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From Raman Gupta <rocketra...@gmail.com>
Subject Re: Race condition with stream use of Global KTable
Date Mon, 08 Apr 2019 19:11:54 GMT
Hmm, not sure I understand what you are suggesting. Let me address
each step in turn:

> 1) materialize the topic-1 into a state store

Ok, I think that's basically what we have with the global k-table I
showed in the topology, or did you mean something else, like using the
Processor API to populate our own state store?

> 2) use the same key as the partitioning key when writing to topic-2 to make sure it is
co-partitioned (unless the resulted
stream from topic-2 does not need to rely on it being partitioned by
the key to perform other operations, it is okay)

Our situation is the latter. The key in topic-2 is essentially a
unique id, and partitioning should therefore be pretty random. Also
remember that the cardinality of topic-1 to topic-2 is 1:n, so there
are multiple different keys on topic-2 for each key in topic-1.

> 3) we can just issue `get` as part of the lower-level processor API rather than performing
a join to query the materialized table from topic-1.

Right now, we're already doing a `get` on the global k-table as part
of the DSL API. Does it make a difference whether we use the Processor
API? How does that resolve the race condition?

Regards,
Raman



On Fri, Apr 5, 2019 at 12:00 PM Guozhang Wang <wangguoz@gmail.com> wrote:
>
> I see.
>
> So back to your original question, yes there will be a race condition since
> the global ktable is updated with a separate thread other than the one
> which is reading from topic-2 and process the record and query the global
> ktable.
>
> What I can think of on top of my head, is to 1) materialize the topic-1
> into a state store, and 2) use the same key as the partitioning key when
> writing to topic-2 to make sure it is co-partitioned (unless the resulted
> stream from topic-2 does not need to rely on it being partitioned by the
> key to perform other operations, it is okay), and then 3) we can just issue
> `get` as part of the lower-level processor API rather than performing a
> join to query the materialized table from topic-1. Does that make sense?
>
> Guozhang
>
> On Thu, Apr 4, 2019 at 7:38 AM Raman Gupta <rocketraman@gmail.com> wrote:
>
> > Yes, the stream transformation of `topic-1` to `topic-2` is a
> > heavyweight operation producing completely different information on
> > topic-2 than is contained on topic-1 (the cardinality is 1-n as well,
> > not 1-1). The schema evolution I am attempting to perform should have
> > captured the data at time of write of topic-2 but didn't. It is easily
> > available in topic-1 though, using some other information in the
> > payload of topic-2.
> >
> > Regards,
> > Raman
> >
> > On Thu, Apr 4, 2019 at 12:57 AM Guozhang Wang <wangguoz@gmail.com> wrote:
> > >
> > > Hi Raman,
> > >
> > > What I'm not clear is that since topic-2 is a transformed topic of
> > topic-1
> > > via "other stream", then why do you still need to join it with topic-1?
> > Or
> > > in other words, are topic-1 and topic-2 containing different data, or
> > > topic-2 is just storing similar data of topic-1 but just in different
> > > format (since it was a transformation result of topic-1 via "other
> > stream")?
> > >
> > >
> > >
> > > Guozhang
> > >
> > > On Tue, Apr 2, 2019 at 4:07 PM Raman Gupta <rocketraman@gmail.com>
> > wrote:
> > >
> > > > Yes, I forgot to show an item on the topology:
> > > >
> > > >    +-----------> global-ktable +---------+
> > > >    |                                     |
> > > >    +                                     v
> > > > topic-1                                stream +----> topic-3
> > > >    +                                     ^
> > > >    |                                     |
> > > >    +----> other stream +--> topic-2 +----+
> > > >
> > > > My use case is a "schema evolution" of the data in topic-2, to produce
> > > > topic-3 via "stream". In order to perform this schema evolution, I
> > > > need to pull some attributes from the payloads in topic-1. I can't
> > > > simply join topic-1 and topic-2 because they do not share keys. The
> > > > global-ktable allows me to easily look up the values I need from
> > > > topic-1 using an attribute from the payload of topic-2, and combine
> > > > those to write to topic-3.
> > > >
> > > > Regards,
> > > > Raman
> > > >
> > > > On Tue, Apr 2, 2019 at 6:56 PM Guozhang Wang <wangguoz@gmail.com>
> > wrote:
> > > > >
> > > > > Hello Raman,
> > > > >
> > > > > It seems from your case that `topic-1` is used for both the global
> > ktable
> > > > > as well as another stream, which then be transformed to a new stream
> > that
> > > > > will be "joined" somehow with the global ktable. Could you elaborate
> > your
> > > > > case a bit more on why do you want to use the same source topic for
> > two
> > > > > entities in your topology?
> > > > >
> > > > >
> > > > > Guozhang
> > > > >
> > > > > On Tue, Apr 2, 2019 at 3:41 PM Raman Gupta <rocketraman@gmail.com>
> > > > wrote:
> > > > >
> > > > > > I have a topology like this:
> > > > > >
> > > > > >    +-----------> global-ktable +---------+
> > > > > >    |                                     |
> > > > > >    +                                     v
> > > > > > topic-1                                stream
> > > > > >    +                                     ^
> > > > > >    |                                     |
> > > > > >    +----> other stream +--> topic-2 +----+
> > > > > >
> > > > > > IOW, a global ktable is built from topic-1. Meanwhile, "other
> > stream"
> > > > > > transforms topic-1 to topic-2. Finally, "stream" operators on
> > topic-2,
> > > > > > and as part of its logic, reads data from "global-ktable".
> > > > > >
> > > > > > I am worried about the race condition present in "stream" between
> > the
> > > > > > message showing up on topic-2, and the "get" from "global-ktable".
> > Is
> > > > > > there a way, other than retrying the `get`, to avoid this race?
> > > > > >
> > > > > > Regards,
> > > > > > Raman
> > > > > >
> > > > >
> > > > >
> > > > > --
> > > > > -- Guozhang
> > > >
> > >
> > >
> > > --
> > > -- Guozhang
> >
>
>
> --
> -- Guozhang

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