On Thu, Jul 23, 2020, at 23:02, Boyang Chen wrote: > Hey Colin, > > some more questions I have about the proposal: > > 1. We mentioned in the networking section that "The only time when clients > should contact a controller node directly is when they are debugging system > issues". But later we didn't talk about how to enable this debug mode, > could you consider getting a section about that? >
Hi Boyang, Thanks for the review. There isn't a separate debug mode or anything like that. The assumption here is that when debugging system issues, we are inside the cluster, or at least have access to the private controller network. This is pretty similar to how most administrators run ZooKeeper. > > 2. “When the active controller decides that a standby controller should > start a snapshot, it will communicate that information in its response to > the periodic heartbeat sent by that node.“ In the KIP-595, we provide an > RPC called `EndQuorumEpoch` which would transfer the leadership role to a > dedicated successor, do you think we could reuse that method instead of > piggy-backing on the heartbeat RPC? > I thought about this a little bit more, and I think the active controller should not need to give up the leadership in order to snapshot. So I removed the part about renouncing the controllership. > > 3. The `DeleteBroker` record is listed but not mentioned in details for the > KIP. Are we going to support removing a broker in runtime, or this record > is just for the sake of removing an obsolete broker due to heartbeat > failure? > This record is about removing a broker from the cluster due to heartbeat failure, or because it's shutting down. I have renamed it to FenceBroker to make that clearer. > > 4. In the rejected alternatives, we mentioned we don't want to combine > heartbeats and fetch and listed out the reason was due to extra complexity. > However, we should also mention some cons caused by this model, for example > we are doing 2X round trips to maintain a liveness, where as a regular > follower it should always send out fetch, for sure. If we are combining the > two, what are the heartbeat request fields we need to populate in the Fetch > protocol to make it work? Could we piggy-back on the UpdateMetadata RPC to > propagate the broker state change for listeners separately to the > controller? I'm not buying either approach here, just hope we could list > out more reasoning for separating the heartbeat RPC from Fetch, pros and > cons. > The UpdateMetadata RPC is not used in the post-KIP-500 world. This is mentioned later in KIP-631 where it says that "we will no longer need to send out LeaderAndIsrRequest, UpdateMetadataRequest, and StopReplicaRequest" We could combine the heartbeat with the fetch request. It would basically mean moving all the heartbeat fields into the fetch request. As the KIP says, this would be pretty messy. Another reason why it would be messy is because of the timing. Fetch requests can get delayed when they're fetching a lot of data. If this delays heartbeats then it could cause brokers to get fenced unnecessarily. This is something that we've gone back and forth about, but overall I think it's good to at least implement the simple thing first. best, Colin > > Boyang > > On Wed, Jul 15, 2020 at 5:30 PM Colin McCabe <cmcc...@apache.org> wrote: > > > On Mon, Jul 13, 2020, at 11:08, Boyang Chen wrote: > > > Hey Colin, some quick questions, > > > > > > 1. I looked around and didn't find a config for broker heartbeat > > interval, > > > are we piggy-back on some existing configs? > > > > > > > Good point. I meant to add this, but I forgot. I added > > registration.heartbeat.interval.ms in the table. > > > > > > > > 2. We only mentioned that the lease time is 10X of the heartbeat > > interval, > > > could we also include why we chose this value? > > > > > > > I will add registration.lease.timeout.ms so that this can be set > > separately from registration.heartbeat.interval.ms. The choice of value > > is a balance between not timing out brokers too soon, and not keeping > > unavailable brokers around too long. > > > > best, > > Colin > > > > > > > > On Mon, Jul 13, 2020 at 10:09 AM Jason Gustafson <ja...