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
