Hi Guozhang, Thanks for the explanation! This is a very good point. I have updated the KIP incorporating the proposed idea. We now maintain/serve MAX as well as MIN version levels of finalized features. So, the client will get to know both these values in the ApiVersionsResponse. This serves as a solution to the problem that you explained earlier.
It is important to note the following explanation. We only allow the finalized feature MAX version level to be increased/decreased dynamically via the controller API. Contrastingly, the MIN version level can not be mutated via the Controller API. This is because, MIN version level is usually increased only to indicate the intent to stop support for a certain feature version. We would usually deprecate features during broker releases, after prior announcements. Therefore, the facility to mutate MIN version level need not be made available through the controller API to the cluster operator. Instead it is sufficient if such changes can be done directly by the controller i.e. during a certain Kafka release we would change the controller code to mutate the '/features' ZK node increasing the MIN version level of one or more finalized features (this will be a planned change, as determined by Kafka developers). Then, as this Broker release gets rolled out to a cluster, the feature versions will become permanently deprecated. Here are links to the specific sub-sections with the changes including MIN/MAX version levels: Goals: https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features#KIP-584:Versioningschemeforfeatures-Goals Non-goals (see point #2): https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features#KIP-584:Versioningschemeforfeatures-Non-goals Feature version deprecation: https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features#KIP-584:Versioningschemeforfeatures-Featureversiondeprecation Admin API changes: https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features#KIP-584:Versioningschemeforfeatures-AdminAPIchanges Cheers, Kowshik On Mon, Apr 6, 2020 at 3:28 PM Guozhang Wang <wangg...@gmail.com> wrote: > Hello Kowshik, > > For 2) above, my motivations is more from the flexibility on client side > instead of version deprecation: let's say a client talks to the cluster > learned that the cluster-wide version for feature is X, while the client > itself only knows how to execute the feature up to version Y ( < X), then > at the moment the client has to give up leveraging that since it is not > sure if all brokers actually supports version Y or not. This is because the > version X is only guaranteed to be within the common overlapping range of > all [low, high] across brokers where "low" is not always 0, so the client > cannot safely assume that any versions smaller than X are also supported on > the cluster. > > If we assume that when cluster-wide version is X, then all versions smaller > than X are guaranteed to be supported, then it means all broker's supported > version range is like [0, high], which I think is not realistic? > > > Guozhang > > > > On Mon, Apr 6, 2020 at 12:06 PM Jun Rao <j...@confluent.io> wrote: > > > Hi, Kowshik, > > > > Thanks for the reply. A few more replies below. > > > > 100.6 You can look for the sentence "This operation requires ALTER on > > CLUSTER." in KIP-455 <https://issues.apache.org/jira/browse/KIP-455>. > Also, you can check its usage in > > KafkaApis.authorize(). > > > > 110. From the external client/tooling perspective, it's more natural to > use > > the release version for features. If we can use the same release version > > for internal representation, it seems simpler (easier to understand, no > > mapping overhead, etc). Is there a benefit with separate external and > > internal versioning schemes? > > > > 111. To put this in context, when we had IBP, the default value is the > > current released version. So, if you are a brand new user, you don't need > > to configure IBP and all new features will be immediately available in > the > > new cluster. If you are upgrading from an old version, you do need to > > understand and configure IBP. I see a similar pattern here for > > features. From the ease of use perspective, ideally, we shouldn't > require a > > new user to have an extra step such as running a bootstrap script unless > > it's truly necessary. If someone has a special need (all the cases you > > mentioned seem special cases?), they can configure a mode such that > > features are enabled/disabled manually. > > > > Jun > > > > On Fri, Apr 3, 2020 at 5:54 PM Kowshik Prakasam <kpraka...