@confluent.io> > > wrote: > > > > > > > Hi Colin, > > > > > > > > Thanks for the proposal. A few initial comments comments/questions > > below: > > > > > > > > 1. I don't follow why we need a separate configuration for > > > > `controller.listeners`. The current listener configuration already > > allows > > > > users to specify multiple listeners, which allows them to define > > internal > > > > endpoints that are not exposed to clients. Can you explain what the new > > > > configuration gives us that we don't already have? > > > > 2. What is the advantage of creating a separate `controller.id` > > instead of > > > > just using `broker.id`? > > > > 3. It sounds like you are imagining a stop-the-world approach to > > > > snapshotting, which is why we need the controller micromanaging > > snapshots > > > > on all followers. Alternatives include fuzzy snapshots which can be > > done > > > > concurrently. If this has been rejected, can you add some detail about > > why? > > > > 4. More of a nit, but should `DeleteBrokerRecord` be > > > > `ShutdownBrokerRecord`? The broker is just getting removed from ISRs, > > but > > > > it would still be present in the replica set (I assume). > > > > > > > > Thanks, > > > > Jason > > > > > > > > On Sun, Jul 12, 2020 at 12:24 AM Colin McCabe <cmcc...@apache.org> > > wrote: > > > > > > > > > Hi Unmesh, > > > > > > > > > > That's an interesting idea, but I think it would be best to strive > > for > > > > > single metadata events that are complete in themselves, rather than > > > > trying > > > > > to do something transactional or EOS-like. For example, we could > > have a > > > > > create event that contains all the partitions to be created. > > > > > > > > > > best, > > > > > Colin > > > > > > > > > > > > > > > On Fri, Jul 10, 2020, at 04:12, Unmesh Joshi wrote: > > > > > > I was thinking that we might need something like multi-operation > > > > > > <https://issues.apache.org/jira/browse/ZOOKEEPER-965> record in > > > > > zookeeper > > > > > > to atomically create topic and partition records when this multi > > record > > > > > is > > > > > > committed. This way metadata will have both the TopicRecord and > > > > > > PartitionRecord together always, and in no situation we can have > > > > > > TopicRecord without PartitionRecord. Not sure if there are other > > > > > situations > > > > > > where multi-operation is needed. > > > > > > <https://issues.apache.org/jira/browse/ZOOKEEPER-965> > > > > > > > > > > > > Thanks, > > > > > > Unmesh > > > > > > > > > > > > On Fri, Jul 10, 2020 at 11:32 AM Colin McCabe <cmcc...@apache.org> > > > > > wrote: > > > > > > > > > > > > > Hi Unmesh, > > > > > > > > > > > > > > Yes, once the last stable offset advanced, we would consider the > > > > topic > > > > > > > creation to be done, and then we could return success to the > > client. > > > > > > > > > > > > > > best, > > > > > > > Colin > > > > > > > > > > > > > > On Thu, Jul 9, 2020, at 19:44, Unmesh Joshi wrote: > > > > > > > > It still needs HighWaterMark / LastStableOffset to be advanced > > by > > > > two > > > > > > > > records? Something like following? > > > > > > > > > > > > > > > > > > > > > > > > | | > > > > > > > > <------------------ |----------------| HighWaterMark > > > > > > > > Response |PartitionRecord | > > > > > > > > | | > > > > > > > > -----------------| > > > > > > > > | TopicRecord | > > > > - > > > > > > > > | | > > > > > > > > -------------------> ------------------ Previous > > HighWaterMark > > > > > > > > CreateTopic | | > > > > > > > > | | > > > > > > > > | | > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Fri, Jul 10, 2020 at 1:30 AM Colin McCabe < > > cmcc...