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From Nacho Solis <nso...@linkedin.com.INVALID>
Subject Re: [DISCUSS] KIP-82 - Add Record Headers
Date Thu, 06 Oct 2016 21:46:13 GMT
I'm also

1. no  (ordered keys)
2. yes (propose key space)


1. I don't think there is going to be much savings in ordering the keys.
I'm assuming some parsing will happen either way. Ordering the keys would
be useful if we were doing linear search on the headers, and even then, the
performance difference would be small for any reasonable number of headers
(even anything that fits in 1 meg).

However, I think that it's likely that whoever is looking at the headers is
going to want to search for plugins for any header in existence, as such,
it's going to have to iterate over the whole header set. So, for every
header, look up the plugin, not the other way around.  Even if we did it
the other way around (for every plugin, search if there is a header
present) we would expect only to have an algorithm that is O(n) and only
iterate once over the list. We wouldn't need to iterate more than once.

Given this, the code overhead of ordering the headers when something is
inserted and such is a bigger pain than dealing with a potentially
unordered list.


2. I like structure and to reduce the play space for potential keys.  This
will allow us to do filter and know when we're testing. At the same time,
we're reserving a lot of space for future usages. However, if there is no
agreement on this I don't think it would be a blocker.  I just want to make
sure we have some order and if possible contiguous ranges for similar
usages.

Nacho


On Thu, Oct 6, 2016 at 2:26 PM, radai <radai.rosenblatt@gmail.com> wrote:

> 1. tending towards no, but I dont have any strong opinions on header
> ordering. it offers a potential speedup for header lookup in a serialized
> blob (in wire format) but that goes away if the headers are fully
> serialized/deserialized always. on the downside its an implementation
> detail that 3rd party impls would need to worry about, and would be hard to
> diagnose if they fail to. its also less friendly to high performance io
> (think about appending headers to an existing blob in pass-through
> components like mirror-maker vs write to the middle) - its still possible
> though. however, the kafka code base is far from being iovec friendly
> anyway.
>
> 2. yes.
>
>
>
>
>
> On Thu, Oct 6, 2016 at 8:58 AM, K Burstev <k.burstev@yandex.com> wrote:
>
> > @Mayuresh
> >
> > Yes exactly, it is a real nasty race issue.
> >
> > This is why I look forward to being able to trash our custom workaround
> :)
> >
> > Kostya
> >
> >
> > 06.10.2016, 02:36, "Mayuresh Gharat" <gharatmayuresh15@gmail.com>:
> > > @Kostya
> > >
> > > Regarding "To get around this we have an awful *cough* solution whereby
> > we
> > > have to send our message wrapper with the headers and null content, and
> > > then we have an application that has to consume from all the compacted
> > > topics and when it sees this message it produces back in a null payload
> > > record to make the broker compact it out."
> > >
> > >  ---> This has a race condition, right?
> > >
> > > Suppose the producer produces a message with headers and null content
> at
> > > time To to Kafka.
> > >
> > > Then the producer, at time To + 1, sends another message with headers
> and
> > > actual content to Kafka.
> > >
> > > What we expect is that the application that is consuming and then
> > producing
> > > same message with null payload should happen at time To + 0.5, so that
> > the
> > > message at To + 1 is not deleted.
> > >
> > > But there is no guarantee here.
> > >
> > > If the null payload goes in to Kafka at time To + 2, then essentially
> you
> > > loose the second message produced by the producer at time To + 1.
> > >
> > > Thanks,
> > >
> > > Mayuresh
> > >
> > > On Wed, Oct 5, 2016 at 6:13 PM, Joel Koshy <jjkoshy.w@gmail.com>
> wrote:
> > >
> > >>  @Nacho
> > >>
> > >>  > > - Brokers can't see the headers (part of the "V" black box)>
> > >>  >
> > >>
> > >>  > (Also, it would be nice if we had a way to access the headers from
> > the
> > >>  > > brokers, something that is not trivial at this time with the
> > current
> > >>  > broker
> > >>  > > architecture).
> > >>  >
> > >>  >
> > >>
> > >>  I think this can be addressed with broker interceptors which we
> > touched on
> > >>  in KIP-42
> > >>  <https://cwiki.apache.org/confluence/display/KAFKA/KIP-
> > >>  42%3A+Add+Producer+and+Consumer+Interceptors>
> > >>  .
> > >>
> > >>  @Gwen
> > >>
> > >>  You are right that the wrapper thingy “works”, but there are some
> > drawbacks
> > >>  that Nacho and Radai have covered in detail that I can add a few more
> > >>  comments to.
> > >>
> > >>  At LinkedIn, we *get by* without the proposed Kafka record headers by
> > >>  dumping such metadata in one or two places:
> > >>
> > >>     - Most of our applications use Avro, so for the most part we can
> > use an
> > >>     explicit header field in the Avro schema. Topic owners are
> supposed
> > to
> > >>     include this header in their schemas.
> > >>     - A prefix to the payload that primarily contains the schema’s ID
> > so we
> > >>     can deserialize the Avro. (We could use this for other use-cases
> as
> > >>  well -
> > >>     i.e., move some of the above into this prefix blob.)
> > >>
> > >>  Dumping headers in the Avro schema pollutes the application’s data
> > model
> > >>  with data/service-infra-related fields that are unrelated to the
> > underlying
> > >>  topic; and forces the application to deserialize the entire blob
> > whether or
> > >>  not the headers are actually used. Conversely from an infrastructure
> > >>  perspective, we would really like to not touch any application data.
> > Our
> > >>  infiltration of the application’s schema is a major reason why many
> at
> > >>  LinkedIn sometimes assume that we (Kafka folks) are the shepherds for
> > all
> > >>  things Avro :)
> > >>
> > >>  Another drawback is that all this only works if everyone in the
> > >>  organization is a good citizen and includes the header; and uses our
> > >>  wrapper libraries - which is a good practice IMO - but may not always
> > be
> > >>  easy for open source projects that wish to directly use the Apache
> > >>  producer/client. If instead we allow these headers to be inserted via
> > >>  suitable interceptors outside the application payloads it would
> remove
> > such
> > >>  issues of separation in the data model and choice of clients.
> > >>
> > >>  Radai has enumerated a number of use-cases
> > >>  <https://cwiki.apache.org/confluence/display/KAFKA/A+
> > >>  Case+for+Kafka+Headers>
> > >>  and
> > >>  I’m sure the broader community will have a lot more to add. The
> > feature as
> > >>  such would enable an ecosystem of plugins from different vendors that
> > users
> > >>  can mix and match in their data pipelines without requiring any
> > specific
> > >>  payload formats or client libraries.
> > >>
> > >>  Thanks,
> > >>
> > >>  Joel
> > >>
> > >>  > >
> > >>  > >
> > >>  > > On Wed, Oct 5, 2016 at 2:20 PM, Gwen Shapira <gwen@confluent.io>
> > >>  wrote:
> > >>  > >
> > >>  > > > Since LinkedIn has some kind of wrapper thingy that adds
the
> > headers,
> > >>  > > > where they could have added them to Apache Kafka - I'm
very
> > curious
> > >>  to
> > >>  > > > hear what drove that decision and the pros/cons of managing
the
> > >>  > > > headers outside Kafka itself.
> > >>  > > >
> > >>  >
> > >
> > > --
> > > -Regards,
> > > Mayuresh R. Gharat
> > > (862) 250-7125
> >
>



-- 
Nacho (Ignacio) Solis
Kafka
nsolis@linkedin.com

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