We have proposed a few significant changes to the message format in KIP-98 which now seems likely to pass (perhaps with some iterations on implementation details). It would be good to try and coordinate the changes in both of the proposals to make sure they are consistent and compatible.
I think using the attributes to indicate null headers is a reasonable approach. We have proposed to do the same thing for the message key and value. That said, I sympathize with Jay's argument. Having multiple ways to specify a null value increases the overall complexity of the protocol. You can see this just from the fact that you need the extra verbiage in the protocol specification in this KIP and in KIP-98 to describe the dependence between the fields and the attributes. It seems like a slippery slope if you start allowing different request types to implement the protocol specification differently. You can also argue that the messages already are and are likely to remain a special case. For example, there is currently no generality in how compressed message sets are represented that would be applicable for other request types. Some might see this divergence as an unfortunate protocol deficiency which should be fixed; others might see it as sort of the inevitability of needing to optimize where it counts most. I'm probably somewhere in between, but I think we probably all share the intuition that the protocol should be kept as consistent as possible. With that in mind, here are a few comments: 1. One thing I found a little odd when reading the current proposal is that the headers are both represented as an array of bytes and as an array of key/value pairs. I'd probably suggest something like this: Headers => [HeaderKey HeaderValue] HeaderKey => String HeaderValue => Bytes An array in the Kafka protocol is represented as a 4-byte integer indicating the number of elements in the array followed by the serialization of the elements. Unless I'm misunderstanding, what you have instead is the total size of the headers in bytes followed by the elements. I'm not sure I see any reason for this inconsistency. 2. In KIP-98, we've introduced variable-length integer fields. Effectively, we've enriched (or "complicated" as Jay might say ;) the protocol specification to include the following types: VarInt, VarLong, UnsignedVarInt and UnsignedVarLong. Along with these primitives, we could introduce the following types: VarSizeArray => NumberOfItems Item1 Item2 .. ItemN NumberOfItems => UnsignedVarInt VarSizeNullableArray => NumberOfItemsOrNull Item1 Item2 .. ItemN NumberOfItemsOrNull => VarInt (-1 means null) And similarly for the `String` and `Bytes` types. These types can save a considerable amount of space in this proposal because they can be used for both the number of headers included in the message and the lengths of the header keys and values. We could do this instead: Headers => VarSizeArray[HeaderKey HeaderValue] HeaderKey => VarSizeString HeaderValue => VarSizeBytes Combining the savings from the use of variable length fields, the benefit of using the attributes to represent null seems pretty small. 3. Whichever way we go (whether we use the attributes or not), we should at least be consistent between this KIP and KIP-98. It would be very strange to have two ways to represent null values in the same schema. Either way is OK with me. I think some message-level optimizations are justifiable, but the savings here seem minimal (a few bytes per message), so maybe it's not worth the cost of letting the message diverge even further from the rest of the protocol. -Jason On Wed, Feb 15, 2017 at 8:52 AM, radai <radai.rosenbl...@gmail.com> wrote: > I've trimmed the inline contents as this mail is getting too big for the > apache mailing list software to deliver :-( > > 1. the important thing for interoperability is for different "interested > parties" (plugins, infra layers/wrappers, user-code) to be able to stick > pieces of metadata onto msgs without getting in each other's way. a common > key scheme (Strings, as of the time of this writing?) is all thats required > for that. it is assumed that the other end interested in any such piece of > metadata knows the encoding, and byte[] provides for the most flexibility. > i believe this is the same logic behind core kafka being byte[]/byte[] - > Strings are more "usable" but bytes are flexible and so were chosen. > Also - core kafka doesnt even do that good of a job on usability of the > payload (example - i have to specify the nop byte[] "decoders" explicitly > in conf), and again sacrificies usability for the sake of performance (no > convenient single-record processing as poll is a batch, lots of obscure > little config details exposing internals of the batching mechanism, etc) > > this is also why i really dislike the idea of a "type system" for header > values, it further degrades the usability, adds complexity and will > eventually get in people's way, also, it would be the 2nd/3rd home-group > serialization mechanism in core kafka (counting 2 iterations of the "type > definition DSL") > > 2. this is an implementation detail, and not even a very "user facing" one? > to the best of my understanding the vote process is on proposed > API/behaviour. also - since we're willing to go with strings just serialize > a 0-sized header blob and IIUC you dont need any optionals anymore. > > 3. yes, we can :-) > > On Tue, Feb 14, 2017 at 11:56 PM, Michael Pearce <michael.pea...@ig.com> > wrote: > > > Hi Jay, > > > > 1) There was some initial debate on the value part, as youll note String, > > String headers were discounted early on. The reason for this is > flexibility > > and keeping in line with the flexibility of key, value of the message > > object itself. I don’t think it takes away from an ecosystem as each > plugin > > will care for their own key, this way ints, booleans , exotic custom > binary > > can all be catered for=. > > a. If you really wanted to push for a typed value interface, I wouldn’t > > want just String values supported, but the the primatives plus string and > > also still keeping the ability to have a binary for custom binaries that > > some organisations may have. > > i. I have written this slight alternative here, > https://cwiki.apache.org/ > > confluence/display/KAFKA/KIP-82+-+Add+Record+Headers+-+Typed > > ii. Essentially the value bytes, has a leading byte overhead. > > 1. This tells you what type the value is, before reading the rest of the > > bytes, allowing serialisation/deserialization to and from the primitives, > > string and byte[]. This is akin to some other messaging systems. > > 2) We are making it optional, so that for those not wanting headers have > 0 > > bytes overhead (think of it as a feature flag), I don’t think this is > > complex, especially if comparing to changes proposed in other kips like > > kip-98. > > a. If you really really don’t like this, we can drop it, but it would > mean > > buying into 4 bytes extra overhead for users who do not want to use > headers. > > 3) In the summary yes, it is at a higher level, but I think this is well > > documented in the proposed changes section. > > a. Added getHeaders method to Producer/Consumer record (that is it) > > b. We’ve also detailed the new Headers class that this method returns > that > > encapsulates the headers protocol and logic. > > > > Best, > > Mike > > > > ==Original questions from the vote thread from Jay.== > > > > Couple of things I think we still need to work out: > > > > 1. I think we agree about the key, but I think we haven't talked about > > the value yet. I think if our goal is an open ecosystem of these > header > > spread across many plugins from many systems we should consider making > > this > > a string as well so it can be printed, set via a UI, set in config, > etc. > > Basically encouraging pluggable serialization formats here will lead > to > > a > > bit of a tower of babel. > > 2. This proposal still includes a pretty big change to our > serialization > > and protocol definition layer. Essentially it is introducing an > optional > > type, where the format is data dependent. I think this is actually a > big > > change though it doesn't seem like it. It means you can no longer > > specify > > this type with our type definition DSL, and likewise it requires > custom > > handling in client libs. This isn't a huge thing, since the Record > > definition is custom anyway, but I think this kind of protocol > > inconsistency is very non-desirable and ties you to hand-coding > things. > > I > > think the type should instead by [Key Value] in our BNF, where key and > > value are both short strings as used elsewhere. This brings it in line > > with > > the rest of the protocol. > > 3. Could we get more specific about the exact Java API change to > > ProducerRecord, ConsumerRecord, Record, etc? > > > > -Jay > > >
