Hey Jeff, Going off of Alexandre's point -- if we have a tagged field in one version but not another (or even in the same version we could have one with and one without) -- are we simply checking if the tagged field exists in the message before we try to read it? I think this is the sort of thing we already do, but just wanted to clarify. We don't need to distinguish versions since we just need to distinguish if the tagged field is there or not.
Thanks, Justine On Tue, Mar 28, 2023 at 3:42 PM Jeff Kim <jeff....@confluent.io.invalid> wrote: > Hi Alexandre, > > Thanks for the feedback. > > 100. You're correct. I updated the KIP to explicitly state that once we > introduce a new tagged field in a later version, that version can never be > downgraded to the listed versions. > > 101. I don't think there is a way to differentiate the two but also not > sure if there will be a need to. > > Best, > Jeff > > On Tue, Mar 28, 2023 at 2:13 PM Alexandre Dupriez < > alexandre.dupr...@gmail.com> wrote: > > > Hi Jeff, > > > > Thank you for the fast answer! > > > > 100. Got it, I think I am starting to understand based on your example > > exposing three Kafka versions. Please let me know if the following > > corresponds to the correct interpretation. > > > > Taking OffsetCommitValue as an example, the idea is to bump the schema > > to version 4 to make the record flexible. The schema version 4 will be > > back-ported to versions 3.0.3, 3.1.3, 3.2.4, 3.3.3, 3.4.1 and > > potentially 3.5. These versions will continue to serialise the record > > with version 3 so that downgrades from there to earlier minor and/or > > major versions are possible. And, these versions will deserialize the > > records with version 4 so that they can support flexible records, > > although they do not make use of tagged fields (if any is present). > > > > Then, in a future version, 3.x, x >= 6, a tag field will be added to > > the record - say, topic id. Downgrade from 3.x to 3.0.3, 3.1.3, 3.2.4, > > 3.3.3, 3.4.1 and 3.5 will be possible because these versions will > > perform deserialisation with the record schema version 4, that is, > > supporting tagged fields. > > > > Hence, this approach is both backward and forward looking and allows > > to extend the scope of compatible versions for downgrades. > > > > [N] 3.x, x >= 6 --> downgradable to [I] 3.0.3, 3.1.3, 3.2.4, 3.3.3, > > 3.4.1, 3.5 --> downgradable to e.g. [O] 3.0.0. > > > > One note though is that a transitive downgrade from 3.x, x >= 6, to > > 3.0.0 via the version domain [I], will not be supported. Should it be > > explicitly mentioned in the KIP that downgrades from 3.0.3, 3.1.3, > > 3.2.4, 3.3.3, 3.4.1, 3.5 to earlier versions may not be possible (if > > offsets or group metadata records were generated by a higher version > > 3.x, x >= 6)? Or am I still misunderstanding? > > > > 101. I will try to provide an example to illustrate what I mean by > > version. Consider the GroupMetadataValue schema version 4 and the > > tagged field topicId introduced in 3.6. Let's say a new optional field > > needs to be added in 3.7. We will have two records version 4, one with > > topic id, the other with topic id and the new field. The new field is > > semantically optional (structurally it is always optional since it is > > a tagged field) but we want to make the distinction between a record > > generated by 3.6 and one generated by 3.7. How do we resolve the > > ambiguity? > > > > Thanks! > > Alexandre > > > > Le mar. 28 mars 2023 à 16:13, Jeff Kim <jeff....@confluent.io.invalid> a > > écrit : > > > > > > Hi Alexandre, > > > > > > Thank you for taking a look! > > > > > > 100. I am not sure I fully understand what you mean by forcefully > adding > > > tagged fields. Let's say VX does not have a flexible version, > > > VY allows deserialization but serializes with a non-flexible version, > and > > > VZ introduces a new tagged field. > > > VX upgrade to VY then downgrade back to VX works because even if group > > > metadata changes VY will serialize with > > > the highest non-flexible version. VZ to VY back to VZ also works > because > > > even if VY serializes with a non-flexible field > > > VZ will be able to deserialize it as it is a supported version. Does > this > > > answer your question? > > > > > > 101. The future versioning scheme needs to be backward compatible with > > > older coordinators. Wouldn't segregating into 2 versions > > > be incompatible? > > > > > > Thanks, > > > Jeff > > > > > > On Tue, Mar 28, 2023 at 5:47 AM Alexandre Dupriez < > > > alexandre.dupr...@gmail.com> wrote: > > > > > > > Hi Jeff, Team, > > > > > > > > Thank you for the KIP. This is a very interesting approach. I feel it > > > > is simpler than the described alternative although it comes with > > > > tradeoffs, thanks for highlighting those. If I may, I would like to > > > > share two naive questions. > > > > > > > > 100. The KIP mentions that records will be serialised with the > highest > > > > non-flexible version (e.g. 3 for GroupMetadataValue and > > > > OffsetCommitValue) so that records can be deserialized with earlier > > > > versions of Kafka. I am not sure I understand correctly: is the idea > > > > to forcefully add tagged fields at the end of the records while > > > > maintaining the existing version (3 for the two record types just > > > > mentioned) so that they can be deserialized by existing Kafka > versions > > > > for which the version of these record types is not known as flexible, > > > > while at the same time preserving the new tagged fields to new Kafka > > > > versions abreast of the addition of a new flexible version for these > > > > record types? If so, is it "bypassing" the protocol convention which > > > > prescribes the use of a flexible version to allow the use of tagged > > > > fields? > > > > > > > > 101. After the bump of the records to a new version indicated as > > > > flexible, the record version is expected to never change while the > > > > underlying tagged fields could potentially still evolve over time. > One > > > > potential downside is that we lose the benefits of the versioning > > > > scheme enforced by the serde protocol. Could this become a problem in > > > > the future if there is ever a need to segregate two distinct > > > > "versions" of the appended record structure held by the tagged > fields? > > > > > > > > Thanks, > > > > Alexandre > > > > > > > > Le jeu. 23 mars 2023 à 18:15, Jeff Kim <jeff....@confluent.io.invalid > > > > a > > > > écrit : > > > > > > > > > > Hi Yi, > > > > > > > > > > > Does it mean with a flexible version, the future > > > > > version of these value types will stay at the version where the > > > > flexibility > > > > > is first introduced? Will there be any need to bump the version > > again in > > > > > the future? > > > > > > > > > > Yes, there will be no need to bump the version since we will only > be > > > > adding > > > > > tagged fields but in the chance the version is bumped, we will > > > > deserialize > > > > > to the highest known (flexible) version which will ignore unknown > > tagged > > > > > fields. > > > > > > > > > > > To enforce the version not bumping, is it possible to have a unit > > test > > > > to > > > > > gate? > > > > > > > > > > Do you have some tests in mind? I don't think we can tell whether a > > > > version > > > > > was bumped or not. > > > > > > > > > > Best, > > > > > Jeff > > > > > > > > > > On Thu, Mar 23, 2023 at 12:07 PM Yi Ding > <yd...@confluent.io.invalid > > > > > > > wrote: > > > > > > > > > > > Hi Jeff, > > > > > > > > > > > > Thanks for the update. Does it mean with a flexible version, the > > future > > > > > > version of these value types will stay at the version where the > > > > flexibility > > > > > > is first introduced? Will there be any need to bump the version > > again > > > > in > > > > > > the future? > > > > > > To enforce the version not bumping, is it possible to have a unit > > test > > > > to > > > > > > gate? > > > > > > > > > > > > > > > > > > On Wed, Mar 22, 2023 at 3:19 PM Jeff Kim > > <jeff....@confluent.io.invalid > > > > > > > > > > > wrote: > > > > > > > > > > > > > Hi all, > > > > > > > > > > > > > > After discussing with my colleagues, I have repurposed the KIP > > for a > > > > > > > general downgrade solution for both transaction and group > > > > coordinators. > > > > > > The > > > > > > > KIP no longer discusses the downgrade path but instead lays out > > the > > > > > > > foundation for future downgrade solutions. > > > > > > > > > > > > > > Link: > > > > > > > > > > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-915%3A+Txn+and+Group+Coordinator+Downgrade+Foundation > > > > > > > > > > > > > > Thanks, > > > > > > > Jeff > > > > > > > > > > > > > > On Mon, Mar 20, 2023 at 7:37 PM Jeff Kim < > jeff....@confluent.io> > > > > wrote: > > > > > > > > > > > > > > > Hi David and Justine, > > > > > > > > > > > > > > > > Thank you both for the detailed feedback. > > > > > > > > > > > > > > > > David, > > > > > > > > > > > > > > > > 1. That makes sense. I revised the "Reading new fields" > section > > > > with > > > > > > how > > > > > > > > we can downgrade to the highest known version and that this > was > > > > > > confirmed > > > > > > > > via unit testing. I also attempted to dive deeper into using > > tagged > > > > > > > fields > > > > > > > > and the rejected alternative. Please let me know what you > > think. > > > > > > > > > > > > > > > > 2. Under "Restrictions and Guidelines" I updated the > paragraph > > to > > > > > > clearly > > > > > > > > state that we want to use tagged fields across all record > types > > > > > > > introduced > > > > > > > > in KIP-848 including OffsetCommitValue. > > > > > > > > > > > > > > > > 3. Would it be possible to bump the OffsetCommitValue record > > > > version to > > > > > > > > make it flexible along with the changes to parse with the > > highest > > > > known > > > > > > > > version? I'm not sure I understand why we cannot make both > > changes > > > > > > > together. > > > > > > > > > > > > > > > > 4. I completely missed this. Added some notes at the end of > > > > > > "Restrictions > > > > > > > > and Guidelines". Unfortunately I can't think of a solution at > > the > > > > > > moment. > > > > > > > > Will get back to you. > > > > > > > > > > > > > > > > 5. I have a section under "Identifying New Record Types" that > > > > discusses > > > > > > > > this: > > > > > > > > > We can automate the cleanup by writing tombstones when the > > > > > > coordinator > > > > > > > > reads unrecognizable records. This may add duplicate work if > > > > tombstones > > > > > > > > were already added but not yet pruned by the log cleaner. > > > > > > > > This is a sure way to delete any unknown record types even if > > the > > > > > > > operator > > > > > > > > does not follow the steps. > > > > > > > > > > > > > > > > 6. Thanks, I have expanded on the section on transactional > > offset > > > > > > > commits. > > > > > > > > As for log compaction, my understanding was that we can > > control the > > > > > > > process > > > > > > > > by forcing compaction. Is my understanding incorrect? > > > > > > > > > > > > > > > > 7. We throw an exception if ConfigResource.type is unknown > > which is > > > > > > true > > > > > > > > in our case because KIP-848 introduces a new GROUP resource > > type. > > > > We > > > > > > will > > > > > > > > need to add some sort of safeguard if the dynamic configs are > > not > > > > > > deleted > > > > > > > > before the server downgrade. I'll give you an update once I > > update > > > > the > > > > > > > > section. > > > > > > > > > > > > > > > > Justine, > > > > > > > > > > > > > > > > On the transactional offset commits, I have updated the KIP > to > > > > reflect > > > > > > > > your points after our offline discussion (much appreciated). > It > > > > seems > > > > > > > that > > > > > > > > the work required to abort from the server side is fairly big > > and > > > > will > > > > > > > > require additional investigation if we are to go down this > > path. > > > > > > > > > > > > > > > > As for downgrade limitations, I missed that part so thanks > for > > the > > > > > > catch > > > > > > > > (along with David). Unfortunately, the proposed design won't > > allow > > > > > > > > downgrades even after the new record types are deleted > because > > the > > > > > > > existing > > > > > > > > OffsetCommitValue record is not flexible and will not be able > > to > > > > parse > > > > > > > > the topicId tagged field. I'll think more about this and get > > back > > > > to > > > > > > you. > > > > > > > > > > > > > > > > Best, > > > > > > > > Jeff > > > > > > > > > > > > > > > > On Mon, Mar 20, 2023 at 2:16 PM Justine Olshan > > > > > > > > <jols...