@confluent.io> > > wrote: > > > > > Hi Jun, > > > > > > Thanks for the feedback and suggestions. Please find my response below. > > > > > > > 100.6 For every new request, the admin needs to control who is > allowed > > to > > > > issue that request if security is enabled. So, we need to assign the > > new > > > > request a ResourceType and possible AclOperations. See > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-455%3A+Create+an+Administrative+API+for+Replica+Reassignment > > > > as an example. > > > > > > (Kowshik): I don't see any reference to the words ResourceType or > > > AclOperations > > > in the KIP. Please let me know how I can use the KIP that you linked to > > > know how to > > > setup the appropriate ResourceType and/or ClusterOperation? > > > > > > > 105. If we change delete to disable, it's better to do this > > consistently > > > in > > > > request protocol and admin api as well. > > > > > > (Kowshik): The API shouldn't be called 'disable' when it is deleting a > > > feature. > > > I've just changed the KIP to use 'delete'. I don't have a strong > > > preference. > > > > > > > 110. The minVersion/maxVersion for features use int64. Currently, our > > > > release version schema is major.minor.bugfix (e.g. 2.5.0). It's > > possible > > > > for new features to be included in minor releases too. Should we make > > the > > > > feature versioning match the release versioning? > > > > > > (Kowshik): The release version can be mapped to a set of feature > > versions, > > > and this can be done, for example in the tool (or even external to the > > > tool). > > > Can you please clarify what I'm missing? > > > > > > > 111. "During regular operations, the data in the ZK node can be > mutated > > > > only via a specific admin API served only by the controller." I am > > > > wondering why can't the controller auto finalize a feature version > > after > > > > all brokers are upgraded? For new users who download the latest > version > > > to > > > > build a new cluster, it's inconvenient for them to have to manually > > > enable > > > > each feature. > > > > > > (Kowshik): I agree that there is a trade-off here, but it will help > > > to decide whether the automation can be thought through in the future > > > in a follow up KIP, or right now in this KIP. We may invest > > > in automation, but we have to decide whether we should do it > > > now or later. > > > > > > For the inconvenience that you mentioned, do you think the problem that > > you > > > mentioned can be overcome by asking for the cluster operator to run a > > > bootstrap script when he/she knows that a specific AK release has been > > > almost completely deployed in a cluster for the first time? Idea is > that > > > the > > > bootstrap script will know how to map a specific AK release to > finalized > > > feature versions, and run the `kafka-features.sh` tool appropriately > > > against > > > the cluster. > > > > > > Now, coming back to your automation proposal/question. > > > I do see the value of automated feature version finalization, but I > also > > > see > > > that this will open up several questions and some risks, as explained > > > below. > > > The answers to these depend on the definition of the automation we > choose > > > to build, and how well does it fit into a kafka deployment. > > > Basically, it can be unsafe for the controller to finalize feature > > version > > > upgrades automatically, without learning about the intent of the > cluster > > > operator. > > > 1. We would sometimes want to lock feature versions only when we have > > > externally verified > > > the stability of the broker binary. > > > 2. Sometimes only the cluster operator knows that a cluster upgrade is > > > complete, > > > and new brokers are highly unlikely to join the cluster. > > > 3. Only the cluster operator knows that the intent is to deploy the > same > > > version > > > of the new broker release across the entire cluster (i.e. the latest > > > downloaded version). > > > 4. For downgrades, it appears the controller still needs some external > > > input > > > (such as the proposed tool) to finalize a feature version downgrade. > > > > > > If we have automation, that automation can end up failing in some of > the > > > cases > > > above. Then, we need a way to declare that the cluster is "not ready" > if > > > the > > > controller cannot automatically finalize some basic required feature > > > version > > > upgrades across the cluster. We need to make the cluster operator aware > > in > > > such a scenario (raise an alert or alike). > > > > > > > 112. DeleteFeaturesResponse: It seems the apiKey should be 49 instead > > of > > > 48. > > > > > > (Kowshik): Done. > > > > > > > > > Cheers, > > > Kowshik > > > > > > On Fri, Apr 3, 2020 at 11:24 AM Jun Rao <j...@confluent.io> wrote: > > > > > > > Hi, Kowshik, > > > > > > > > Thanks for the reply. A few more comments below. > > > > > > > > 100.6 For every new request, the admin needs to control who is > allowed > > to > > > > issue that request if security is enabled. So, we need to assign the > > new > > > > request a ResourceType and possible AclOperations. See > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-455%3A+Create+an+Administrative+API+for+Replica+Reassignment > > > > as > > > > an example. > > > > > > > > 105. If we change delete to disable, it's better to do this > > consistently > > > in > > > > request protocol and admin api as well. > > > > > > > > 110. The minVersion/maxVersion for features use int64. Currently, our > > > > release version schema is major.minor.bugfix (e.g. 2.5.0). It's > > possible > > > > for new features to be included in minor releases too. Should we make > > the > > > > feature versioning match the release versioning? > > > > > > > > 111. "During regular operations, the data in the ZK node can be > mutated > > > > only via a specific admin API served only by the controller." I am > > > > wondering why can't the controller auto finalize a feature version > > after > > > > all brokers are upgraded? For new users who download the latest > version > > > to > > > > build a new cluster, it's inconvenient for them to have to manually > > > enable > > > > each feature. > > > > > > > > 112. DeleteFeaturesResponse: It seems the apiKey should be 49 instead > > of > > > > 48. > > > > > > > > Jun > > > > > > > > > > > > On Fri, Apr 3, 2020 at 1:27 AM Kowshik Prakasam < > > kpraka...@confluent.io> > > > > wrote: > > > > > > > > > Hey Jun, > > > > > > > > > > Thanks a lot for the great feedback! Please note that the design > > > > > has changed a little bit on the KIP, and we now propagate the > > finalized > > > > > features metadata only via ZK watches (instead of > > UpdateMetadataRequest > > > > > from the controller). > > > > > > > > > > Please find below my response to your questions/feedback, with the > > > prefix > > > > > "(Kowshik):". > > > > > > > > > > > 100. UpdateFeaturesRequest/UpdateFeaturesResponse > > > > > > 100.1 Since this request waits for responses from brokers, should > > we > > > > add > > > > > a > > > > > > timeout in the request (like createTopicRequest)? > > > > > > > > > > (Kowshik): Great point! Done. I have added a timeout field. Note: > we > > no > > > > > longer > > > > > wait for responses from brokers, since the design has been changed > so > > > > that > > > > > the > > > > > features information is propagated via ZK. Nevertheless, it is > right > > to > > > > > have a timeout > > > > > for the request. > > > > > > > > > > > 100.2 The response schema is a bit weird. Typically, the response > > > just > > > > > > shows an error code and an error message, instead of echoing the > > > > request. > > > > > > > > > > (Kowshik): Great point! Yeah, I have modified it to just return an > > > error > > > > > code and a message. > > > > > Previously it was not echoing the "request", rather it was > returning > > > the > > > > > latest set of > > > > > cluster-wide finalized features (after applying the updates). But > you > > > are > > > > > right, > > > > > the additional info is not required, so I have removed it from the > > > > response > > > > > schema. > > > > > > > > > > > 100.3 Should we add a separate request to list/describe the > > existing > > > > > > features? > > > > > > > > > > (Kowshik): This is already present in the KIP via the > > > 'DescribeFeatures' > > > > > Admin API, > > > > > which, underneath covers uses the ApiVersionsRequest to > list/describe > > > the > > > > > existing features. Please read the 'Tooling support' section. > > > > > > > > > > > 100.4 We are mixing ADD_OR_UPDATE and DELETE in a single request. > > For > > > > > > DELETE, the version field doesn't make sense. So, I guess the > > broker > > > > just > > > > > > ignores this? An alternative way is to have a separate > > > > > DeleteFeaturesRequest > > > > > > > > > > (Kowshik): Great point! I have modified the KIP now to have 2 > > separate > > > > > controller APIs > > > > > serving these different purposes: > > > > > 1. updateFeatures > > > > > 2. deleteFeatures > > > > > > > > > > > 100.5 In UpdateFeaturesResponse, we have "The monotonically > > > increasing > > > > > > version of the metadata for finalized features." I am wondering > why > > > the > > > > > > ordering is important? > > > > > > > > > > (Kowshik): In the latest KIP write-up, it is called epoch (instead > of > > > > > version), and > > > > > it is just the ZK node version. Basically, this is the epoch for > the > > > > > cluster-wide > > > > > finalized feature version metadata. This metadata is served to > > clients > > > > via > > > > > the > > > > > ApiVersionsResponse (for reads). We propagate updates from the > > > > '/features' > > > > > ZK node > > > > > to all brokers, via ZK watches setup by each broker on the > > '/features' > > > > > node. > > > > > > > > > > Now here is why the ordering is important: > > > > > ZK watches don't propagate at the same time. As a result, the > > > > > ApiVersionsResponse > > > > > is eventually consistent across brokers. This can introduce cases > > > > > where clients see an older lower epoch of the features metadata, > > after > > > a > > > > > more recent > > > > > higher epoch was returned at a previous point in time. We expect > > > clients > > > > > to always employ the rule that the latest received higher epoch of > > > > metadata > > > > > always trumps an older smaller epoch. Those clients that are > external > > > to > > > > > Kafka should strongly consider discovering the latest metadata once > > > > during > > > > > startup from the brokers, and if required refresh the metadata > > > > periodically > > > > > (to get the latest metadata). > > > > > > > > > > > 100.6 Could you specify the required ACL for this new request? > > > > > > > > > > (Kowshik): What is ACL, and how could I find out which one to > > specify? > > > > > Please could you provide me some pointers? I'll be glad to update > the > > > > > KIP once I know the next steps. > > > > > > > > > > > 101. For the broker registration ZK node, should we bump up the > > > version > > > > > in > > > > > the json? > > > > > > > > > > (Kowshik): Great point! Done. I've increased the version in the > > broker > > > > json > > > > > by 1. > > > > > > > > > > > 102. For the /features ZK node, not sure if we need the epoch > > field. > > > > Each > > > > > > ZK node has an internal version field that is incremented on > every > > > > > update. > > > > > > > > > > (Kowshik): Great point! Done. I'm using the ZK node version now, > > > instead > > > > of > > > > > explicitly > > > > > incremented epoch. > > > > > > > > > > > 103. "Enabling the actual semantics of a feature version > > cluster-wide > > > > is > > > > > > left to the discretion of the logic implementing the feature (ex: > > can > > > > be > > > > > > done via dynamic broker config)." Does that mean the broker > > > > registration > > > > > ZK > > > > > > node will be updated dynamically when this happens? > > > > > > > > > > (Kowshik): Not really. The text was just conveying that a broker > > could > > > > > "know" of > > > > > a new feature version, but it does not mean the broker should have > > also > > > > > activated the effects of the feature version. Knowing vs activation > > > are 2 > > > > > separate things, > > > > > and the latter can be achieved by dynamic config. I have reworded > the > > > > text > > > > > to > > > > > make this clear to the reader. > > > > > > > > > > > > > > > > 104. UpdateMetadataRequest > > > > > > 104.1 It would be useful to describe when the feature metadata is > > > > > included > > > > > > in the request. My understanding is that it's only included if > (1) > > > > there > > > > > is > > > > > > a change to the finalized feature; (2) broker restart; (3) > > controller > > > > > > failover. > > > > > > 104.2 The new fields have the following versions. Why are the > > > versions > > > > 3+ > > > > > > when the top version is bumped to 6? > > > > > > "fields": [ > > > > > > {"name": "Name", "type": "string", "versions": "3+", > > > > > > "about": "The name of the feature."}, > > > > > > {"name": "Version", "type": "int64", "versions": "3+", > > > > > > "about": "The finalized version for the feature."} > > > > > > ] > > > > > > > > > > (Kowshik): With the new improved design, we have completely > > eliminated > > > > the > > > > > need to > > > > > use UpdateMetadataRequest. This is because we now rely on ZK to > > deliver > > > > the > > > > > notifications for changes to the '/features' ZK node. > > > > > > > > > > > 105. kafka-features.sh: Instead of using update/delete, perhaps > > it's > > > > > better > > > > > > to use enable/disable? > > > > > > > > > > (Kowshik): For delete, yes, I have changed it so that we instead > call > > > it > > > > > 'disable'. > > > > > However for 'update', it can now also refer to either an upgrade > or a > > > > > forced downgrade. > > > > > Therefore, I have left it the way it is, just calling it as just > > > > 'update'. > > > > > > > > > > > > > > > Cheers, > > > > > Kowshik > > > > > > > > > > On Tue, Mar 31, 2020 at 6:51 PM Jun Rao <j...@confluent.io> wrote: > > > > > > > > > > > Hi, Kowshik, > > > > > > > > > > > > Thanks for the KIP. Looks good overall. A few comments below. > > > > > > > > > > > > 100. UpdateFeaturesRequest/UpdateFeaturesResponse > > > > > > 100.1 Since this request waits for responses from brokers, should > > we > > > > add > > > > > a > > > > > > timeout in the request (like createTopicRequest)? > > > > > > 100.2 The response schema is a bit weird. Typically, the response > > > just > > > > > > shows an error code and an error message, instead of echoing the > > > > request. > > > > > > 100.3 Should we add a separate request to list/describe the > > existing > > > > > > features? > > > > > > 100.4 We are mixing ADD_OR_UPDATE and DELETE in a single request. > > For > > > > > > DELETE, the version field doesn't make sense. So, I guess the > > broker > > > > just > > > > > > ignores this? An alternative way is to have a separate > > > > > > DeleteFeaturesRequest > > > > > > 100.5 In UpdateFeaturesResponse, we have "The monotonically > > > increasing > > > > > > version of the metadata for finalized features." I am wondering > why > > > the > > > > > > ordering is important? > > > > > > 100.6 Could you specify the required ACL for this new request? > > > > > > > > > > > > 101. For the broker registration ZK node, should we bump up the > > > version > > > > > in > > > > > > the json? > > > > > > > > > > > > 102. For the /features ZK node, not sure if we need the epoch > > field. > > > > Each > > > > > > ZK node has an internal version field that is incremented on > every > > > > > update. > > > > > > > > > > > > 103. "Enabling the actual semantics of a feature version > > cluster-wide > > > > is > > > > > > left to the discretion of the logic implementing the feature (ex: > > can > > > > be > > > > > > done via dynamic broker config)." Does that mean the broker > > > > registration > > > > > ZK > > > > > > node will be updated dynamically when this happens? > > > > > > > > > > > > 104. UpdateMetadataRequest > > > > > > 104.1 It would be useful to describe when the feature metadata is > > > > > included > > > > > > in the request. My understanding is that it's only included if > (1) > > > > there > > > > > is > > > > > > a change to the finalized feature; (2) broker restart; (3) > > controller > > > > > > failover. > > > > > > 104.2 The new fields have the following versions. Why are the > > > versions > > > > 3+ > > > > > > when the top version is bumped to 6? > > > > > > "fields": [ > > > > > > {"name": "Name", "type": "string", "versions": "3+", > > > > > > "about": "The name of the feature."}, > > > > > > {"name": "Version", "type": "int64", "versions": "3+", > > > > > > "about": "The finalized version for the feature."} > > > > > > ] > > > > > > > > > > > > 105. kafka-features.sh: Instead of using update/delete, perhaps > > it's > > > > > better > > > > > > to use enable/disable? > > > > > > > > > > > > Jun > > > > > > > > > > > > On Tue, Mar 31, 2020 at 5:29 PM Kowshik Prakasam < > > > > kpraka...@confluent.io > > > > > > > > > > > > wrote: > > > > > > > > > > > > > Hey Boyang, > > > > > > > > > > > > > > Thanks for the great feedback! I have updated the KIP based on > > your > > > > > > > feedback. > > > > > > > Please find my response below for your comments, look for > > sentences > > > > > > > starting > > > > > > > with "(Kowshik)" below. > > > > > > > > > > > > > > > > > > > > > > 1. "When is it safe for the brokers to begin handling EOS > > > traffic" > > > > > > could > > > > > > > be > > > > > > > > converted as "When is it safe for the brokers to start > serving > > > new > > > > > > > > Exactly-Once(EOS) semantics" since EOS is not explained > earlier > > > in > > > > > the > > > > > > > > context. > > > > > > > > > > > > > > (Kowshik): Great point! Done. > > > > > > > > > > > > > > > 2. In the *Explanation *section, the metadata version number > > part > > > > > > seems a > > > > > > > > bit blurred. Could you point a reference to later section > that > > we > > > > > going > > > > > > > to > > > > > > > > store it in Zookeeper and update it every time when there is > a > > > > > feature > > > > > > > > change? > > > > > > > > > > > > > > (Kowshik): Great point! Done. I've added a reference in the > KIP. > > > > > > > > > > > > > > > > > > > > > > 3. For the feature downgrade, although it's a Non-goal of the > > > KIP, > > > > > for > > > > > > > > features such as group coordinator semantics, there is no > legal > > > > > > scenario > > > > > > > to > > > > > > > > perform a downgrade at all. So having downgrade door open is > > > pretty > > > > > > > > error-prone as human faults happen all the time. I'm assuming > > as > > > > new > > > > > > > > features are implemented, it's not very hard to add a flag > > during > > > > > > feature > > > > > > > > creation to indicate whether this feature is "downgradable". > > > Could > > > > > you > > > > > > > > explain a bit more on the extra engineering effort for > shipping > > > > this > > > > > > KIP > > > > > > > > with downgrade protection in place? > > > > > > > > > > > > > > (Kowshik): Great point! I'd agree and disagree here. While I > > agree > > > > that > > > > > > > accidental > > > > > > > downgrades can cause problems, I also think sometimes > downgrades > > > > should > > > > > > > be allowed for emergency reasons (not all downgrades cause > > issues). > > > > > > > It is just subjective to the feature being downgraded. > > > > > > > > > > > > > > To be more strict about feature version downgrades, I have > > modified > > > > the > > > > > > KIP > > > > > > > proposing that we mandate a `--force-downgrade` flag be used in > > the > > > > > > > UPDATE_FEATURES api > > > > > > > and the tooling, whenever the human is downgrading a finalized > > > > feature > > > > > > > version. > > > > > > > Hopefully this should cover the requirement, until we find the > > need > > > > for > > > > > > > advanced downgrade support. > > > > > > > > > > > > > > > 4. "Each broker’s supported dictionary of feature versions > will > > > be > > > > > > > defined > > > > > > > > in the broker code." So this means in order to restrict a > > certain > > > > > > > feature, > > > > > > > > we need to start the broker first and then send a feature > > gating > > > > > > request > > > > > > > > immediately, which introduces a time gap and the > > > intended-to-close > > > > > > > feature > > > > > > > > could actually serve request during this phase. Do you think > we > > > > > should > > > > > > > also > > > > > > > > support configurations as well so that admin user could > freely > > > roll > > > > > up > > > > > > a > > > > > > > > cluster with all nodes complying the same feature gating, > > without > > > > > > > worrying > > > > > > > > about the turnaround time to propagate the message only after > > the > > > > > > cluster > > > > > > > > starts up? > > > > > > > > > > > > > > (Kowshik): This is a great point/question. One of the > > expectations > > > > out > > > > > of > > > > > > > this KIP, which is > > > > > > > already followed in the broker, is the following. > > > > > > > - Imagine at time T1 the broker starts up and registers it’s > > > > presence > > > > > in > > > > > > > ZK, > > > > > > > along with advertising it’s supported features. > > > > > > > - Imagine at a future time T2 the broker receives the > > > > > > > UpdateMetadataRequest > > > > > > > from the controller, which contains the latest finalized > > > features > > > > as > > > > > > > seen by > > > > > > > the controller. The broker validates this data against it’s > > > > > supported > > > > > > > features to > > > > > > > make sure there is no mismatch (it will shutdown if there is > > an > > > > > > > incompatibility). > > > > > > > > > > > > > > It is expected that during the time between the 2 events T1 and > > T2, > > > > the > > > > > > > broker is > > > > > > > almost a silent entity in the cluster. It does not add any > value > > to > > > > the > > > > > > > cluster, or carry > > > > > > > out any important broker activities. By “important”, I mean it > is > > > not > > > > > > doing > > > > > > > mutations > > > > > > > on it’s persistence, not mutating critical in-memory state, > won’t > > > be > > > > > > > serving > > > > > > > produce/fetch requests. Note it doesn’t even know it’s assigned > > > > > > partitions > > > > > > > until > > > > > > > it receives UpdateMetadataRequest from controller. Anything the > > > > broker > > > > > is > > > > > > > doing up > > > > > > > until this point is not damaging/useful. > > > > > > > > > > > > > > I’ve clarified the above in the KIP, see this new section: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features#KIP-584:Versioningschemeforfeatures-Incompatiblebrokerlifetime > > > > > > > . > > > > > > > > > > > > > > > 5. "adding a new Feature, updating or deleting an existing > > > > Feature", > > > > > > may > > > > > > > be > > > > > > > > I misunderstood something, I thought the features are defined > > in > > > > > broker > > > > > > > > code, so admin could not really create a new feature? > > > > > > > > > > > > > > (Kowshik): Great point! You understood this right. Here adding > a > > > > > feature > > > > > > > means we are > > > > > > > adding a cluster-wide finalized *max* version for a feature > that > > > was > > > > > > > previously never finalized. > > > > > > > I have clarified this in the KIP now. > > > > > > > > > > > > > > > 6. I think we need a separate error code like > > > > > > FEATURE_UPDATE_IN_PROGRESS > > > > > > > to > > > > > > > > reject a concurrent feature update request. > > > > > > > > > > > > > > (Kowshik): Great point! I have modified the KIP adding the > above > > > (see > > > > > > > 'Tooling support -> Admin API changes'). > > > > > > > > > > > > > > > 7. I think we haven't discussed the alternative solution to > > pass > > > > the > > > > > > > > feature information through Zookeeper. Is that mentioned in > the > > > KIP > > > > > to > > > > > > > > justify why using UpdateMetadata is more favorable? > > > > > > > > > > > > > > (Kowshik): Nice question! The broker reads finalized feature > info > > > > > stored > > > > > > in > > > > > > > ZK, > > > > > > > only during startup when it does a validation. When serving > > > > > > > `ApiVersionsRequest`, the > > > > > > > broker does not read this info from ZK directly. I'd imagine > the > > > risk > > > > > is > > > > > > > that it can increase > > > > > > > the ZK read QPS which can be a bottleneck for the system. > Today, > > in > > > > > Kafka > > > > > > > we use the > > > > > > > controller to fan out ZK updates to brokers and we want to > stick > > to > > > > > that > > > > > > > pattern to avoid > > > > > > > the ZK read bottleneck when serving `ApiVersionsRequest`. > > > > > > > > > > > > > > > 8. I was under the impression that user could configure a > range > > > of > > > > > > > > supported versions, what's the trade-off for allowing single > > > > > finalized > > > > > > > > version only? > > > > > > > > > > > > > > (Kowshik): Great question! The finalized version of a feature > > > > basically > > > > > > > refers to > > > > > > > the cluster-wide finalized feature "maximum" version. For > > example, > > > if > > > > > the > > > > > > > 'group_coordinator' feature > > > > > > > has the finalized version set to 10, then, it means that > > > cluster-wide > > > > > all > > > > > > > versions upto v10 are > > > > > > > supported for this feature. However, note that if some version > > (ex: > > > > v0) > > > > > > > gets deprecated > > > > > > > for this feature, then we don’t convey that using this scheme > > (also > > > > > > > supporting deprecation is a non-goal). > > > > > > > > > > > > > > (Kowshik): I’ve now modified the KIP at all points, refering to > > > > > finalized > > > > > > > feature "maximum" versions. > > > > > > > > > > > > > > > 9. One minor syntax fix: Note that here the "client" here may > > be > > > a > > > > > > > producer > > > > > > > > > > > > > > (Kowshik): Great point! Done. > > > > > > > > > > > > > > > > > > > > > Cheers, > > > > > > > Kowshik > > > > > > > > > > > > > > > > > > > > > On Tue, Mar 31, 2020 at 1:17 PM Boyang Chen < > > > > > reluctanthero...