Hi Jay, Thanks for the comments and feedback.
I think its quite clear that if a problem keeps arising then it is clear that it needs resolving, and addressing properly. Fair enough at linkedIn, and historically for the very first use cases addressing this maybe not have been a big priority. But as Kafka is now Apache open source and being picked up by many including my company, it is clear and evident that this is a requirement and issue that needs to be now addressed to address these needs. The fact in almost every transport mechanism including networking layers in the enterprise ive worked in, there has always been headers i think clearly shows their need and success for a transport mechanism. I understand some concerns with regards to impact for others not needing it. What we are proposing is flexible solution that provides no overhead on storage or network traffic layers if you chose not to use headers, but does enable those who need or want it to use it. On your response to 1), there is nothing saying that it should be put in any faster or without diligence and the same KIP process can still apply for adding kafka-scope headers, having headers, just makes it easier to add, without constant message and record changes. Timestamp is a clear real example of actually what should be in a header (along with other fields) but as such the whole message/record object needed to be changed to add this, as will any further headers deemed needed by kafka. On response to 2) why within my company as a platforms designer should i enforce that all teams use the same serialization for their payloads? But what i do need is some core cross cutting concerns and information addressed at my platform level and i don't want to impose onto my development teams. This is the same argument why byte[] is the exposed value and key because as a messaging platform you dont want to impose that on my company. On response to 3) Actually this isnt true, there are many 3rd party tools, we need to hook into our messaging flows that they only build onto standardised interfaces as obviously the cost to have a custom implementation for every company would be very high. APM tooling is a clear case in point, every enterprise level APM tool on the market is able to stitch in transaction flow end 2 end over a platform over http, jms because they can stitch in some "magic" data in a uniform/standardised for the two mentioned they stitch this into the headers. It is current form they cannot do this with Kafka. Providing a standardised interface will i believe actually benefit the project as commercial companies like these will now be able to plugin their tooling uniformly, making it attractive and possible. Some of you other concerns as Joel mentions these are more implementation details, that i think should be agreed upon, but i think can be addressed. e.g. re your concern on the hashmap. it is more than possible not to have every record have to have a hashmap unless it actually has a header (just like we have managed to do on the serialized meesage) so if theres a concern on the in memory record size for those using kafka without headers. On your second to last comment about every team choosing their own format, actually we do want this a little, as very first mentioned, no we don't want a free for all, but some freedom to have different serialization has different benefits and draw backs across our business. I can iterate these if needed. One of the use case for headers provided by linkedIn on top of my KIP even shows where headers could be beneficial here as a header could be used to detail which data format the message is serialized to allowing me to consume different formats. Also we have some systems that we need to integrate that pretty near impossible to wrap or touch their binary payloads, or we’re not allowed to touch them (historic system, or inter/intra corporate) Headers really gives as a solution to provide a pluggable platform, and standardisation that allows users to build platforms that adapt to their needs. Cheers Mike ________________________________________ From: Jay Kreps <j...@confluent.io> Sent: Friday, October 7, 2016 4:45 PM To: dev@kafka.apache.org Subject: Re: [DISCUSS] KIP-82 - Add Record Headers Hey guys, This discussion has come up a number of times and we've always passed. One of things that has helped keep Kafka simple is not adding in new abstractions and concepts except when the proposal is really elegant and makes things simpler. Consider three use cases for headers: 1. Kafka-scope: We want to add a feature to Kafka that needs a particular field. 2. Company-scope: You want to add a header to be shared by everyone in your company. 3. World-wide scope: You are building a third party tool and want to add some kind of header. For the case of (1) you should not use headers, you should just add a field to the record format. Having a second way of encoding things doesn't make sense. Occasionally people have complained that adding to the record format is hard and it would be nice to just shove lots of things in quickly. I think a better solution would be to make it easy to add to the record format, and I think we've made progress on that. I also think we should be insanely focused on the simplicity of the abstraction and not adding in new thingies often---we thought about time for years before adding a timestamp and I guarantee you we would have goofed it up if we'd gone with the earlier proposals. These things end up being long term commitments so it's really worth being thoughtful. For case (2) just use the body of the message. You don't need a globally agreed on definition of headers, just standardize on a header you want to include in the value in your company. Since this is just used by code in your company having a more standard header format doesn't really help you. In fact by using something like Avro you can define exactly the types you want, the required header fields, etc. The only case that headers help is (3). This is a bit of a niche case and i think is easily solved just making the reading and writing of given required fields pluggable to work with the header you have. A couple of specific problems with this proposal: 1. A global registry of numeric keys is super super ugly. This seems silly compared to the Avro (or whatever) header solution which gives more compact encoding, rich types, etc. 2. Using byte arrays for header values means they aren't really interoperable for case (3). E.g. I can't make a UI that displays headers, or allow you to set them in config. To work with third party headers, the only case I think this really helps, you need the union of all serialization schemes people have used for any tool. 3. For case (2) and (3) your key numbers are going to collide like crazy. I don't think a global registry of magic numbers maintained either by word of mouth or checking in changes to kafka source is the right thing to do. 4. We are introducing a new serialization primitive which makes fields disappear conditional on the contents of other fields. This breaks the whole serialization/schema system we have today. 5. We're adding a hashmap to each record 6. This proposes making the ProducerRecord and ConsumerRecord mutable and adding setters and getters (which we try to avoid). For context on LinkedIn: I set up the system there, but it may have changed since i left. The header is maintained with the record schemas in the avro schema registry and is required for all records. Essentially all messages must have a field named "header" of type EventHeader which is itself a record schema with a handful of fields (time, host, etc). The header follows the same compatibility rules as other avro fields, so it can be evolved in a compatible way gradually across apps. Avro is typed and doesn't require deserializing the full record to read the header. The header information is (timestamp, host, etc) is important and needs to propagate into other systems like Hadoop which don't have a concept of headers for records, so I doubt it could move out of the value in any case. Not allowing teams to chose a data format other than avro was considered a feature, not a bug, since the whole point was to be able to share data, which doesn't work if every team chooses their own format. I agree with the critique of compaction not having a value. I think we should consider fixing that directly. -Jay On Thu, Sep 22, 2016 at 12:31 PM, Michael Pearce <michael.pea...@ig.com> wrote: > Hi All, > > > I would like to discuss the following KIP proposal: > > https://cwiki.apache.org/confluence/display/KAFKA/KIP- > 82+-+Add+Record+Headers > > > > I have some initial ?drafts of roughly the changes that would be needed. > This is no where finalized and look forward to the discussion especially as > some bits I'm personally in two minds about. > > https://github.com/michaelandrepearce/kafka/tree/kafka-headers-properties > > > > Here is a link to a alternative option mentioned in the kip but one i > would personally would discard (disadvantages mentioned in kip) > > https://github.com/michaelandrepearce/kafka/tree/kafka-headers-full? > > > Thanks > > Mike > > > > > > The information contained in this email is strictly confidential and for > the use of the addressee only, unless otherwise indicated. 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