ID is also kept for the life of the JVM but is generated by the broker itself upon startup, and the combination of the two allows the Controller to act idempotently if any previously-sent registration response gets lost. Makes sense.
One thing I wonder about is if it might be helpful for the broker to send the Cluster ID as determined from its meta.properties file in its registration request. Does it even make sense for the broker to successfully register and enter the Fenced state if it has the wrong Cluster ID? The nextMetadatOffset value that the broker communicates in its registration request only has meaning within the correct cluster, so it feels to me that the Controller should have some way to perform this sanity check. There is currently (pre-KIP 500) a check in the broker to make sure its configured cluster ID matches the one stored in ZooKeeper, and we will have to perform this validation somewhere in the KIP-500 world. If the Controller doesn't do it within the registration request then the broker will have to make a metadata request to the Controller, retrieve the Cluster ID, and perform the check itself. It feels to me that it might be better for the Controller to just do it, and then the broker doesn't have to worry about it anymore once it successfully registers. I also have a question about the broker.id value and meta.properties. The KIP now says "In version 0 of meta.properties, there is a broker.id field. Version 1 does not have this field. It is no longer needed because we no longer support dynamic broker id assignment." But then there is an example version 1 meta.properties file that shows the broker.id value. I actually wonder if maybe the broker.id value would be good to keep in the version 1 meta.properties file because it currently (pre-KIP 500, version 0) acts as a sanity check to make sure the broker is using the correct log directory. Similarly with the controller.id value on controllers -- it would allow the same type of sanity check for quorum controllers. Thanks again for your diligent leadership on this KIP. Ron On Mon, Nov 30, 2020 at 7:41 PM Colin McCabe <cmcc...@apache.org> wrote: > > On Fri, Oct 23, 2020, at 16:10, Jun Rao wrote: > > Hi, Colin, > > > > Thanks for the reply. A few more comments. > > Hi Jun, > > Thanks again for the reply. Sorry for the long hiatus. I was on vacation > for a while. > > > > > 55. There is still text that favors new broker registration. "When a broker > > first starts up, when it is in the INITIAL state, it will always "win" > > broker ID conflicts. However, once it is granted a lease, it transitions > > out of the INITIAL state. Thereafter, it may lose subsequent conflicts if > > its broker epoch is stale. (See KIP-380 for some background on broker > > epoch.) The reason for favoring new processes is to accommodate the common > > case where a process is killed with kill -9 and then restarted. We want it > > to be able to reclaim its old ID quickly in this case." > > > > Thanks for the reminder. I have clarified the language here. Hopefully now > it is clear that we don't allow quick re-use of broker IDs. > > > 80.1 Sounds good. Could you document that listeners is a required config > > now? It would also be useful to annotate other required configs. For > > example, controller.connect should be required. > > > > I added a note specifying that these are required. > > > 80.2 Could you list all deprecated existing configs? Another one is > > control.plane.listener.name since the controller no longer sends > > LeaderAndIsr, UpdateMetadata and StopReplica requests. > > > > I added a section specifying some deprecated configs. > > > 83.1 It seems that the broker can transition from FENCED to RUNNING without > > registering for a new broker epoch. I am not sure how this works. Once the > > controller fences a broker, there is no need for the controller to keep the > > boker epoch around. So, if the fenced broker's heartbeat request with the > > existing broker epoch will be rejected, leading the broker back to the > > FENCED state again. > > > > The broker epoch refers to the broker registration. So we DO keep the broker > epoch around even while the broker is fenced. > > The broker epoch changes only when there is a new broker registration. > Fencing or unfencing the broker doesn't change the broker epoch. > > > 83.5 Good point on KIP-590. Then should we expose the controller for > > debugging purposes? If not, we should deprecate the controllerID field in > > MetadataResponse? > > > > I think it's OK to expose it for now, with the proviso that it won't be > reachable by clients. > > > 90. We rejected the shared ID with just one reason "This is not a good idea > > because NetworkClient assumes a single ID space. So if there is both a > > controller 1 and a broker 1, we don't have a way of picking the "right" > > one." This doesn't seem to be a strong reason. For example, we could > > address the NetworkClient issue with the node type as you pointed out or > > using the negative value of a broker ID as the controller ID. > > > > It would require a lot of code changes to support multiple types of node IDs. > It's not clear to me that the end result would be better -- I tend to think > it would be worse, since it would be more complex. In a similar vein, using > negative numbers seems dangerous, since we use negatives or -1 as "special > values" in many places. For example, -1 often represents "no such node." > > One important thing to keep in mind is that we want to be able to transition > from a broker and a controller being co-located to them no longer being > co-located. This is much easier to do when they have separate IDs. > > > 100. In KIP-589 > > <https://cwiki.apache.org/confluence/display/KAFKA/KIP-589+Add+API+to+update+Replica+state+in+Controller>, > > the broker reports all offline replicas due to a disk failure to the > > controller. It seems this information needs to be persisted to the > > metadata > > log. Do we have a corresponding record for that? > > > > Hmm, I have to look into this a little bit more. We may need a new record > type. > > > 101. Currently, StopReplica request has 2 modes, without deletion and with > > deletion. The former is used for controlled shutdown and handling disk > > failure, and causes the follower to stop. The latter is for topic deletion > > and partition reassignment, and causes the replica to be deleted. Since we > > are deprecating StopReplica, could we document what triggers the stopping > > of a follower and the deleting of a replica now? > > > > RemoveTopic triggers deletion. In general the functionality of StopReplica > is subsumed by the metadata records. > > > 102. Should we include the metadata topic in the MetadataResponse? If so, > > when it will be included and what will the metadata response look like? > > > > No, it won't be included in the metadata response sent back from the brokers. > > > 103. "The active controller assigns the broker a new broker epoch, based on > > the latest committed offset in the log." This seems inaccurate since the > > latest committed offset doesn't always advance on every log append. > > > > Given that the new broker epoch won't be visible until the commit has > happened, I have changed this to "the next available offset in the log" > > > 104. REGISTERING(1) : It says "Otherwise, the broker moves into the FENCED > > state.". It seems this should be RUNNING? > > > > 105. RUNNING: Should we require the broker to catch up to the metadata log > > to get into this state? > > For 104 and 105, these sections have been reworked. > > best, > Colin > > > > > Thanks, > > > > Jun > > > > > > > > On Fri, Oct 23, 2020 at 1:20 PM Colin McCabe <cmcc...@apache.org> wrote: > > > > > On Wed, Oct 21, 2020, at 05:51, Tom Bentley wrote: > > > > Hi Colin, > > > > > > > > On Mon, Oct 19, 2020, at 08:59, Ron Dagostino wrote: > > > > > > Hi Colin. Thanks for the hard work on this KIP. > > > > > > > > > > > > I have some questions about what happens to a broker when it becomes > > > > > > fenced (e.g. because it can't send a heartbeat request to keep its > > > > > > lease). The KIP says "When a broker is fenced, it cannot process > > > > > > any > > > > > > client requests. This prevents brokers which are not receiving > > > > > > metadata updates or that are not receiving and processing them fast > > > > > > enough from causing issues to clients." And in the description of > > > > > > the > > > > > > FENCED(4) state it likewise says "While in this state, the broker > > > does > > > > > > not respond to client requests." It makes sense that a fenced > > > > > > broker > > > > > > should not accept producer requests -- I assume any such requests > > > > > > would result in NotLeaderOrFollowerException. But what about > > > > > > KIP-392 > > > > > > (fetch from follower) consumer requests? It is conceivable that > > > these > > > > > > could continue. Related to that, would a fenced broker continue to > > > > > > fetch data for partitions where it thinks it is a follower? Even if > > > > > > it rejects consumer requests it might still continue to fetch as a > > > > > > follower. Might it be helpful to clarify both decisions here? > > > > > > > > > > Hi Ron, > > > > > > > > > > Good question. I think a fenced broker should continue to fetch on > > > > > partitions it was already fetching before it was fenced, unless it > > > hits a > > > > > problem. At that point it won't be able to continue, since it doesn't > > > have > > > > > the new metadata. For example, it won't know about leadership changes > > > in > > > > > the partitions it's fetching. The rationale for continuing to fetch > > > is to > > > > > try to avoid disruptions as much as possible. > > > > > > > > > > I don't think fenced brokers should accept client requests. The issue > > > is > > > > > that the fenced broker may or may not have any data it is supposed to > > > > > have. It may or may not have applied any configuration changes, etc. > > > that > > > > > it is supposed to have applied. So it could get pretty confusing, and > > > also > > > > > potentially waste the client's time. > > > > > > > > > > > > > > When fenced, how would the broker reply to a client which did make a > > > > request? > > > > > > > > > > Hi Tom, > > > > > > The broker will respond with a retryable error in that case. Once the > > > client has re-fetched its metadata, it will no longer see the fenced > > > broker > > > as part of the cluster. I added a note to the KIP. > > > > > > best, > > > Colin > > > > > > > > > > > Thanks, > > > > > > > > Tom > > > > > > > > >