@apache.org> > > > > > wrote: > > > > > > > > > > > > > > > > > On Thu, Jul 9, 2020, at 04:37, Unmesh Joshi wrote: > > > > > > > > > > I see that, when a new topic is created, two metadata > > records, > > > > a > > > > > > > > > > TopicRecord (just the name and id of the topic) and a > > > > > PartitionRecord > > > > > > > > > (more > > > > > > > > > > like LeaderAndIsr, with leader id and replica ids for the > > > > > partition) > > > > > > > are > > > > > > > > > > created. > > > > > > > > > > While creating the topic, log entries for both the records > > need > > > > > to be > > > > > > > > > > committed in RAFT core. Will it need something like a > > > > > > > > > MultiOperationRecord > > > > > > > > > > in zookeeper. Then, we can have a single log entry with > > both > > > > the > > > > > > > records, > > > > > > > > > > and the create topic request can be fulfilled atomically > > when > > > > > both > > > > > > > the > > > > > > > > > > records are committed? > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hi Unmesh, > > > > > > > > > > > > > > > > > > Since the active controller is the only node writing to the > > log, > > > > > there > > > > > > > is > > > > > > > > > no need for any kind of synchronization or access control at > > the > > > > > log > > > > > > > level. > > > > > > > > > > > > > > > > > > best, > > > > > > > > > Colin > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > Unmesh > > > > > > > > > > > > > > > > > > > > On Wed, Jul 8, 2020 at 6:57 AM Ron Dagostino < > > > > rndg...@gmail.com> > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > HI Colin. Thanks for the KIP. Here is some feedback and > > > > > various > > > > > > > > > > > questions. > > > > > > > > > > > > > > > > > > > > > > "*Controller processes will listen on a separate port > > from > > > > > brokers. > > > > > > > > > This > > > > > > > > > > > will be true even when the broker and controller are > > > > > co-located in > > > > > > > the > > > > > > > > > same > > > > > > > > > > > JVM*". I assume it is possible that the port numbers > > could be > > > > > the > > > > > > > same > > > > > > > > > when > > > > > > > > > > > using separate JVMs (i.e. broker uses port 9192 and > > > > controller > > > > > also > > > > > > > > > uses > > > > > > > > > > > port 9192). I think it would be clearer to state this > > along > > > > > these > > > > > > > > > > > lines: "Controller > > > > > > > > > > > nodes will listen on a port, and the controller port must > > > > > differ > > > > > > > from > > > > > > > > > any > > > > > > > > > > > port that a broker in the same JVM is listening on. In > > other > > > > > > > words, a > > > > > > > > > > > controller and a broker node, when in the same JVM, do > > not > > > > > share > > > > > > > ports" > > > > > > > > > > > > > > > > > > > > > > I think the sentence "*In the realm of ACLs, this > > translates > > > > to > > > > > > > > > controllers > > > > > > > > > > > requiring CLUSTERACTION on CLUSTER for all operations*" > > is > > > > > > > confusing. > > > > > > > > > It > > > > > > > > > > > feels to me that you can just delete it. Am I missing > > > > > something > > > > > > > here? > > > > > > > > > > > > > > > > > > > > > > The KIP states "*The metadata will be stored in memory > > on all > > > > > the > > > > > > > > > active > > > > > > > > > > > controllers.*" Can there be multiple active controllers? > > > > > Should > > > > > > > it > > > > > > > > > > > instead read "The metadata will be stored in memory on > > all > > > > > > > potential > > > > > > > > > > > controllers." (or something like that)? > > > > > > > > > > > > > > > > > > > > > > KIP-595 states "*we have assumed the name > > __cluster_metadata > > > > > for > > > > > > > this > > > > > > > > > > > topic, but this is not a formal part of this proposal*". > > > > This > > > > > > > KIP-631 > > > > > > > > > > > states "*Metadata changes need to be persisted to the > > > > > __metadata > > > > > > > log > > > > > > > > > before > > > > > > > > > > > we propagate them to the other nodes in the cluster. > > This > > > > > means > > > > > > > > > waiting > > > > > > > > > > > for the metadata log's last stable offset to advance to > > the > > > > > offset > > > > > > > of > > > > > > > > > the > > > > > > > > > > > change.