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From Chen Qin <qinnc...@gmail.com>
Subject Re: Cassandra statebackend
Date Sat, 06 Aug 2016 05:31:24 GMT
Aljoscha

Sorry about late reply.

David and I drafted a design doc with some diagrams. We may not work on it
immediately, but we thought it would be valuable to share our thoughts and
hear feedbacks.

https://docs.google.com/document/d/1diHQyOPZVxgmnmYfiTa6glLf-FlFjSHcL8J3YR2xLdk/edit#heading=h.12fh7saw98iz

>about sate lineage:

One approach might add pointer to keep data lineage between updated key in
first checkpoint and t's restored checkpoint_id correspondent. It assume
restore from a save point will not cause job manager re instrument already
used checkpoint id.

>clean up old states,

Since job manager already knew save points and latests successful
checkpoint. When a save point is created, it could be good time for job
manager to instrument clean up message and ask each states to move
effective key/values up to current save point and delete anything before.
That's doesn't need to be synchronized since both before and after
compaction will not change states value but location where that value
stored. Delete a save point / checkpoint can also trigger compaction.

Thanks,
Chen

On Thu, Jul 28, 2016 at 6:59 AM, Aljoscha Krettek <aljoscha@apache.org>
wrote:

> Hi,
> thanks for opening the Jira issue. I'll continue the discussion here
> instead of in the Jira, I hope that's OK.
>
> That last paragraph of yours is the most interesting. We will have to adapt
> the way that checkpoints are stored to accommodate state backends that
> store state in some external system, such as Cassandra. Right now, each
> Checkpoint/Savepoint is stored in isolation and the system does not know
> about any relation between them. We have to introduce such a relation,
> basically putting the checkpoints into a graph structure that shows the
> lineage of the checkpoints. Then, when we are cleaning up old checkpoints
> we check the ranges of (logical) timestamps of the checkpoints that we can
> remove and instruct the StateBackend to remove the relevant ranges.
>
> This leads to another interesting thing. We might need to have a
> StateBackend component running in the JobManager that we can invoke to
> delete ranges of checkpoints. Right now, a StateBackend only lives on the
> TaskManager, in the operators. Cleanup of time ranges, however, should
> probably happen in some centralized location.
>
> Cheers,
> Aljoscha
>
> On Mon, 25 Jul 2016 at 22:38 Chen Qin <qinnchen@gmail.com> wrote:
>
> > Hi Aljoscha,
> >
> > Cool! I created a JIRA for this.
> > https://issues.apache.org/jira/browse/FLINK-4266
> > Some comments inline.
> >
> > Chen
> >
> > On Mon, Jul 25, 2016 at 2:41 AM, Aljoscha Krettek <aljoscha@apache.org>
> > wrote:
> >
> > > Hi,
> > > I thought there was a Jira for that but I looked and couldn't find it.
> If
> > > you'd like you can create one and we can discuss the design. Do you
> have
> > > any ideas yet?
> > >
> > > The tricky things I see in this are:
> > >  - Knowing which data is the current data. This will require some kind
> of
> > > timestamps or increasing IDs.
> > >
> >
> > ​We are thinking of leveraging client assigned timestamp from
> > checkpoint_timestamp.
> > ​
> >
> > >  - Knowing when you can retire data from Cassandra
> > >
> > ​That's interesting part, each state checkpoint snapshot might reference
> > t's previous snapshot​. Delete/Consolidate rows previous snapshot with
> > eventual consistency can be tricky.
> >  ​
> >
> > > Some of these might require some changes to how Flink handles
> checkpoints
> > > and it somewhat goes into the direction of incremental checkpoints.
> That
> > > last part is especially important once you deal with savepoints, which
> > can
> > > stay around indefinitely.
> > >
> > > Cheers,
> > > Aljoscha
> > >
> > > On Mon, 25 Jul 2016 at 08:31 Tai Gordon <tzulitai@gmail.com> wrote:
> > >
> > > > Hi Chen,
> > > >
> > > > AFAIK, there currently isn’t any FLIP / JIRA / work currently for a
> > > > Cassandra state backend. I think it’ll definitely by interesting to
> > have
> > > > one in Flink.
> > > >
> > > > Regards,
> > > > Gordon
> > > >
> > > >
> > > > On July 25, 2016 at 10:24:32 AM, Chen Qin (qinnchen@gmail.com)
> wrote:
> > > >
> > > > ​Hi there,
> > > >
> > > > Is there any design docs or on going efforts there?
> > > >
> > > > Thanks,
> > > > Chen ​
> > > >
> > >
> >
>

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