This will change some logic though, right? IIRC, right now produce/fetch requests to marked-for-deletion topics fail because the topics are simple not around. You get a generic "doesn't exist" error. If we keep these topics and add a flag, we'll need to find all the places with this implicit logic and correct for it.
And since our tests for topic deletion are clearly inadequate... I'm a bit scared :) Gwen On Thu, Apr 7, 2016 at 10:34 AM, Guozhang Wang <wangg...@gmail.com> wrote: > Hmm, I think since in the original protocol, metadata response do not have > information for "marked for deleted topics" and hence we want to remove > that topic from returning in response by cleaning the metadata cache once > it is marked to deletion. > > With the new format, I think it is OK to delay the metadata cleaning. > > Guozhang > > On Thu, Apr 7, 2016 at 8:35 AM, Grant Henke <ghe...@cloudera.com> wrote: > > > I am testing the marked for deletion flag in the metadata and ran into > some > > challenges. > > > > It turns out that as soon as a topic is marked for deletion it may be > > purged from the metadata cache. This means that Metadata responses > > can't/don't return the topic. Though the topic may still exist if its not > > ready to be completely deleted or is in the process of being deleted. > > > > This poses a challenge because a user would have no way to tell if a > topic > > still exists, and is marked for deletion, other than to try and recreate > it > > and see a failure. I could change the logic to no longer purge a message > > from the cache until its completely deleted, but I am not sure if that > > would impact the clients in some way negatively. Does anyone have enough > > background to say? > > > > I will dig into this a bit more today, but wanted to throw this out there > > for some early feedback. > > > > Thank you, > > Grant > > > > > > On Tue, Apr 5, 2016 at 8:02 PM, Jun Rao <j...@confluent.io> wrote: > > > > > 5. You will return no error and 4,5,6 as replicas. The response also > > > includes a list of live brokers. So the client can figure out 5 is not > > live > > > directly w/o relying on the error code. > > > > > > Thanks, > > > > > > Jun > > > > > > On Tue, Apr 5, 2016 at 5:05 PM, Grant Henke <ghe...@cloudera.com> > wrote: > > > > > > > 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 > > > > > > > > > > > > > > > -- > > Grant Henke > > Software Engineer | Cloudera > > gr...@cloudera.com | twitter.com/gchenke | linkedin.com/in/granthenke > > > > > > -- > -- Guozhang >
