Hey Joe, Yeah I think a lot of those items are limitations in that document that we should definitely fix.
The first issue you point out is a serious one: We give the total list of errors but don't list which errors can result from which APIs. This is a big issue because actually no one knows and even if you know the code base, determining that from the code is not trivial (since errors can percolate from lower layers). If you are writing a client, in practice, you just try stuff and handle the errors that you've seen and add some generic catch all for any new errors (which is actually a good forward-compatability practice). But it would be a lot easier if this kind of trial and error wasn't required. Having just done the Java producer and consumer I definitely felt that pain. The second issue I think we kind of tried to address by giving basic usage info for things like metadata requests etc. But I think what you are pointing out is that this just isn't nearly detailed enough. Ideally we should give a lot of guidance on implementation options, optimizations, best practices, etc. I agree with this. Especially as we start to get the new consumer protocols in shape having this is really important for helping people make use of them as there are several apis that work together. I think we could expand this section of the docs a lot. I think it also might be a good idea to move this document out of wiki and into the main docs. This way we can version it with releases. Currently there is no way to tell which api versions are supported in which Kafka version as the document is always the current state of the protocol minus stuff on trunk that isn't released yet. This mostly works since in practice if you are developing a client you should probably target the latest release, but it would be better to be able to tell what was in each release. -Jay On Mon, Jan 12, 2015 at 5:50 PM, Joe Stein <joe.st...@stealth.ly> wrote: > Having an index for every protocol/API change (like > https://www.python.org/dev/peps/pep-0257/ ) will be much better than the > flat wire protocol doc we have now. It is impossible ( without jumping into > code ) right now to know if an error is supported in one version of Kafka > vs another or different messages even. Having something that is iterative > for each change that is explicit, clear and concise for developers for > client development would be wonderful. Some folks just try to keep pace > with the wire protocol doc regardless and often develop the wrong > functionality expected because functionality is not always part of the > protocol but an expectation / extension of the producer and/or consumer > layer from the project code. > > The "expected behavior" I think is a huge gap between the project and > client implementations. When you are a Kafka user you have certain > expectations when working with producers and consumers. e.g. if you fail a > produced message the expectation is to retry X times with a Y backoff > between each try. The wire protocol doc doesn't always expose these > "features" that are expected behaviors and often get missed. Assumptions > get made and in clients developed very large features take a while (often > seen via production issues) to get found out. I think this problem (which > is a big one IMHO) also will be better resolved with the KIP process. > Client application developers can look at new features, understand the > goals and expectations, develop those goals in the language/system required > and support the byte structure(s) for a complete use case. > > I think child pages from > > https://cwiki.apache.org/confluence/display/KAFKA/A+Guide+To+The+Kafka+Protocol > might be a way to go. I only suggest that because people already use that > page now and we can keep it as a high level "here is what you do" and then > sub link to the child pages when appropriate. I hate completely abandoning > something that is not entirely bad but just missing some updates in > different ways. So, maybe something like that or having a committed > specific part under git or svn might also make sense also. > > I am not really opinionated on how we implement as long as we do implement > something for these issues. > > Feature and/or byte changes should bump the version number, +1 > > /******************************************* > Joe Stein > Founder, Principal Consultant > Big Data Open Source Security LLC > http://www.stealth.ly > Twitter: @allthingshadoop <http://www.twitter.com/allthingshadoop> > ********************************************/ > > On Mon, Jan 12, 2015 at 8:27 PM, Jay Kreps <jay.kr...@gmail.com> wrote: > > > Yeah I think this makes sense. Some of the crazy nesting will get better > > when we move to the new protocol definition I think, but we will always > > need some kind of if statement that branches for the different behavior > and > > this makes testing difficult. > > > > Probably the best thing to do would be to announce a version deprecated > > which will have no function but will serve as a warning that it is going > > away and then remove it some time later. This would mean including > > something that notes this in the protocol docs and maybe the release > notes. > > We should probably just always do this for all but the latest version of > > all apis. I think probably a year of deprecation should be sufficient > prior > > to removal? > > > > I also think we can maybe use some common sense in deciding this. > Removing > > older versions will always be bad for users and client developers and > > always be good for Kafka committers. I think we can be more aggressive on > > things that are not heavily used (and hence less bad for users) or for > > which supporting multiple versions is particularly onerous. > > > > -Jay > > > > On Mon, Jan 12, 2015 at 5:02 PM, Guozhang Wang <wangg...@gmail.com> > wrote: > > > > > +1 on version evolving with any protocol / data format / functionality > > > changes, and I am wondering if we have a standard process of > deprecating > > > old versions? Today with just a couple of versions for the protocol > (e.g. > > > offset commit) the code on the server side is already pretty nested and > > > complicated in order to support different version supports. > > > > > > On Mon, Jan 12, 2015 at 9:21 AM, Jay Kreps <j...