> > Would you be proposing in KIP-98 to convert the other message int’s (key > length, value length) also to varint to keep it uniform. > Also I assume there will be a static or helper method made to write/read > these in the client and server.
Yes, that is what we are proposing, so using varints for headers would be consistent with the rest of the message. We have used static helper methods in our prototype implementation. The cost of parsing, we want to parse/interpret the headers lazily (this is > a key point brought up earlier in discussions) I'm a bit skeptical of this. Has anyone done the performance testing? I can probably implement it and test it if no one else has. I was also under the impression that there may be use cases down the road where the broker would need to interpret headers. That wouldn't be off the table in the future if it's represented as bytes, but it would be quite a bit more awkward, right? By the way, one question I have been wondering about. My understanding is that headers are primarily for use cases where a third-party components wants to enrich messages without needing to understand or modify the schema of the message key and value. For the applications which directly produce and consume the messages and control the key/value schema directly, it seems we would rather have them implement headers directly in their own schema. Supposing for the sake of argument that it was possible, my question is whether it be sufficient to expose the headers in the interceptor API and not in the common API? -Jason On Fri, Feb 17, 2017 at 3:26 AM, Michael Pearce <michael.pea...@ig.com> wrote: > On the point of varInts > > Would you be proposing in KIP-98 to convert the other message int’s (key > length, value length) also to varint to keep it uniform. > Also I assume there will be a static or helper method made to write/read > these in the client and server. > > Cheers > Mike > > > > On 17/02/2017, 11:22, "Michael Pearce" <michael.pea...@ig.com> wrote: > > On the point re: headers in the message protocol being a byte array > and not a count of elements followed by the elements. Again this was > discussed/argued previously. > > It was agreed on for a few reasons some of which you have obviously > picked up on: > > Broker is able to pass it through opaquely > The cost of parsing, we want to parse/interpret the headers lazily > (this is a key point brought up earlier in discussions) > Headers can be copied from consumer record to producer record (aka > mirror makers etc) without parsing if no changes are being made or being > looked at. > Keeps the broker agnostic to the format > You need an int32 either for the byte size of the headers, or for the > count of elements, so overheads are the same, but with going with an opaque > byte array has the above advantages. > > Cheers > Mike > > > On 17/02/2017, 02:50, "Jason Gustafson" <ja...@confluent.io> wrote: > > Sorry, should have noted that the performance testing was done > using the > producer performance tool shipped with Kafka. > > -Jason > > On Thu, Feb 16, 2017 at 6:44 PM, Jason Gustafson < > ja...@confluent.io> wrote: > > > Hey Nacho, > > > > I've compared performance of our KIP-98 implementation with and > without > > varints. For messages around 128 bytes, we see an increase in > throughput of > > about 30% using the default configuration settings. At 256 > bytes, the > > increase is around 16%. Obviously the performance converge as > messages get > > larger, but it seems well worth the cost. Note that we are also > seeing a > > substantial performance increase against trunk primarily because > of the > > much more efficient packing that varints provide us. Anything > adding to > > message overhead, such as record headers, would only increase > the relative > > difference. (Of course take these numbers with a grain of salt > since I have > > only used the default settings with both the producer and broker > on my > > local machine. We intend to provide more extensive performance > details as > > part of the work for KIP-98.) > > > > The implementation we are using is from protobuf ( > > https://developers.google.com/protocol-buffers/docs/encoding), > which is > > also used in HBase. It is trivial to implement and as far as I > know doesn't > > suffer from the aliasing problem you are describing. I checked > with Magnus > > (the author of librdkafka) and he agreed that the savings seemed > worth the > > cost of implementation. > > > > -Jason > > > > On Thu, Feb 16, 2017 at 4:32 PM, Ignacio Solis <iso...@igso.net> > wrote: > > > >> -VarInts > >> > >> I'm one of the people (if not the most) opposed to VarInts. > VarInts > >> have a place, but this is not it. (We had a large discussion > about > >> them at the beginning of KIP-82 time) > >> > >> If anybody has real life performance numbers of VarInts > improving > >> things or significantly reducing resources I would like to know > what > >> that case may be. Yes, you can save some bytes here and there, > but > >> this is probably insignificant to the overall system behavior > and > >> storage requirements. -- I say this with respect to using > VarInts in > >> the protocol itself, not as part of the data. > >> > >> VarInts require you to parse the Int before using it and > depending on > >> the encoding they can suffer from aliasing (multiple > representations > >> for the same value). > >> > >> Why add complexity? > >> > >> Nacho > >> > >> > >> On Thu, Feb 16, 2017 at 10:29 AM, Colin McCabe < > cmcc...@apache.org> > >> wrote: > >> > +1 for varints here-- it would save quite a bit of space. > They are > >> > pretty quick to implement as well. > >> > > >> > I think it makes sense for values to be byte arrays. Users > might want > >> > to attach arbitrary payloads; they shouldn't be forced to > serialize > >> > everything to Java strings. > >> > > >> > best, > >> > Colin > >> > > >> > > >> > On Thu, Feb 16, 2017, at 09:52, Jason Gustafson wrote: > >> >> Hey Michael, > >> >> > >> >> Hmm, I guess the point of representing it as bytes is to > allow the > >> broker > >> >> to pass it through opaquely? Is the cost of parsing them a > concern, or > >> >> are > >> >> we simply trying to ensure that the broker stays agnostic to > the > >> format? > >> >> > >> >> On varints, I think adding support for them makes less sense > for an > >> >> isolated use case, but as part of a more holistic change > (such as what > >> we > >> >> have proposed in KIP-98), I think they are justifiable. If > we add them, > >> >> then the need to use attributes becomes quite a bit weaker, > right? The > >> >> other thing I find slightly odd is the fact that null > headers has no > >> >> actual > >> >> semantic meaning for the message (unlike null keys and > values). It is > >> >> just > >> >> a space optimization. It seems a bit better to always use > size 0 to > >> >> indicate having no headers. > >> >> > >> >> Overall, the main point is ensuring that the message schema > remains > >> >> consistent, either within the larger protocol, or at a > minimum within > >> the > >> >> message itself. > >> >> > >> >> -Jason > >> >> > >> >> On Thu, Feb 16, 2017 at 6:39 AM, Michael Pearce < > michael.pea...@ig.com > >> > > >> >> wrote: > >> >> > >> >> > Hi Jason, > >> >> > > >> >> > On point 1) in the message protocol the headers are simply > a byte > >> array, > >> >> > as like the key or value, this is to clearly demarcate the > header in > >> the > >> >> > core message. Then the header byte array in the core > message is an > >> array of > >> >> > key, value pairs. This is what it is denoting. > >> >> > > >> >> > Then this would be I guess in the given notation: > >> >> > > >> >> > Headers => [KeyLength, Key, ValueLength, Value] > >> >> > KeyLength => int32 <-----------------NEW size of the > byte[] of > >> the > >> >> > serialised key value > >> >> > Key => bytes <---------------------- NEW serialised > string (UTF8) > >> >> > bytes of the header key > >> >> > ValueLength => int32 <-------------- NEW size of the > byte[] of > >> the > >> >> > serialised header value > >> >> > Value => bytes <-------------------- NEW serialised > form of the > >> header > >> >> > value > >> >> > > >> >> > The key length and value length is matching the way the > protocol is > >> >> > defined in the core message currently. > >> >> > > >> >> > > >> >> > > >> >> > > >> >> > On point 2) > >> >> > Var sized ints, this was discussed much earlier on, in > fact I had > >> >> > suggested it myself (with Hadoop references), the > complexity of this > >> >> > compared to having a simpler protocol was argued and > agreed it > >> wasn’t worth > >> >> > the complexity as all other clients in other languages > would need to > >> ensure > >> >> > theyre using the right var size algorithm, as there is a > few. > >> >> > > >> >> > On point 3) > >> >> > We did the attributes, optional approach as originally > there was > >> marked > >> >> > concern that headers would cause a message size overhead > for others, > >> who > >> >> > don’t want them. As such this is the clean solution to > achieve that. > >> If > >> >> > that no longer holds, and we don’t care that we add 4bytes > overhead, > >> then > >> >> > im happy to remove. > >> >> > > >> >> > I’m personally in favour of keeping the message as small > as possible > >> so > >> >> > people don’t get shocks in perf and throughputs dues to > message size, > >> >> > unless they actively use the feature, as such I do prefer > the > >> attribute bit > >> >> > wise feature flag approach myself. > >> >> > > >> >> > > >> >> > > >> >> > > >> >> > On 16/02/2017, 05:40, "Jason Gustafson" < > ja...@confluent.io> wrote: > >> >> > > >> >> > 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 > >> >> > > > > >> >> > > > >> >> > > >> >> > > >> >> > The information contained in this email is strictly > confidential and > >> for > >> >> > the use of the addressee only, unless otherwise indicated. > If you > >> are not > >> >> > the intended recipient, please do not read, copy, use or > disclose to > >> others > >> >> > this message or any attachment. Please also notify the > sender by > >> replying > >> >> > to this email or by telephone (+44(020 7896 0011) and then > delete > >> the email > >> >> > and any copies of it. Opinions, conclusion (etc) that do > not relate > >> to the > >> >> > official business of this company shall be understood as > neither > >> given nor > >> >> > endorsed by it. IG is a trading name of IG Markets Limited > (a company > >> >> > registered in England and Wales, company number 04008957) > and IG > >> Index > >> >> > Limited (a company registered in England and Wales, > company number > >> >> > 01190902). Registered address at Cannon Bridge House, 25 > Dowgate > >> Hill, > >> >> > London EC4R 2YA. Both IG Markets Limited (register number > 195355) > >> and IG > >> >> > Index Limited (register number 114059) are authorised and > regulated > >> by the > >> >> > Financial Conduct Authority. > >> >> > > >> > >> > >> > >> -- > >> Nacho - Ignacio Solis - iso...@igso.net > >> > > > > > > > > > The information contained in this email is strictly confidential and for > the use of the addressee only, unless otherwise indicated. If you are not > the intended recipient, please do not read, copy, use or disclose to others > this message or any attachment. Please also notify the sender by replying > to this email or by telephone (+44(020 7896 0011) and then delete the email > and any copies of it. Opinions, conclusion (etc) that do not relate to the > official business of this company shall be understood as neither given nor > endorsed by it. IG is a trading name of IG Markets Limited (a company > registered in England and Wales, company number 04008957) and IG Index > Limited (a company registered in England and Wales, company number > 01190902). Registered address at Cannon Bridge House, 25 Dowgate Hill, > London EC4R 2YA. Both IG Markets Limited (register number 195355) and IG > Index Limited (register number 114059) are authorised and regulated by the > Financial Conduct Authority. >
