Re: MongoDB Kafka Connect driver

[Sunny Shah <su...@tinyowl.co.in>]

Hello Everyone,

Thanks a lot for your valuable responses.

We will use external database to store Key and Kafka-offset, We won't set
any preference on which database to use, We will leave it to the
driver-user by using a flexible data-access-model like Apache Metamodel.

@James, Even for MongoDB connect driver till time T2 it won't be in the
consistent stage.

On Sat, Jan 30, 2016 at 3:43 AM, James Cheng <jc...@tivo.com> wrote:

> Not sure if this will help anything, but just throwing it out there.
>
> The Maxwell and mypipe projects both do CDC from MySQL and support
> bootstrapping. The way they do it is kind of "eventually consistent".
>
> 1) At time T1, record coordinates of the end of the binlog as of T1.
> 2) At time T2, do a full dump of the database into Kafka.
> 3) Connect back to the binlog in the coordinates recorded in step #1, and
> emit all those records into Kafka.
>
> As Jay mentioned, MySQL supports full row images. At the start of step #3,
> the kafka topic contains all rows as of time T2. It is possible that during
> step #3, that you will emit rows that changed between T1 and T2. From the
> point of view of the consumer of the kafka topic, they would see rows that
> went "back in time". However, as step #3 progresses, and the consumer keeps
> reading, those rows would eventually converge down to their final state.
>
> Maxwell: https://github.com/zendesk/maxwell
> mypipe: https://github.com/mardambey/mypipe
>
> Does that idea help in any way? Btw, a reason it is done this way is that
> it is "difficult" to do #1 and #2 above in a coordinated way without
> locking the database or without adding additional outside dependencies (LVM
> snapshots, being a specific one).
>
> Btw, I glanced at some docs about the Mongodb oplog. It seems that each
> oplog contains
> 1) A way to identify the document that the change applies to.
> 2) A series of mongodb commands (set, unset) to alter the document in #1
> to become the new document.
>
> Thoughts:
> For #1, does it identify a particular "version" of a document? (I don't
> know much about mongodb). If so, you might be able to use it to determine
> if the change should even be attempted to be applied to the object.
> For #2, doesn't that mean you'll need "understand" mongodb's syntax and
> commands? Although maybe it is simply sets/unsets/deletes, in which case
> it's maybe pretty simple.
>
> -James
>
> > On Jan 29, 2016, at 9:39 AM, Jay Kreps <ja...@confluent.io> wrote:
> >
> > Also, most database provide a "full logging" option that let's you
> capture
> > the whole row in the log (I know Oracle and MySQL have this) but it
> sounds
> > like Mongo doesn't yet. That would be the ideal solution.
> >
> > -Jay
> >
> > On Fri, Jan 29, 2016 at 9:38 AM, Jay Kreps <ja...@confluent.io> wrote:
> >
> >> Ah, agreed. This approach is actually quite common in change capture,
> >> though. For many use cases getting the final value is actually
> preferable
> >> to getting intermediates. The exception is usually if you want to do
> >> analytics on something like number of changes.
> >>
> >> On Fri, Jan 29, 2016 at 9:35 AM, Ewen Cheslack-Postava <
> ewen@confluent.io>
> >> wrote:
> >>
> >>> Jay,
> >>>
> >>> You can query after the fact, but you're not necessarily going to get
> the
> >>> same value back. There could easily be dozens of changes to the
> document
> >>> in
> >>> the oplog so the delta you see may not even make sense given the
> current
> >>> state of the document. Even if you can apply it the delta, you'd still
> be
> >>> seeing data that is newer than the update. You can of course take this
> >>> shortcut, but it won't give correct results. And if the data has been
> >>> deleted since then, you won't even be able to write the full record...
> As
> >>> far as I know, the way the op log is exposed won't let you do something
> >>> like pin a query to the state of the db at a specific point in the op
> log
> >>> and you may be reading from the beginning of the op log, so I don't
> think
> >>> there's a way to get correct results by just querying the DB for the
> full
> >>> documents.
> >>>
> >>> Strictly speaking you don't need to get all the data in memory, you
> just
> >>> need a record of the current set of values somewhere. This is what I
> was
> >>> describing following those two options -- if you do an initial dump to
> >>> Kafka, you could track only offsets in memory and read back full
> values as
> >>> needed to apply deltas, but this of course requires random reads into
> your
> >>> Kafka topic (but may perform fine in practice depending on the
> workload).
> >>>
> >>> -Ewen
> >>>
> >>> On Fri, Jan 29, 2016 at 9:12 AM, Jay Kreps <ja...@confluent.io> wrote:
> >>>