@confluent.io.invalid> wrote: > > > > > > > > > > > > > > > >> Hey Jeff and David, > > > > > > > >> > > > > > > > >> Thanks for the KIP! I was also looking into this a bit since > > I may > > > > > > want > > > > > > > to > > > > > > > >> change the record format for KIP-890 as well (finally > > > > implementing the > > > > > > > >> record change from KIP-360 to better support the epoch bump. > > This > > > > will > > > > > > > >> potentially be helpful for me to implement that work. > > > > > > > >> > > > > > > > >> I discussed some aspects with David offline. Namely, the > > > > alternative > > > > > > > >> solution to rewrite the records in the old format upon > > downgrade. > > > > One > > > > > > > plus > > > > > > > >> to this approach is that it should trigger compaction for > all > > the > > > > > > > existing > > > > > > > >> (new format) records. > > > > > > > >> The main issue we ran into was how to handle ongoing > > transactions. > > > > > > (Ie, > > > > > > > >> could we abort and rewrite with the old format) I think > that's > > > > one of > > > > > > > the > > > > > > > >> main downsides. I think it could potentially be possible to > do > > > > this, > > > > > > but > > > > > > > >> we'd definitely need a server-side mechanism to abort and > > rewrite > > > > the > > > > > > > >> records. > > > > > > > >> > > > > > > > >> I think the trouble I was running into with the current > > solution > > > > is > > > > > > that > > > > > > > >> we > > > > > > > >> can only downgrade to a version slightly before the new > group > > > > > > > coordinator. > > > > > > > >> If someone was running 3.2 and they upgrade to 3.6, but > find a > > > > bug in > > > > > > > 3.4 > > > > > > > >> (or any version between 3.5 and the one they upgraded from), > > then > > > > they > > > > > > > can > > > > > > > >> only downgrade to 3.5. This puts us in a difficult spot. I > > guess > > > > in > > > > > > this > > > > > > > >> scenario, they will need to wait for the new format records > > to get > > > > > > > cleared > > > > > > > >> away before downgrading again. Is that correct? > > > > > > > >> > > > > > > > >> Thanks, > > > > > > > >> Justine > > > > > > > >> > > > > > > > >> On Thu, Mar 16, 2023 at 10:41 AM David Jacot > > > > > > > <dja...@confluent.io.invalid > > > > > > > >> > > > > > > > > >> wrote: > > > > > > > >> > > > > > > > >> > Hi Jeff, > > > > > > > >> > > > > > > > > >> > Thanks for the KIP! I am really glad that we are finally > > > > addressing > > > > > > > this > > > > > > > >> > gap in KIP-848. I have a few general comments. > > > > > > > >> > > > > > > > > >> > 1. Overall, I feel like the important bits are not bold > > enough > > > > in > > > > > > the > > > > > > > >> KIP. > > > > > > > >> > I think that it is good to explain the overall > > upgrade/downgrade > > > > > > > process > > > > > > > >> > and to highlight where the issues are but I think that the > > core > > > > bits > > > > > > > >> should > > > > > > > >> > give more details. For instance, we should explain why > > relying > > > > on > > > > > > tag > > > > > > > >> > fields works to ignore fields added in future releases. My > > > > > > > >> understanding is > > > > > > > >> > that it works because the buffer for the tagged fields is > > > > serialized > > > > > > > at > > > > > > > >> the > > > > > > > >> > end so reading with the old version, which is a prefix of > > the > > > > new > > > > > > one, > > > > > > > >> > works. > > > > > > > >> > > > > > > > > >> > 2. Moreover, it would be great if we could make the > > principle > > > > more > > > > > > > >> general. > > > > > > > >> > My hope is that we can keep reusing the principles > > introduced > > > > in the > > > > > > > >> KIP in > > > > > > > >> > future releases as well. For instance, let's say that we > > need to > > > > > > add a > > > > > > > >> new > > > > > > > >> > field to one of the new records introduced by KIP-848 or > > that we > > > > > > will > > > > > > > >> have > > > > > > > >> > to introduce a new record type as well. Would it work for > > those > > > > > > cases > > > > > > > as > > > > > > > >> > well? > > > > > > > >> > > > > > > > > >> > 3. Regarding enabling support for tagged fields for the > > > > > > > >> OffsetCommitValue > > > > > > > >> > record, it would be great if you could give more details > on > > the > > > > > > steps > > > > > > > to > > > > > > > >> > get there in the KIP. My understanding is that we would > > have to > > > > do > > > > > > the > > > > > > > >> > following: 1) Update the code which reads the records to > > fail > > > > back > > > > > > to > > > > > > > >> the > > > > > > > >> > highest known version if the version stored in the log is > > > > unknown. > > > > > > > Let's > > > > > > > >> > say that we do this in AK 3.5. 2) We need to turn on > tagged > > > > fields > > > > > > for > > > > > > > >> the > > > > > > > >> > record. I think that we can only do this in AK 3.6+. > > > > > > > >> > > > > > > > > >> > 4. I may have missed this part but we should clearly > > explain the > > > > > > > >> drawback > > > > > > > >> > of the proposed approach as well. Say that we enable > tagged > > > > fields > > > > > > for > > > > > > > >> > OffsetCommitValue in AK 3.6. This means that it won't be > > > > possible to > > > > > > > >> > downgrade a cluster from 3.6 to a version earlier than > 3.5. > > > > This is > > > > > > a > > > > > > > >> > significant limitation in my opinion because, I think, > users > > > > don't > > > > > > > >> > necessarily upgrade to all versions. > > > > > > > >> > > > > > > > > >> > 5. In the proposal, it is not clear about whether the old > > > > software > > > > > > > will > > > > > > > >> > delete unknown records or not. It is true that new records > > will > > > > be > > > > > > > >> deleted > > > > > > > >> > when the group is downgraded but this only works if the > > operator > > > > > > > >> respects > > > > > > > >> > the process. > > > > > > > >> > > > > > > > > >> > 6. It would be great if we could extend the rejected > > > > alternative. > > > > > > The > > > > > > > >> > alternative sounds clearly better when you read it so we > > should > > > > > > really > > > > > > > >> > explain the reason to reject it. 1) One issue that you > > mention > > > > is > > > > > > that > > > > > > > >> the > > > > > > > >> > log must be compacted before downgrading and we don't > really > > > > control > > > > > > > >> this > > > > > > > >> > process. 2) Transactions may be difficult to handle. I > > suppose > > > > that > > > > > > it > > > > > > > >> is > > > > > > > >> > possible to handle them though. Have you thought about > this? > > > > > > > >> > > > > > > > > >> > 7. For the new dynamic configs, what happens if they are > > kept > > > > and > > > > > > the > > > > > > > >> > quorum controller is downgraded? > > > > > > > >> > > > > > > > > >> > Best, > > > > > > > >> > David > > > > > > > >> > > > > > > > > >> > On Thu, Mar 16, 2023 at 12:56 AM Jeff Kim > > > > > > > <jeff....@confluent.io.invalid > > > > > > > >> > > > > > > > > >> > wrote: > > > > > > > >> > > > > > > > > >> > > Hi folks, > > > > > > > >> > > > > > > > > > >> > > I would like to start a discussion thread for KIP-915: > > Next > > > > Gen > > > > > > > Group > > > > > > > >> > > Coordinator Downgrade Path which proposes the downgrade > > > > design for > > > > > > > the > > > > > > > >> > new > > > > > > > >> > > group coordinator introduced in KIP-848 > > > > > > > >> > > < > > > > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-848%3A+The+Next+Generation+of+the+Consumer+Rebalance+Protocol > > > > > > > >> > > > > > > > > > > >> > > . > > > > > > > >> > > > > > > > > > >> > > KIP: > > > > > > > >> > > > > > > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-915%3A+Next+Gen+Group+Coordinator+Downgrade+Path > > > > > > > >> > > > > > > > > > >> > > Thanks, > > > > > > > >> > > Jeff > > > > > > > >> > > > > > > > > > >> > > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > > > > > >
