Why would the replica placement logic run in the controller rather than in the AdminManager?
My understanding, and correct me if I got it wrong, is that we are aiming at better separation of concerns. The controller job will be managing consensus and consistency of metadata, but creating the metadata itself will be in the AdminManager. On Mon, Nov 16, 2020, 5:31 AM David Jacot <dja...@confluent.io> wrote: > Hi Mickael, > > Thanks for the KIP. It is an interesting one. > > I imagine that custom assignors may use a rather complex model of the > cluster in order > to be able to allocate partitions in smarter ways. For instance, one may > use the distribution > of leaders in the cluster to allocate the new leaders. With the current > interface, that > means computing the distribution based on the Cluster received for every > assignment > request. That could become pretty inefficient in clusters with a large > number of nodes > and/or partitions. That could become even worse if the model is more > complicated. > > I wonder if you have thought about this or experienced this with your > prototype? > > Have you considered going with an approach à la ClientQuotaCallback where > the plugin > is updated when the Cluster has changed? That would allow to keep an > internal model > ready. Another way would be to use batching as suggested as it would allow > to amortize > the cost of building a model for the current request/user. > > I also wonder if using Cluster is a good idea here. With KIP-500, I can > imagine that this > plugin will run in the controller directly instead of running in the > AdminManager as today. > In this case, we could obviously continue to build that Cluster object but > we may have > better ways. Therefore, I wonder if having an interface to represent the > cluster may be > better from an evolution perspective. Have you considered this? > > Best, > David > > On Mon, Nov 16, 2020 at 12:10 PM Mickael Maison <mickael.mai...@gmail.com> > wrote: > > > Hi all, > > > > If I don't see additional feedback in the next few days, I'll start a > vote. > > Thanks > > > > On Thu, Nov 5, 2020 at 6:29 PM Mickael Maison <mickael.mai...@gmail.com> > > wrote: > > > > > > Hi all, > > > > > > I've updated the KIP to reflect the latest discussions. > > > > > > Tom, > > > 2) Updated > > > 4) I decided against switching to a "batch interface" and added the > > > reasons in the Rejected Alternatives section > > > > > > Please take a look and let me know if you have any feedback. > > > Thanks > > > > > > On Tue, Oct 6, 2020 at 9:43 AM Mickael Maison < > mickael.mai...@gmail.com> > > wrote: > > > > > > > > Hi Efe, > > > > > > > > Thanks for the feedback. > > > > We also need to assign replicas when adding partitions to an existing > > > > topic. This is why I choose to use a list of partition ids. Otherwise > > > > we'd need the number of partitions and the starting partition id. > > > > > > > > Let me know if you have more questions > > > > > > > > On Tue, Oct 6, 2020 at 2:16 AM Efe Gencer > <agen...@linkedin.com.invalid> > > wrote: > > > > > > > > > > Hi Mickael, > > > > > > > > > > Thanks for the KIP! > > > > > A call to an external system, e.g. Cruise Control, in the > > implementation of the provided interface can indeed help with the initial > > assignment of partitions. > > > > > > > > > > I am curious why the proposed > > `ReplicaAssignor#assignReplicasToBrokers` receives a list of partition > ids > > as opposed to the number of partitions to create the topic with? > > > > > > > > > > Would you clarify if this API is expected to be used (1) only for > > new topics or (2) also for existing topics? > > > > > > > > > > Best, > > > > > Efe > > > > > ________________________________ > > > > > From: Mickael Maison <mickael.mai...@gmail.com> > > > > > Sent: Thursday, October 1, 2020 9:43 AM > > > > > To: dev <dev@kafka.apache.org> > > > > > Subject: Re: [DISCUSS] KIP-660: Pluggable ReplicaAssignor > > > > > > > > > > Thanks Tom for the feedback! > > > > > > > > > > 1. If the data returned by the ReplicaAssignor implementation does > > not > > > > > match that was requested, we'll also throw a > ReplicaAssignorException > > > > > > > > > > 2. Good point, I'll update the KIP > > > > > > > > > > 3. The KIP mentions an error code associated with > > > > > ReplicaAssignorException: REPLICA_ASSIGNOR_FAILED > > > > > > > > > > 4. (I'm naming your last question 4.) I spent some time looking at > > it. > > > > > Initially I wanted to follow the model from the topic policies. But > > as > > > > > you said, computing assignments for the whole batch may be more > > > > > desirable and also avoids incrementally updating the cluster state. > > > > > The logic in AdminManager is very much centered around doing 1 > topic > > > > > at a time but as far as I can tell we should be able to update it > to > > > > > compute assignments for the whole batch. > > > > > > > > > > I'll play a bit more with 4. and I'll update the KIP in the next > few > > days > > > > > > > > > > On Mon, Sep 21, 2020 at 10:29 AM Tom Bentley <tbent...@redhat.com> > > wrote: > > > > > > > > > > > > Hi Mickael, > > > > > > > > > > > > A few thoughts about the ReplicaAssignor contract: > > > > > > > > > > > > 1. What happens if a ReplicaAssignor impl returns a Map where > some > > > > > > assignments don't meet the given replication factor? > > > > > > 2. Fixing the signature of assignReplicasToBrokers() as you have > > would make > > > > > > it hard to pass extra information in the future (e.g. maybe > > someone comes > > > > > > up with a use case where passing the clientId would be needed) > > because it > > > > > > would require the interface be changed. If you factored all the > > parameters > > > > > > into some new type then the signature could be > > > > > > assignReplicasToBrokers(RequiredReplicaAssignment) and adding any > > new > > > > > > properties to RequiredReplicaAssignment wouldn't break the > > contract. > > > > > > 3. When an assignor throws RepliacAssignorException what error > > code will be > > > > > > returned to the client? > > > > > > > > > > > > Also, this sentence got me thinking: > > > > > > > > > > > > > If multiple topics are present in the request, AdminManager > will > > update > > > > > > the Cluster object so the ReplicaAssignor class has access to the > > up to > > > > > > date cluster metadata. > > > > > > > > > > > > Previously I've looked at how we can improve Kafka's pluggable > > policy > > > > > > support to pass the more of the cluster state to policy > > implementations. A > > > > > > similar problem exists there, but the more cluster state you pass > > the > > > > > > harder it is to incrementally change it as you iterate through > the > > topics > > > > > > to be created/modified. This likely isn't a problem here and now, > > but it > > > > > > could limit any future changes to the pluggable assignors. Did > you > > consider > > > > > > the alternative of the assignor just being passed a Set of > > assignments? > > > > > > That means you can just pass the cluster state as it exists at > the > > time. It > > > > > > also gives the implementation more information to work with to > > find more > > > > > > optimal assignments. For example, it could perform a bin packing > > type > > > > > > assignment which found a better optimum for the whole collection > > of topics > > > > > > than one which was only told about all the topics in the request > > > > > > sequentially. > > > > > > > > > > > > Otherwise this looks like a valuable feature to me. > > > > > > > > > > > > Kind regards, > > > > > > > > > > > > Tom > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Fri, Sep 11, 2020 at 6:19 PM Robert Barrett < > > bob.barr...@confluent.io> > > > > > > wrote: > > > > > > > > > > > > > Thanks Mickael, I think adding the new Exception resolves my > > concerns. > > > > > > > > > > > > > > On Thu, Sep 3, 2020 at 9:47 AM Mickael Maison < > > mickael.mai...@gmail.com> > > > > > > > wrote: > > > > > > > > > > > > > > > Thanks Robert and Ryanne for the feedback. > > > > > > > > > > > > > > > > ReplicaAssignor implementations can throw an exception to > > indicate an > > > > > > > > assignment can't be computed. This is already what the > current > > round > > > > > > > > robin assignor does. Unfortunately at the moment, there are > no > > generic > > > > > > > > error codes if it fails, it's either INVALID_PARTITIONS, > > > > > > > > INVALID_REPLICATION_FACTOR or worse UNKNOWN_SERVER_ERROR. > > > > > > > > > > > > > > > > So I think it would be nice to introduce a new > Exception/Error > > code to > > > > > > > > cover any failures in the assignor and avoid > > UNKNOWN_SERVER_ERROR. > > > > > > > > > > > > > > > > I've updated the KIP accordingly, let me know if you have > more > > questions. > > > > > > > > > > > > > > > > On Fri, Aug 28, 2020 at 4:49 PM Ryanne Dolan < > > ryannedo...@gmail.com> > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > Thanks Mickael, the KIP makes sense to me, esp for cases > > where an > > > > > > > > external > > > > > > > > > system (like cruise control or an operator) knows more > about > > the target > > > > > > > > > cluster state than the broker does. > > > > > > > > > > > > > > > > > > Ryanne > > > > > > > > > > > > > > > > > > On Thu, Aug 20, 2020, 10:46 AM Mickael Maison < > > > > > > > mickael.mai...@gmail.com> > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > Hi, > > > > > > > > > > > > > > > > > > > > I've created KIP-660 to make the replica assignment logic > > pluggable. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcwiki.apache.org%2Fconfluence%2Fdisplay%2FKAFKA%2FKIP-660%253A%2BPluggable%2BReplicaAssignor&data=02%7C01%7Cagencer%40linkedin.com%7Ca156bf97031b4100b62d08d866293434%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637371674445085863&sdata=Cz1u3y1M%2BH5dFIx%2BHkQwugN%2FqTH1ugjXaaBhbToCkDM%3D&reserved=0 > > > > > > > > > > > > > > > > > > > > Please take a look and let me know if you have any > > feedback. > > > > > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > > >
