Hi Jun, See my responses below:
2. The issues that I was thinking are the following. (a) Say the controller > has topic deletion disabled and a topic deletion request is submitted to > ZK. In this case, the controller will ignore this request. However, the > broker may pick up the topic deletion marker in a transient window. (b) > Suppose that a topic is deleted and then recreated immediately. It is > possible for a broker to see the newly created topic and then the previous > topic deletion marker in a transient window. Thinking about this a bit > more. Both seem to be transient. So, it may not be a big concern. So, I am > ok with this as long as the interim solution is not too complicated. > Another thing to think through. If a topic is marked for deletion, do we > still return the partition level metadata? I am not changing anything about the metadata content, only adding a boolean based on the marked for deletion flag in zookeeper. This is maintaining the same method that the topics script does today. I do think delete improvements should be considered/reviewed. The goal here is to allow the broker to report the value that its sees, which is the value in zookeeper. 5. The issue is the following. If you have a partition with 3 replicas > 4,5,6, leader is on replica 4 and replica 5 is down. Currently, the broker > will send a REPLICA_NOT_AVAILABLE error code and only replicas 4,6 in the > assigned replicas. It's more intuitive to send no error code and 4,5,6 in > the assigned replicas in this case. Should the list with no error code just be 4,6 since 5 is not available? On Mon, Apr 4, 2016 at 1:34 PM, Jun Rao <j...@confluent.io> wrote: > Grant, > > 2. The issues that I was thinking are the following. (a) Say the controller > has topic deletion disabled and a topic deletion request is submitted to > ZK. In this case, the controller will ignore this request. However, the > broker may pick up the topic deletion marker in a transient window. (b) > Suppose that a topic is deleted and then recreated immediately. It is > possible for a broker to see the newly created topic and then the previous > topic deletion marker in a transient window. Thinking about this a bit > more. Both seem to be transient. So, it may not be a big concern. So, I am > ok with this as long as the interim solution is not too complicated. > Another thing to think through. If a topic is marked for deletion, do we > still return the partition level metadata? > > 3. Your explanation on controller id seems reasonable to me. > > 5. The issue is the following. If you have a partition with 3 replicas > 4,5,6, leader is on replica 4 and replica 5 is down. Currently, the broker > will send a REPLICA_NOT_AVAILABLE error code and only replicas 4,6 in the > assigned replicas. It's more intuitive to send no error code and 4,5,6 in > the assigned replicas in this case. > > Thanks, > > Jun > > On Mon, Apr 4, 2016 at 8:33 AM, Grant Henke <ghe...@cloudera.com> wrote: > > > Hi Jun, > > > > Please See my responses below: > > > > Hmm, I am not sure about the listener approach. It ignores configs like > > > enable.topic.deletion and also opens the door for potential ordering > > issues > > > since now there are two separate paths for propagating the metadata to > > the > > > brokers. > > > > > > This mechanism is very similar to how deletes are tracked on the > controller > > itself. It is also the same way ACLs are tracked on brokers in the > default > > implementation. I am not sure I understand what ordering issue there > could > > be. This is used to report what topics are marked for deletion, which > today > > has no dependency on enable.topic.deletion. I agree that the delete > > mechanism in Kafka has a lot of room for improvement, but the goal in > this > > change is just to enable reporting it to the user, not to fix/improve > > existing issues. If you have an alternate approach that does not require > > major changes to the controller code, I would be open to investigate it. > > > > Could we just leave out markedForDeletion for now? In the common > > > case, if a topic is deleted, it will only be in markedForDeletion state > > for > > > a few milli seconds anyway. > > > > > > I don't think we should leave it out. The point of these changes is to > > prevent a user from needing to talk directly to zookeeper. We need a way > > for a user to see if a topic has been marked for deletion. Given the > issues > > with the current delete implementation, its fairly common for a topic to > > remain marked as deleted for quite some time. > > > > Yes, for those usage, it just seems it's a bit weird for the client to > > > issue a MetadataRequest to get the controller info since it doesn't > need > > > any topic metadata. > > > > > > Why does this seam weird? The MetadataRequest is the request used to > > discover the cluster and metadata about that cluster regardless of the > > topics you are interested in, if any. In fact, a big motivation for the > > change to allow requesting "no topics" is because the existing producer > and > > consumer often want to learn about the cluster without asking for topic > > metadata and today that means that they request all topics. > > > > 5. The issue is that for a client, when handling a metadata response, the > > > natural logic is if there is any error in the response, go to the error > > > handling path (e.g., back off and refresh metadata). Otherwise, get the > > > leader info and initiate a request to the leader if leader is > available. > > If > > > you look at the current logic in MetadataCache.getPartitionMetadata(), > if > > > an assigned replica is not alive, we will send a REPLICA_NOT_AVAILABLE > > > error code in the response. If the client follows the above logic, it > > will > > > keep doing the error handling even though there is nothing wrong with > the > > > leader. A better behavior is to simply return the list of replica ids > > with > > > no error code in this case. > > > > > > To be sure I understand this correctly. Instead of returning the complete > > list of replicas, including the ones that errored as unavailable. You are > > suggesting to drop the unavailable ones and return just the replicas with > > no-errors and return no error code on the partition. Is that correct? > > > > Under what scenario does the MetadataCache have a replica that is not > > available? > > > > Thanks, > > Grant > > > > > > > > > > > > On Mon, Apr 4, 2016 at 12:25 AM, Jun Rao <j...@confluent.io> wrote: > > > > > Grant, > > > > > > 2. Hmm, I am not sure about the listener approach. It ignores configs > > like > > > enable.topic.deletion and also opens the door for potential ordering > > issues > > > since now there are two separate paths for propagating the metadata to > > the > > > brokers. Could we just leave out markedForDeletion for now? In the > common > > > case, if a topic is deleted, it will only be in markedForDeletion state > > for > > > a few milli seconds anyway. > > > > > > 3. Yes, for those usage, it just seems it's a bit weird for the client > to > > > issue a MetadataRequest to get the controller info since it doesn't > need > > > any topic metadata. > > > > > > 5. The issue is that for a client, when handling a metadata response, > the > > > natural logic is if there is any error in the response, go to the error > > > handling path (e.g., back off and refresh metadata). Otherwise, get the > > > leader info and initiate a request to the leader if leader is > available. > > If > > > you look at the current logic in MetadataCache.getPartitionMetadata(), > if > > > an assigned replica is not alive, we will send a REPLICA_NOT_AVAILABLE > > > error code in the response. If the client follows the above logic, it > > will > > > keep doing the error handling even though there is nothing wrong with > the > > > leader. A better behavior is to simply return the list of replica ids > > with > > > no error code in this case. > > > > > > Thanks, > > > > > > Jun > > > > > > > > > > > > On Sun, Apr 3, 2016 at 6:29 PM, Grant Henke <ghe...@cloudera.com> > wrote: > > > > > > > Responding to a few of the other comments: > > > > > > > > it seems that you propagated > > > > > the topic deletion marker by having the replicaManager read from ZK > > > > > directly. It seems that it would be simpler/consistent if the > > > controller > > > > > propagates that information directly through UpdateMetaRequest. > > > > > > > > > > > > I was told that I should not try and modify controller logic with > KIP-4 > > > > changes. It was indicated that a larger controller rewrite and > testing > > > was > > > > planned and those changes should be considered then. Since marking a > > > topic > > > > for deletion doesn't flow through the controller and therefore the > > > > UpdateMetadataRequest, > > > > it would take quite a bit of change. We would need to trigger a new > > > > UpdateMetadataRequest > > > > every time a new topic is marked for deletion. > > > > > > > > Instead I added a listener to maintain a cache of the topic deletion > > > znodes > > > > in the ReplicaManager where the existing UpdateMetadataRequests are > > > > handled. This would make it easy to swap out later once the data is a > > > part > > > > of that request and have minimal impact in the mean time. > > > > > > > > > > > > > Could you add a description on how controller id will be used in > the > > > > > client? > > > > > > > > > > > > I will add it to the wiki. Today metrics are the only way to access > > this > > > > piece of data. It is useful information about the cluster for many > > > reasons. > > > > Having programatic access to identify the controller is helpful for > > > > automation. For example, It can be used during rolling restart logic > to > > > > shutdown the controller last to prevent multiple fail overs. Beyond > > > > automation, it can be leveraged in KIP-4 to route admin requests to > the > > > > controller broker. > > > > > > > > We had a weird semantic in version 0 of MetadataRequest. If a replica > > is > > > > > not live, but the leader is live, we return an > > > > > error ReplicaNotAvailableException in the partition metadata. This > > > makes > > > > it > > > > > a bit confusing for the client to parse since it has to first check > > > > whether > > > > > leader is available or not before error code checking. We were > > thinking > > > > of > > > > > changing that behavior the next time we bump up the version of > > > > > MetadataRequest. > > > > > Now that time has come, could you include that in the proposal? > > > > > > > > > > > > I am not sure I completely follow the issue and requested change. > Could > > > you > > > > point me to the discussion? > > > > > > > > Thank you, > > > > Grant > > > > > > > > On Sun, Apr 3, 2016 at 8:09 PM, Grant Henke <ghe...@cloudera.com> > > wrote: > > > > > > > > > Hi Jun and Ismael, > > > > > > > > > > Initially I had 2 booleans used to indicate if a topic was internal > > and > > > > if > > > > > a topic was marked for deletion. To save space on large > deployments, > > > > Ismael > > > > > suggested I break out the internal topics and deleted topics into > > their > > > > own > > > > > lists. The idea was that instead of 2 bytes added per topic, in the > > > > general > > > > > case the lists would be empty. Even in those lists I still only > > return > > > > > topics that were requested. In fact on the client side they are > just > > > > > utilized to translate back to booleans. I do prefer the booleans > from > > > an > > > > > expressiveness standpoint but was not strongly opinionated on the > > > > > structure. > > > > > > > > > > Thank you, > > > > > Grant > > > > > > > > > > On Sun, Apr 3, 2016 at 8:00 PM, Ismael Juma <ism...@juma.me.uk> > > wrote: > > > > > > > > > >> Hi Jun, > > > > >> > > > > >> A couple of comments inline. > > > > >> > > > > >> On Sun, Apr 3, 2016 at 5:55 PM, Jun Rao <j...@confluent.io> wrote: > > > > >> > > > > >> > 1. It seems a bit weird to return just a list of internal topics > > w/o > > > > the > > > > >> > corresponding metadata. It also seems a bit weird to return the > > > > internal > > > > >> > topics even if the client doesn't ask for it. > > > > >> > > > > >> > > > > >> Good point. > > > > >> > > > > >> > > > > >> > Would it be better to just > > > > >> > add a flag in topic_metadata to indicate whether it's an > internal > > > > topic > > > > >> or > > > > >> > not, and only include the internal topics when thy are asked (or > > all > > > > >> topics > > > > >> > are requested) for? > > > > >> > > > > > >> > > > > >> The disadvantage of this is that we are adding one byte per topic > > even > > > > >> though we have a very small number of internal topics (currently a > > > > single > > > > >> internal topic). It seems a bit wasteful and particularly so when > > > using > > > > >> regex subscriptions (since we have to retrieve all topics in that > > > case). > > > > >> > > > > >> 2. A similar comment on topics_marked_for_deletion. Would it be > > better > > > > to > > > > >> > only return them when asked for and just return a new > TopicDeleted > > > > error > > > > >> > code in topic_metadata? > > > > >> > > > > >> > > > > >> I agree that this seems better. > > > > >> > > > > >> Ismael > > > > >> > > > > > > > > > > > > > > > > > > > > -- > > > > > Grant Henke > > > > > Software Engineer | Cloudera > > > > > gr...@cloudera.com | twitter.com/gchenke | > > linkedin.com/in/granthenke > > > > > > > > > > > > > > > > > > > > > -- > > > > Grant Henke > > > > Software Engineer | Cloudera > > > > gr...@cloudera.com | twitter.com/gchenke | > linkedin.com/in/granthenke > > > > > > > > > > > > > > > -- > > Grant Henke > > Software Engineer | Cloudera > > gr...@cloudera.com | twitter.com/gchenke | linkedin.com/in/granthenke > > > -- Grant Henke Software Engineer | Cloudera gr...@cloudera.com | twitter.com/gchenke | linkedin.com/in/granthenke