@gmail.com> > > > > > > > wrote: > > > > > > > > > > > > > > > Hey Kowshik, > > > > > > > > > > > > > > > > thanks for the revised KIP. Got a couple of questions: > > > > > > > > > > > > > > > > 1. "When is it safe for the brokers to begin handling EOS > > > traffic" > > > > > > could > > > > > > > be > > > > > > > > converted as "When is it safe for the brokers to start > serving > > > new > > > > > > > > Exactly-Once(EOS) semantics" since EOS is not explained > earlier > > > in > > > > > the > > > > > > > > context. > > > > > > > > > > > > > > > > 2. In the *Explanation *section, the metadata version number > > part > > > > > > seems a > > > > > > > > bit blurred. Could you point a reference to later section > that > > we > > > > > going > > > > > > > to > > > > > > > > store it in Zookeeper and update it every time when there is > a > > > > > feature > > > > > > > > change? > > > > > > > > > > > > > > > > 3. For the feature downgrade, although it's a Non-goal of the > > > KIP, > > > > > for > > > > > > > > features such as group coordinator semantics, there is no > legal > > > > > > scenario > > > > > > > to > > > > > > > > perform a downgrade at all. So having downgrade door open is > > > pretty > > > > > > > > error-prone as human faults happen all the time. I'm assuming > > as > > > > new > > > > > > > > features are implemented, it's not very hard to add a flag > > during > > > > > > feature > > > > > > > > creation to indicate whether this feature is "downgradable". > > > Could > > > > > you > > > > > > > > explain a bit more on the extra engineering effort for > shipping > > > > this > > > > > > KIP > > > > > > > > with downgrade protection in place? > > > > > > > > > > > > > > > > 4. "Each broker’s supported dictionary of feature versions > will > > > be > > > > > > > defined > > > > > > > > in the broker code." So this means in order to restrict a > > certain > > > > > > > feature, > > > > > > > > we need to start the broker first and then send a feature > > gating > > > > > > request > > > > > > > > immediately, which introduces a time gap and the > > > intended-to-close > > > > > > > feature > > > > > > > > could actually serve request during this phase. Do you think > we > > > > > should > > > > > > > also > > > > > > > > support configurations as well so that admin user could > freely > > > roll > > > > > up > > > > > > a > > > > > > > > cluster with all nodes complying the same feature gating, > > without > > > > > > > worrying > > > > > > > > about the turnaround time to propagate the message only after > > the > > > > > > cluster > > > > > > > > starts up? > > > > > > > > > > > > > > > > 5. "adding a new Feature, updating or deleting an existing > > > > Feature", > > > > > > may > > > > > > > be > > > > > > > > I misunderstood something, I thought the features are defined > > in > > > > > broker > > > > > > > > code, so admin could not really create a new feature? > > > > > > > > > > > > > > > > 6. I think we need a separate error code like > > > > > > FEATURE_UPDATE_IN_PROGRESS > > > > > > > to > > > > > > > > reject a concurrent feature update request. > > > > > > > > > > > > > > > > 7. I think we haven't discussed the alternative solution to > > pass > > > > the > > > > > > > > feature information through Zookeeper. Is that mentioned in > the > > > KIP > > > > > to > > > > > > > > justify why using UpdateMetadata is more favorable? > > > > > > > > > > > > > > > > 8. I was under the impression that user could configure a > range > > > of > > > > > > > > supported versions, what's the trade-off for allowing single > > > > > finalized > > > > > > > > version only? > > > > > > > > > > > > > > > > 9. One minor syntax fix: Note that here the "client" here may > > be > > > a > > > > > > > producer > > > > > > > > > > > > > > > > Boyang > > > > > > > > > > > > > > > > On Mon, Mar 30, 2020 at 4:53 PM Colin McCabe < > > cmcc...@apache.org > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > On Thu, Mar 26, 2020, at 19:24, Kowshik Prakasam wrote: > > > > > > > > > > Hi Colin, > > > > > > > > > > > > > > > > > > > > Thanks for the feedback! I've changed the KIP to address > > your > > > > > > > > > > suggestions. > > > > > > > > > > Please find below my explanation. Here is a link to KIP > > 584: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features > > > > > > > > > > . > > > > > > > > > > > > > > > > > > > > 1. '__data_version__' is the version of the finalized > > feature > > > > > > > metadata > > > > > > > > > > (i.e. actual ZK node contents), while the > > > '__schema_version__' > > > > is > > > > > > the > > > > > > > > > > version of the schema of the data persisted in ZK. These > > > serve > > > > > > > > different > > > > > > > > > > purposes. '__data_version__' is is useful mainly to > clients > > > > > during > > > > > > > > reads, > > > > > > > > > > to differentiate between the 2 versions of eventually > > > > consistent > > > > > > > > > 'finalized > > > > > > > > > > features' metadata (i.e. larger metadata version is more > > > > recent). > > > > > > > > > > '__schema_version__' provides an additional degree of > > > > > flexibility, > > > > > > > > where > > > > > > > > > if > > > > > > > > > > we decide to change the schema for '/features' node in ZK > > (in > > > > the > > > > > > > > > future), > > > > > > > > > > then we can manage broker roll outs suitably (i.e. > > > > > > > > > > serialization/deserialization of the ZK data can be > handled > > > > > > safely). > > > > > > > > > > > > > > > > > > Hi Kowshik, > > > > > > > > > > > > > > > > > > If you're talking about a number that lets you know if data > > is > > > > more > > > > > > or > > > > > > > > > less recent, we would typically call that an epoch, and > not a > > > > > > version. > > > > > > > > For > > > > > > > > > the ZK data structures, the word "version" is typically > > > reserved > > > > > for > > > > > > > > > describing changes to the overall schema of the data that > is > > > > > written > > > > > > to > > > > > > > > > ZooKeeper. We don't even really change the "version" of > > those > > > > > > schemas > > > > > > > > that > > > > > > > > > much, since most changes are backwards-compatible. But we > do > > > > > include > > > > > > > > that > > > > > > > > > version field just in case. > > > > > > > > > > > > > > > > > > I don't think we really need an epoch here, though, since > we > > > can > > > > > just > > > > > > > > look > > > > > > > > > at the broker epoch. Whenever the broker registers, its > > epoch > > > > will > > > > > > be > > > > > > > > > greater than the previous broker epoch. And the newly > > > registered > > > > > > data > > > > > > > > will > > > > > > > > > take priority. This will be a lot simpler than adding a > > > separate > > > > > > epoch > > > > > > > > > system, I think. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 2. Regarding admin client needing min and max > information - > > > you > > > > > are > > > > > > > > > right! > > > > > > > > > > I've changed the KIP such that the Admin API also allows > > the > > > > user > > > > > > to > > > > > > > > read > > > > > > > > > > 'supported features' from a specific broker. Please look > at > > > the > > > > > > > section > > > > > > > > > > "Admin API changes". > > > > > > > > > > > > > > > > > > Thanks. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 3. Regarding the use of `long` vs `Long` - it was not > > > > deliberate. > > > > > > > I've > > > > > > > > > > improved the KIP to just use `long` at all places. > > > > > > > > > > > > > > > > > > Sounds good. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 4. Regarding kafka.admin.FeatureCommand tool - you are > > right! > > > > > I've > > > > > > > > > updated > > > > > > > > > > the KIP sketching the functionality provided by this > tool, > > > with > > > > > > some > > > > > > > > > > examples. Please look at the section "Tooling support > > > > examples". > > > > > > > > > > > > > > > > > > > > Thank you! > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, Kowshik. > > > > > > > > > > > > > > > > > > cheers, > > > > > > > > > Colin > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Cheers, > > > > > > > > > > Kowshik > > > > > > > > > > > > > > > > > > > > On Wed, Mar 25, 2020 at 11:31 PM Colin McCabe < > > > > > cmcc...@apache.org> > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > Thanks, Kowshik, this looks good. > > > > > > > > > > > > > > > > > > > > > > In the "Schema" section, do we really need both > > > > > > __schema_version__ > > > > > > > > and > > > > > > > > > > > __data_version__? Can we just have a single version > > field > > > > > here? > > > > > > > > > > > > > > > > > > > > > > Shouldn't the Admin(Client) function have some way to > get > > > the > > > > > min > > > > > > > and > > > > > > > > > max > > > > > > > > > > > information that we're exposing as well? I guess we > > could > > > > have > > > > > > > min, > > > > > > > > > max, > > > > > > > > > > > and current. Unrelated: is the use of Long rather than > > > long > > > > > > > > deliberate > > > > > > > > > > > here? > > > > > > > > > > > > > > > > > > > > > > It would be good to describe how the command line tool > > > > > > > > > > > kafka.admin.FeatureCommand will work. For example the > > > flags > > > > > that > > > > > > > it > > > > > > > > > will > > > > > > > > > > > take and the output that it will generate to STDOUT. > > > > > > > > > > > > > > > > > > > > > > cheers, > > > > > > > > > > > Colin > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Tue, Mar 24, 2020, at 17:08, Kowshik Prakasam wrote: > > > > > > > > > > > > Hi all, > > > > > > > > > > > > > > > > > > > > > > > > I've opened KIP-584 > <https://issues.apache.org/jira/browse/KIP-584> > > > > > > <https://issues.apache.org/jira/browse/KIP-584> < > > > > > > > https://issues.apache.org/jira/browse/KIP-584 > > > > > > > > > > > > > > > > > > > > > which > > > > > > > > > > > > is intended to provide a versioning scheme for > > features. > > > > I'd > > > > > > like > > > > > > > > to > > > > > > > > > use > > > > > > > > > > > > this thread to discuss the same. I'd appreciate any > > > > feedback > > > > > on > > > > > > > > this. > > > > > > > > > > > > Here > > > > > > > > > > > > is a link to KIP-584 > <https://issues.apache.org/jira/browse/KIP-584> > > > > > > <https://issues.apache.org/jira/browse/KIP-584>: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-584%3A+Versioning+scheme+for+features > > > > > > > > > > > > . > > > > > > > > > > > > > > > > > > > > > > > > Thank you! > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Cheers, > > > > > > > > > > > > Kowshik > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > -- Guozhang >