@confluent.io> wrote: > > > > > > > Hey Jun, > > > > > > > > Good points. > > > > > > > > I totally agree that the versioning needs to cover both format and > > > behavior > > > > if the behavior change is incompatible. > > > > > > > > I kind of agree about the stable/unstable stuff. What I think this > > means > > > is > > > > not that we would ever evolve the protocol without changing the > > version, > > > > but rather that we would drop support for older versions quicker. On > > one > > > > hand that makes sense and it is often a high bar to get things right > > the > > > > first time. On the other hand I think in practice the set of people > who > > > > interact with the protocol is often different from the end user. So > the > > > > end-user experience may still be "hey my code just broke" because > some > > > > client they use relied on an unstable protocol unbeknownst to them. > > But I > > > > think all that means is that we should be thoughtful about removing > > > support > > > > for old protocol versions even if they were marked unstable. > > > > > > > > Does anyone else have feedback or thoughts on the KIP stuff? > > Objections? > > > > Thoughts on structure? > > > > > > > > -Jay > > > > > > > > On Mon, Jan 12, 2015 at 8:20 AM, Jun Rao <j...@confluent.io> wrote: > > > > > > > > > Jay, > > > > > > > > > > Thanks for bringing this up. Yes, we should increase the level of > > > > awareness > > > > > of compatibility. > > > > > > > > > > For 1 and 2, they probably should include any functional change. > For > > > > > example, even if there is no change in the binary data format, but > > the > > > > > interpretation is changed, we should consider this as a binary > format > > > > > change and bump up the version number. > > > > > > > > > > 3. Having a wider discussion on api/protocol/data changes in the > > > mailing > > > > > list seems like a good idea. > > > > > > > > > > 7. It might be good to also document api/protocol/data format that > > are > > > > > considered stable (or unstable). For example, in 0.8.2 release, we > > will > > > > > have a few new protocols (e.g. HeartBeat) for the development of > the > > > new > > > > > consumer. Those new protocols probably shouldn't be considered > stable > > > > until > > > > > the new consumer is more fully developed. > > > > > > > > > > Thanks, > > > > > > > > > > Jun > > > > > > > > > > > > > > > > > > > > On Fri, Jan 9, 2015 at 4:29 PM, Jay Kreps <j...@confluent.io> > wrote: > > > > > > > > > > > Hey guys, > > > > > > > > > > > > We had a bit of a compatibility slip-up in 0.8.2 with the offset > > > commit > > > > > > stuff. We caught this one before the final release so it's not > too > > > bad. > > > > > But > > > > > > I do think it kind of points to an area we could do better. > > > > > > > > > > > > One piece of feedback we have gotten from going out and talking > to > > > > users > > > > > is > > > > > > that compatibility is really, really important to them. Kafka is > > > > getting > > > > > > deployed in big environments where the clients are embedded in > lots > > > of > > > > > > applications and any kind of incompatibility is a huge pain for > > > people > > > > > > using it and generally makes upgrade difficult or impossible. > > > > > > > > > > > > In practice what I think this means for development is a lot more > > > > > pressure > > > > > > to really think about the public interfaces we are making and try > > our > > > > > best > > > > > > to get them right. This can be hard sometimes as changes come in > > > > patches > > > > > > and it is hard to follow every single rb with enough diligence to > > > know. > > > > > > > > > > > > Compatibility really means a couple things: > > > > > > 1. Protocol changes > > > > > > 2. Binary data format changes > > > > > > 3. Changes in public apis in the clients > > > > > > 4. Configs > > > > > > 5. Metric names > > > > > > 6. Command line tools > > > > > > > > > > > > I think 1-2 are critical. 3 is very important. And 4, 5 and 6 are > > > > pretty > > > > > > important but not critical. > > > > > > > > > > > > One thing this implies is that we are really going to have to do > a > > > good > > > > > job > > > > > > of thinking about apis and use cases. You can definitely see a > > number > > > > of > > > > > > places in the old clients and in a couple of the protocols where > > > enough > > > > > > care was not given to thinking things through. Some of those were > > > from > > > > > long > > > > > > long ago, but we should really try to avoid adding to that set > > > because > > > > > > increasingly we will have to carry around these mistakes for a > long > > > > time. > > > > > > > > > > > > Here are a few things I thought we could do that might help us > get > > > > better > > > > > > in this area: > > > > > > > > > > > > 1. Technically we are just in a really bad place with the > protocol > > > > > because > > > > > > it is defined twice--once in the old scala request objects, and > > once > > > in > > > > > the > > > > > > new protocol format for the clients. This makes changes massively > > > > > painful. > > > > > > The good news is that the new request definition DSL was intended > > to > > > > make > > > > > > adding new protocol versions a lot easier and clearer. It will > also > > > > make > > > > > it > > > > > > a lot more obvious when the protocol is changed since you will be > > > > > checking > > > > > > in or reviewing a change to Protocol.java. Getting the server > moved > > > > over > > > > > to > > > > > > the new request objects and protocol definition will be a bit of > a > > > slog > > > > > but > > > > > > it will really help here I think. > > > > > > > > > > > > 2. We need to get some testing in place on cross-version > > > compatibility. > > > > > > This is work and no tests here will be perfect, but I suspect > with > > > some > > > > > > effort we could catch a lot of things. > > > > > > > > > > > > 3. I was also thinking it might be worth it to get a little bit > > more > > > > > formal > > > > > > about the review and discussion process for things which will > have > > > > impact > > > > > > to these public areas to ensure we end up with something we are > > happy > > > > > with. > > > > > > Python has a PIP process ( > > https://www.python.org/dev/peps/pep-0257/) > > > > by > > > > > > which major changes are made, and it might be worth it for us to > > do a > > > > > > similar thing. We have essentially been doing this already--major > > > > changes > > > > > > almost always have an associated wiki, but I think just getting a > > > > little > > > > > > more rigorous might be good. The idea would be to just call out > > these > > > > > wikis > > > > > > as official proposals and do a full Apache discuss/vote thread > for > > > > these > > > > > > important change. We would use these for big features (security, > > log > > > > > > compaction, etc) as well as for small changes that introduce or > > > change > > > > a > > > > > > public api/config/etc. This is a little heavier weight, but I > think > > > it > > > > is > > > > > > really just critical that we get these things right and this > would > > > be a > > > > > way > > > > > > to call out this kind of change so that everyone would take the > > time > > > to > > > > > > look at them. > > > > > > > > > > > > Thoughts? > > > > > > > > > > > > -Jay > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > -- Guozhang > > > > > >
