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From Michael Pearce <Michael.Pea...@ig.com>
Subject Re: [DISCUSS] KIP-82 - Add Record Headers
Date Fri, 17 Feb 2017 20:48:02 GMT
Re:

“    The point about creation of maps seems orthogonal. We can still represent
    the headers as a slice of bytes until the time it is accessed.”

That’s exactly what we’re doing the headers are a slice of bytes, which then gets parsed later if needed, or can be parsed right away, the headers is part of the protocol, so can still be validated if wanted.

If you had a header count then you would have to go through each header key and value length value to work out how much to skip to get to say the value or any future component in the message after the headers. Having it as a byte[] with length value makes this a lot easier to skip. 


On 17/02/2017, 20:37, "Michael Pearce" <Michael.Pearce@ig.com> wrote:

    What’s the issue with exposing a method getHeaders on the producer/consumer record? It doesn’t break anything. We don’t need any special version.
    
    Current batch consumer model and consumer interceptors don’t work where headers need to be acted on at per message level at time of processing, very case is APM (the core one), where the header value is used to continue tracing. JMS/HTTP etc all expose these, without issues. I would NOT want to lock this down to only be usable accessible via interceptors, as we’d fail on one of the main goals.
    
    Regards
    Mike
    
    
    
    
    On 17/02/2017, 20:21, "Jason Gustafson" <jason@confluent.io> wrote:
    
        The point about creation of maps seems orthogonal. We can still represent
        the headers as a slice of bytes until the time it is accessed.
    
    
        > Yes exactly we have access to the records thus why the header should be
        > accessible via it and not hidden for only interceptors to access.
    
    
        As explained above, the point is to make the intended usage clear.
        Applications should continue to rely on the key/value fields to serialize
        their own headers, and it would be more ideal if we can avoid leaking
        third-party headers into applications. This is difficult to do with the
        current interceptors because they share the record objects with the common
        API. What I had in mind is something like an extension of the current
        interceptors which exposed a different object (e.g. `RecordAndHeaders`).
        The challenge is for MM-like use cases. Let me see if I can come up with a
        concrete proposal for that problem.
    
        -Jason
    
    
    
        On Fri, Feb 17, 2017 at 11:55 AM, Michael Pearce <Michael.Pearce@ig.com>
        wrote:
    
        > I am happy to move the definition of the header into the message body, but
        > would cause us not to lazy initialise/parse the headers, as obviously, we
        > would have to traverse these reading the message.
        >
        > This was actually one of Jay’s requests:
        >
        > “    2. I think we should think about creating the Map lazily to avoid
        >     parsing out all the headers into little objects. HashMaps themselves
        > are
        >     kind of expensive and the consumer is very perf sensitive so and making
        >     gazillions of hashmaps that may or may not get used is probably a bad
        > idea.”
        >
        >
        >
        >
        >
        > On 17/02/2017, 19:44, "Michael Pearce" <Michael.Pearce@ig.com> wrote:
        >
        >     Yes exactly we have access to the records thus why the header should
        > be accessible via it and not hidden for only interceptors to access.
        >
        >     Sent using OWA for iPhone
        >     ________________________________________
        >     From: Magnus Edenhill <magnus@edenhill.se>
        >     Sent: Friday, February 17, 2017 7:34:49 PM
        >     To: dev@kafka.apache.org
        >     Subject: Re: [DISCUSS] KIP-82 - Add Record Headers
        >
        >     Big +1 on VarInts.
        >     CPUs are fast, memory is slow.
        >
        >     I agree with Jason that we'll want to continue verifying messages,
        >     including their headers, so while I appreciate the idea of the opaque
        >     header blob it won't be useful in practice.
        >
        >     /Magnus
        >
        >     2017-02-17 10:41 GMT-08:00 Jason Gustafson <jason@confluent.io>:
        >
        >     > Sorry, my mistake. The consumer interceptor is per batch, though I'm
        > not
        >     > sure that's an actual limitation since you still have access to the
        >     > individual records.
        >     >
        >     > -Jason
        >     >
        >     > On Fri, Feb 17, 2017 at 10:39 AM, Jason Gustafson <
        > jason@confluent.io>
        >     > wrote:
        >     >
        >     > > Re headers as byte array and future use by broker. This doesn't
        > take away
        >     > >> from that at all. Nor makes it difficult at all in my opinion.
        >     > >
        >     > >
        >     > > Yeah, I didn't say it was difficult, only awkward. You wouldn't
        > write the
        >     > > schema that way if you were planning to use it on the brokers from
        > the
        >     > > beginning. Note also that one of the benefits of letting the broker
        >     > > understand headers is that it can validate that they are properly
        >     > > formatted. If cost is the only concern, we should confirm its
        > impact
        >     > > through performance testing.
        >     > >
        >     > > One of the key use cases requires access on consume at per
        > event/message
        >     > >> level at the point that message is being processed, as such the
        > batch
        >     > >> interceptors and batch consume api isn't suitable. It needs to be
        > at the
        >     > >> record level.
        >     > >
        >     > >
        >     > > I'm not sure I understand the point about batching. Interceptors
        > are
        >     > > applied per-message, right?
        >     > >
        >     > > My intent on interceptors is to keep the usage of headers
        > well-defined so
        >     > > that they don't start leaking unnecessarily into applications. My
        > guess
        >     > is
        >     > > that it's probably inevitable, but isolating it in the
        > interceptors would
        >     > > at least give people a second thought before deciding to use it.
        > The main
        >     > > challenge in my mind is figuring out how an MM use case would
        > work. It
        >     > > would be more cumbersome to replicate headers through an
        > interceptor,
        >     > > though arguably MM should be working at a lower level anyway.
        >     > >
        >     > > -Jason
        >     > >
        >     > > On Fri, Feb 17, 2017 at 10:16 AM, Michael Pearce <
        > Michael.Pearce@ig.com>
        >     > > wrote:
        >     > >
        >     > >> Re headers available on the record  va interceptors only
        >     > >>
        >     > >> One of the key use cases requires access on consume at per
        > event/message
        >     > >> level at the point that message is being processed, as such the
        > batch
        >     > >> interceptors and batch consume api isn't suitable. It needs to be
        > at the
        >     > >> record level.
        >     > >>
        >     > >> This anyhow is similar to jms/http/amqp where headers are
        > available to
        >     > >> consuming applications.
        >     > >>
        >     > >> Re headers as byte array and future use by broker. This doesn't
        > take
        >     > away
        >     > >> from that at all. Nor makes it difficult at all in my opinion.
        >     > >>
        >     > >>
        >     > >>
        >     > >> Sent using OWA for iPhone
        >     > >> ________________________________________
        >     > >> From: Jason Gustafson <jason@confluent.io>
        >     > >> Sent: Friday, February 17, 2017 5:55:42 PM
        >     > >> To: dev@kafka.apache.org
        >     > >> Subject: Re: [DISCUSS] KIP-82 - Add Record Headers
        >     > >>
        >     > >> >
        >     > >> > 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.Pearce@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.Pearce@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" <jason@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 <
        >     > >> > jason@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 <
        >     > >> isolis@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 <
        >     > >> > cmccabe@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.Pearce@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" <
        >     > >> > jason@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.rosenblatt@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.Pearce@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
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        >     > >> >         >> >> > the use of the addressee only, unless otherwise
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        >     > >> > delete
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        >     > >> > not relate
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        >     > as
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        >     > >> >         >>
        >     > >> >         >> --
        >     > >> >         >> Nacho - Ignacio Solis - isolis@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.
        >     > >> >
        >     > >> 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.
        >     > >>
        >     > >
        >     > >
        >     >
        >     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.
        >
        >
        >
        >
    
    
    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.
    

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