> >>>> Hey Ewen, how come you need to get it all in memory for approach (1)?
> I
> >>>> guess the obvious thing to do would just be to query for the record
> >>>> after-image when you get the diff--e.g. just read a batch of changes
> and
> >>>> multi-get the final values. I don't know how bad the overhead of this
> >>> would
> >>>> be...batching might reduce it a fair amount. The guarantees for this
> are
> >>>> slightly different than the pure oplog too (you get the current value
> >>> not
> >>>> every necessarily every intermediate) but that should be okay for most
> >>>> uses.
> >>>>
> >>>> -Jay
> >>>>
> >>>> On Fri, Jan 29, 2016 at 8:54 AM, Ewen Cheslack-Postava <
> >>> ewen@confluent.io>
> >>>> wrote:
> >>>>
> >>>>> Sunny,
> >>>>>
> >>>>> As I said on Twitter, I'm stoked to hear you're working on a Mongo
> >>>>> connector! It struck me as a pretty natural source to tackle since it
> >>>> does
> >>>>> such a nice job of cleanly exposing the op log.
> >>>>>
> >>>>> Regarding the problem of only getting deltas, unfortunately there is
> >>> not
> >>>> a
> >>>>> trivial solution here -- if you want to generate the full updated
> >>> record,
> >>>>> you're going to have to have a way to recover the original document.
> >>>>>
> >>>>> In fact, I'm curious how you were thinking of even bootstrapping. Are
> >>> you
> >>>>> going to do a full dump and then start reading the op log? Is there a
> >>>> good
> >>>>> way to do the dump and figure out the exact location in the op log
> >>> that
> >>>> the
> >>>>> query generating the dump was initially performed? I know that
> >>> internally
> >>>>> mongo effectively does these two steps, but I'm not sure if the
> >>> necessary
> >>>>> info is exposed via normal queries.
> >>>>>
> >>>>> If you want to reconstitute the data, I can think of a couple of
> >>> options:
> >>>>>
> >>>>> 1. Try to reconstitute inline in the connector. This seems difficult
> >>> to
> >>>>> make work in practice. At some point you basically have to query for
> >>> the
> >>>>> entire data set to bring it into memory and then the connector is
> >>>>> effectively just applying the deltas to its in memory copy and then
> >>> just
> >>>>> generating one output record containing the full document each time
> it
> >>>>> applies an update.
> >>>>> 2. Make the connector send just the updates and have a separate
> stream
> >>>>> processing job perform the reconstitution and send to another topic.
> >>> In
> >>>>> this case, the first topic should not be compacted, but the second
> one
> >>>>> could be.
> >>>>>
> >>>>> Unfortunately, without additional hooks into the database, there's
> not
> >>>> much
> >>>>> you can do besides this pretty heavyweight process. There may be some
> >>>>> tricks you can use to reduce the amount of memory used during the
> >>> process
> >>>>> (e.g. keep a small cache of actual records and for the rest only
> store
> >>>>> Kafka offsets for the last full value, performing a (possibly
> >>> expensive)
> >>>>> random read as necessary to get the full document value back), but to
> >>> get
> >>>>> full correctness you will need to perform this process.
> >>>>>
> >>>>> In terms of Kafka Connect supporting something like this, I'm not
> sure
> >>>> how
> >>>>> general it could be made, or that you even want to perform the
> process
> >>>>> inline with the Kafka Connect job. If it's an issue that repeatedly
> >>>> arises
> >>>>> across a variety of systems, then we should consider how to address
> it
> >>>> more
> >>>>> generally.
> >>>>>
> >>>>> -Ewen
> >>>>>
> >>>>> On Tue, Jan 26, 2016 at 8:43 PM, Sunny Shah <su...@tinyowl.co.in>
> >>> wrote:
> >>>>>
> >>>>>>
> >>>>>> Hi ,
> >>>>>>
> >>>>>> We are trying to write a Kafka-connect connector for Mongodb. The
> >>> issue
> >>>>>> is, MongoDB does not provide an entire changed document for update
> >>>>>> operations, It just provides the modified fields.
> >>>>>>
> >>>>>> if Kafka allows custom log compaction then It is possible to
> >>> eventually
> >>>>>> merge an entire document and subsequent update to to create an
> >>> entire
> >>>>>> record again.
> >>>>>>
> >>>>>> As Ewen pointed out to me on twitter, this is not possible, then
> >>> What
> >>>> is
> >>>>>> the Kafka-connect way of solving this issue?
> >>>>>>
> >>>>>> @Ewen, Thanks a lot for a really quick answer on twitter.
> >>>>>>
> >>>>>> --
> >>>>>> Thanks and Regards,
> >>>>>> Sunny
> >>>>>>
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> >>>>>
> >>>>>
> >>>>>
> >>>>> --
> >>>>> Thanks,
> >>>>> Ewen
> >>>>>
> >>>>
> >>>
> >>>
> >>>
> >>> --
> >>> Thanks,
> >>> Ewen
> >>>
> >>
> >>
>
>
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-- 
Thanks and Regards,
 Sunny

-- 
The contents of this e-mail and any attachment(s) are confidential and 
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