*" Are we here formally defining "__metadata" as > > the > > > > > topic > > > > > > > > > name, > > > > > > > > > > > and should these sentences refer to "__metadata topic" > > rather > > > > > than > > > > > > > > > > > "__metadata log"? What are the "other nodes in the > > cluster" > > > > > that > > > > > > > are > > > > > > > > > > > referred to? These are not controller nodes but brokers, > > > > > right? > > > > > > > If > > > > > > > > > so, > > > > > > > > > > > then should we say "before we propagate them to the > > brokers"? > > > > > > > > > Technically > > > > > > > > > > > we have a controller cluster and a broker cluster -- two > > > > > separate > > > > > > > > > clusters, > > > > > > > > > > > correct? (Even though we could potentially share JVMs > > and > > > > > > > therefore > > > > > > > > > > > require no additional processes.). If the statement is > > > > > referring to > > > > > > > > > nodes > > > > > > > > > > > in both clusters then maybe we should state "before we > > > > > propagate > > > > > > > them > > > > > > > > > to > > > > > > > > > > > the other nodes in the controller cluster or to brokers." > > > > > > > > > > > > > > > > > > > > > > "*The controller may have several of these uncommitted > > > > changes > > > > > in > > > > > > > > > flight at > > > > > > > > > > > any given time. In essence, the controller's in-memory > > state > > > > > is > > > > > > > > > always a > > > > > > > > > > > little bit in the future compared to the current state. > > This > > > > > > > allows > > > > > > > > > the > > > > > > > > > > > controller to continue doing things while it waits for > > the > > > > > previous > > > > > > > > > changes > > > > > > > > > > > to be committed to the Raft log.*" Should the three > > > > references > > > > > > > above > > > > > > > > > be to > > > > > > > > > > > the active controller rather than just the controller? > > > > > > > > > > > > > > > > > > > > > > "*Therefore, the controller must not make this future > > state > > > > > > > "visible" > > > > > > > > > to > > > > > > > > > > > the rest of the cluster until it has been made > > persistent – > > > > > that > > > > > > > is, > > > > > > > > > until > > > > > > > > > > > it becomes current state*". Again I wonder if this should > > > > > refer to > > > > > > > > > "active" > > > > > > > > > > > controller, and indicate "anyone else" as opposed to "the > > > > rest > > > > > of > > > > > > > the > > > > > > > > > > > cluster" since we are talking about 2 clusters here? > > > > > > > > > > > > > > > > > > > > > > "*When the active controller decides that it itself > > should > > > > > create a > > > > > > > > > > > snapshot, it will first try to give up the leadership of > > the > > > > > Raft > > > > > > > > > quorum.*" > > > > > > > > > > > Why? Is it necessary to state this? It seems like it > > might > > > > > be an > > > > > > > > > > > implementation detail rather than a necessary > > > > > > > constraint/requirement > > > > > > > > > that > > > > > > > > > > > we declare publicly and would have to abide by. > > > > > > > > > > > > > > > > > > > > > > "*It will reject brokers whose metadata is too stale*". > > Why? > > > > > An > > > > > > > > > example > > > > > > > > > > > might be helpful here. > > > > > > > > > > > > > > > > > > > > > > "*it may lose subsequent conflicts if its broker epoch is > > > > > stale*" > > > > > > > This > > > > > > > > > is > > > > > > > > > > > the first time a "broker epoch" is mentioned. I am > > assuming > > > > > it is > > > > > > > the > > > > > > > > > > > controller epoch communicated to it (if any). It would > > be > > > > > good to > > > > > > > > > > > introduce it/explicitly state what it is before > > referring to > > > > > it. > > > > > > > > > > > > > > > > > > > > > > Ron > > > > > > > > > > > > > > > > > > > > > > On Tue, Jul 7, 2020 at 6:48 PM Colin McCabe < > > > > > cmcc...@apache.org> > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > Hi all, > > > > > > > > > > > > > > > > > > > > > > > > I posted a KIP about how the quorum-based controller > > > > > envisioned > > > > > > > in > > > > > > > > > > > KIP-500 > > > > > > > > > > > > will work. Please take a look here: > > > > > > > > > > > > https://cwiki.apache.org/confluence/x/4RV4CQ > > > > > > > > > > > > > > > > > > > > > > > > best, > > > > > > > > > > > > Colin > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > >
