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From Konstantine Karantasis <konstant...@confluent.io>
Subject Re: [DISCUSS] KIP-558: Track the set of actively used topics by connectors in Kafka Connect
Date Wed, 15 Jan 2020 21:18:41 GMT
Hi Tom,

here are my replies to your comments:

a) Interesting point. It's worth indeed adding it in the "Rejected
Alternatives" sections. I did not consider it as an implementation option.
Connect has already a REST API that, as you note, seems a more natural
choice, especially since its already in place and this feature actually
fits well under the connector endpoint namespace. See KIP-495 for an
interesting discussion on doubling down to the initial design decision to
use the REST API instead of jmx for similar things.

b) I thought hard but briefly about this. Although there's potential for
ambiguity if someone inspects the key on its own, I believe that if we
combine this information with what we save in the value, the ambiguity is
removed. With respect to whether a connector can override another
connector's key, I don't think that's possible for any combination of
unique topic name and connector name. I hope I'm not missing anything.

c) That means that worker tasks will produce topic status records once they
detect that the worker they are running on does not include a topic that is
used by the tasks in the list of active topics for this connector. But once
the worker adds this topic to the set of active topics for this connector,
then the worker tasks stop producing those messages. They'll produce one
again if the worker stops including that topic in the active topics set for
some reason. I could improve the wording on the KIP, but I'd also like to
know we are on the same page re: the design here.

Let me know if the above address your questions.
Thanks,
Konstantine



On Wed, Jan 15, 2020 at 12:05 PM Randall Hauch <rhauch@gmail.com> wrote:

> Almog,
>
> You raise some interesting questions. Comments inline below.
>
> On Wed, Jan 15, 2020 at 11:19 AM Almog Gavra <almog@confluent.io> wrote:
>
> > Hi Konstantine,
> >
> > Thanks for the KIP! This is going to make automatic integration with
> > Connect much more powerful.
> >
> > My thoughts are mostly around freshness of the data and being able to
> > expose that to users. Riffing on Randall's timestamp question - have we
> > considered adding some interval at which point a connector will republish
> > any topics that it encounters and update the timestamp? That way we have
> > some refreshing mechanism that isn't as powerful as the complete reset
> > (which may not be practical in many scenarios).
> >
>
> My question about recording the timestamp at which each active topic record
> were (infrequently) written was more about making a bit more information
> available given the current design, and whether recording a bit more
> information (for very little additional storage cost and no extra runtime
> cost) may be worth it if in the future we figure out how to use this
> information.
>
> I think it's more complicated to try to record the history of when topics
> were most recently used, since that requires recording a lot more active
> topic records than the current proposal. Besides, it's not unexpected that
> source and sink connectors sometimes don't use topics for periods of time.
> A sink connector only consumes from a topic when there are additional
> records to consume, and a source connector only needs to write to a topic
> when there is information in the upstream system targeted to that topic. An
> example of the latter is that most database connectors will write to a
> topic for a particular table only when rows in that table have changed.
>
>
> > I also agree with Randall's other point (Would it be better to not
> > automatically reset connector's active topics when a sink connector is
> > restarted?). I think keeping the behavior as symmetrical between sink and
> > source connectors is a good idea.
> >
> > Lastly, with regards to the API, I can imagine it is also pretty useful
> to
> > answer the inverse question: "which connectors write to topic X". Perhaps
> > we can achieve this by letting the users compute it and just expose an
> API
> > that returns the entire mapping at once (instead of needing to call the
> > /connectors/{name}/topics endpoint for each connector).
>
>
> It may be worth considering a method such as /topics that would produce a
> result that is an aggregate of the potentially many
> /connectors/{name}/topic responses, especially if the result of the /topics
> is an array of the individual responses from /connectors/{name}/topic. The
> benefit of a single request is that the answer to "which connectors use
> topic X" can be computed using simple tools such as 'jq'. And, if tooling
> frequently fetches the active topic information for all connectors,
> providing the aggregate method would reduce the load on the tooling and the
> server. It may also be relatively easy to implement.
>
>
> > Otherwise, looks good to me! Hits the requirements that I had in mind on
> > the nose.
> > - Almog
> >
> > On Wed, Jan 15, 2020 at 1:14 AM Tom Bentley <tbentley@redhat.com> wrote:
> >
> > > Hi Konstantine,
> > >
> > > Thanks for the KIP, I can see how it could be useful.
> > >
> > > a) Did you consider using a metric for this? I don't think it would
> > satisfy
> > > all the use cases you have in mind, but you could mention it in the
> > > rejected alternatives.
> > >
> > > b) If the topic name contains the string "-connector" then the key
> format
> > > is ambiguous. This isn't necessarily fatal because the value will
> > > disambiguate, but it could be misleading. Any reason not to just use a
> > JSON
> > > key, and simplify the value?
> > >
> > > c) I didn't understand this part: "As soon as a worker detects the
> > addition
> > > of a topic to a connector's set of active topics, the worker will cease
> > to
> > > post update messages to the status.storage.topic for that connector. ".
> > I'm
> > > sure I've overlooking something but why is this necessary? Is this were
> > the
> > > task id in the value is used?
> > >
> > > Thanks again,
> > >
> > > Tom
> > >
> > > On Wed, Jan 15, 2020 at 12:15 AM Randall Hauch <rhauch@gmail.com>
> wrote:
> > >
> > > > Oh, one more thing:
> > > >
> > > > 9. There's no mention of how the status topic is partitioned, or how
> > > > partitioning will be used by the new topic records. The KIP should
> > > probably
> > > > outline this for clarity and completeness.
> > > >
> > > > Best regards,
> > > >
> > > > Randall
> > > >
> > > > On Tue, Jan 14, 2020 at 5:25 PM Randall Hauch <rhauch@gmail.com>
> > wrote:
> > > >
> > > > > Thanks, Konstantine. Overall, this KIP looks interesting and really
> > > > > useful, and for the most part is spot on. I do have a number of
> > > > > questions/comments about specifics:
> > > > >
> > > > >    1. The topic records have a value that includes the connector
> > name,
> > > > >    task number that last reported the topic is used, and the topic
> > > name.
> > > > >    There's no mention of record timestamps, but I wonder if it'd
be
> > > > useful to
> > > > >    record this. One challenge might be that a connector does not
> > write
> > > > to a
> > > > >    topic for a while or the task remains running for long periods
> of
> > > > time and
> > > > >    therefore the worker doesn't record that this topic has been
> newly
> > > > written
> > > > >    to since it the task was restarted. IOW, the semantics of the
> > > > timestamp may
> > > > >    be a bit murky. Have you thought about recording the timestamp,
> > and
> > > > if so
> > > > >    what are the pros and cons?
> > > > >    - The "Recording active topics" section says the following:
> > > > >       "As soon as a worker detects the addition of a topic to a
> > > > >       connector's set of active topics, all the connector's tasks
> > that
> > > > inspect
> > > > >       source or sink records will cease to post update messages to
> > the
> > > > >       status.storage.topic."
> > > > >       This probably means the timestamp won't be very useful.
> > > > >    2. The KIP says "the Kafka record value stores the ID of the
> task
> > > that
> > > > >    succeeded to store a topic status record last." However, this
> is a
> > > bit
> > > > >    unclear: is it really storing the last task that successfully
> > wrote
> > > > to that
> > > > >    topic (as this would require very frequent writes to this
> topic),
> > or
> > > > is it
> > > > >    more that this is the task that was last *recorded* as having
> > > written
> > > > >    to the topic? (Here, "recorded" could be a bit of a gray area,
> > since
> > > > this
> > > > >    would depend on the how the worker periodically records this
> > > > information.)
> > > > >    Any kind of clarity here might be helpful.
> > > > >    3. In the "Recording active topics" section (and the surrounding
> > > > >    sections), the "task" is used ambiguously. For example, "when
> its
> > > > tasks
> > > > >    start processing their first records ... these tasks will start
> > > > inspecting
> > > > >    which is the Kafka topic of each of these records". IIUC, the
> > first
> > > > "task"
> > > > >    mentioned is the connector's task, and the second is the
> worker's
> > > > task. Do
> > > > >    we need to distinguish this more clearly?
> > > > >    4. Maybe I missed it, but does this KIP explicitly say that the
> > > > >    Connector API is unchanged? It's probably worth pointing out to
> > help
> > > > >    assuage any concerns that connector implementations have to
> change
> > > to
> > > > make
> > > > >    use of this feature.
> > > > >    5. In the "Resetting a connector's set of active topics" section
> > the
> > > > >    behavior is not exactly clear. Consider a user running connector
> > > "A",
> > > > the
> > > > >    connector has been fully started and is processing records, and
> > the
> > > > worker
> > > > >    has recorded topic usage records. Then the user resets the
> active
> > > > topics
> > > > >    for connector A while the connector is still running? If the
> > > connector
> > > > >    writes to no new topics, before the tasks are rebalanced then
is
> > it
> > > > correct
> > > > >    that Connect would report no active topics? And after the tasks
> > are
> > > > >    rebalance, will the worker record any topics used by connector
> A?
> > > > >    6. In the "Restaring" (misspelled) section: "Reconfiguring a
> > source
> > > > >    connector has also no altering effect for a source connector.
> > > > However, when
> > > > >    reconfiguring a sink connector if the new configuration no
> longer
> > > > includes
> > > > >    any of the previously tracked topics, these topics will be
> removed
> > > > from the
> > > > >    set of active topics for this sink connector by appending
> > tombstone
> > > > >    messages appropriately after the reconfiguration of the
> > connector."
> > > > Would
> > > > >    it be better to not automatically reset connector's active
> topics
> > > > when a
> > > > >    sink connector is restarted? Isn't that more consistent with the
> > > > >    "Resetting" behavior and the goals at the top of the KIP: "it'd
> be
> > > > useful
> > > > >    for users, operators and applications to know which are the
> topics
> > > > that a
> > > > >    connector has used since it was first created"?
> > > > >    7. The `PUT /connectors/{name}/topics/reset` endpoint "this
> > request
> > > > >    can be reapplied after the deletion of the connector". IOW, even
> > > > though
> > > > >    connector with that name doesn't exist, we can still make this
> > > > request? How
> > > > >    does this compare with other methods such as "status"?
> > > > >    8. What are the security implications of this proposal?
> > > > >
> > > > > As you can see, most of these can probably be addressed without
> much
> > > > work.
> > > > >
> > > > > Best regards,
> > > > >
> > > > > Randall
> > > > >
> > > > > On Mon, Jan 13, 2020 at 11:05 PM Konstantine Karantasis <
> > > > > konstantine@confluent.io> wrote:
> > > > >
> > > > >> Hi all.
> > > > >>
> > > > >> I just posted KIP-558: Track the set of actively used topics
by
> > > > connectors
> > > > >> in Kafka Connect
> > > > >>
> > > > >> Wiki link here:
> > > > >>
> > > > >>
> > > >
> > >
> >
> https://cwiki.apache.org/confluence/display/KAFKA/KIP-558%3A+Track+the+set+of+actively+used+topics+by+connectors+in+Kafka+Connect
> > > > >>
> > > > >> I think it's a nice extension to follow up on KIP-158 and a useful
> > > > feature
> > > > >> to the ever increasing number of applications that are built
> around
> > > > Kafka
> > > > >> Connect.
> > > > >> Would love to hear what you think.
> > > > >>
> > > > >> Best,
> > > > >> Konstantine
> > > > >>
> > > > >
> > > >
> > >
> >
>

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