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From Chandni Singh <chan...@datatorrent.com>
Subject Re: Supporting iterations in Apex
Date Thu, 17 Sep 2015 00:07:51 GMT
My comments:

1)
As I understand IdempotentStorageManager satisfies this use-case. It has
been used with operators which are dynamically partition-able and has been
integrated with various operators in Malhar to make them idempotent.

So IMO we should not start building another version of recovery mechanism
in Apex.

Once we have hammered out the details of the WAL abstraction (
https://malhar.atlassian.net/browse/APEX-99), we can deprecate
IdempotentStorageManager and use that.

2)
If we use IdempotentStorageManager here, it will give us  a better
understanding of its limitations and therefore help us with the WAL
abstraction.

Thanks,
Chandni


On Wed, Sep 16, 2015 at 4:42 PM, David Yan <david@datatorrent.com> wrote:

> Thanks Chetan.
>
> Can you point me to the location of Deduper code that may be helpful with
> the recovery implementation?
>
> Does anyone have any opinion on the renaming of ITERATION_WINDOW_COUNT?
> DELAY_BY_WINDOW_COUNT? DELAY_WINDOW_COUNT?
>
> David
>
> On Wed, Sep 16, 2015 at 2:21 PM, Chetan Narsude <chetan@datatorrent.com>
> wrote:
>
> > David,
> >
> >  I have 3 comments:
> >
> > 1. The "ahead window" phrase you discussed above is really behind window.
> > With Apex, the windows which are ahead are the windows with smaller
> window
> > Id. smaller window ids are followed by bigger window ids.
> >
> > 2.  ITERATION_WINDOW_COUNT sounds like a misnomer. IMO, It  should be
> > something akin to DELAY_BY_WINDOW_COUNT as you are delaying the events by
> > those many windows. You are not iterating over them as many times. It
> also
> > resonates with PortContext.SLIDE_BY_WINDOW_COUNT
> >
> > 3. Deduper has similar requirement where large amount of data
> (potentially
> > even larger) needs to be partitioned. You can borrow the idea/code from
> > there. And perhaps abstract the code to be reusable.
> >
> > HTH.
> >
> > --
> > Chetan
> >
> > On Wed, Sep 16, 2015 at 1:44 PM, David Yan <david@datatorrent.com>
> wrote:
> >
> > > Hi all,
> > >
> > > One current disadvantage of Apex is the inability to do iterations and
> > > machine learning algorithms because we don't allow loops in the
> > application
> > > DAG (hence the name DAG).  I am proposing that we allow loops in the
> DAG
> > if
> > > the loop advances the window ID by a configured amount.  A JIRA ticket
> > has
> > > been created:
> > >
> > > https://malhar.atlassian.net/browse/APEX-60
> > >
> > > I have started this work in my fork at
> > > https://github.com/davidyan74/incubator-apex-core/tree/APEX-60.
> > >
> > > The current progress is that a simple test case works.  Major work
> still
> > > needs to be done with respect to recovery and partitioning.
> > >
> > > The value ITERATION_WINDOW_COUNT is an attribute to an input port of an
> > > operator.  If the value of the attribute is greater than or equal to 1,
> > any
> > > tuples sent to the input port are treated to be ITERATION_WINDOW_COUNT
> > > windows ahead of what they are.
> > >
> > > For recovery, we will need to checkpoint all the tuples between ports
> > with
> > > the to replay the looped tuples.  During the recovery, if the operator
> > has
> > > an input port, with ITERATION_WINDOW_COUNT=2, is recovering from
> > checkpoint
> > > window 14, the tuples for that input port from window 13 and window 14
> > need
> > > to be replayed to be treated as window 15 and window 16 respectively
> > (13+2
> > > and 14+2).
> > >
> > > In other words, we need to store all the tuples from window with ID
> > > committedWindowId minus ITERATION_WINDOW_COUNT for recovery and purge
> the
> > > tuples earlier than that window.
> > > We can optimize this by only storing the tuples for
> > ITERATION_WINDOW_COUNT
> > > windows prior to any checkpoint.
> > >
> > > For that, we need a storage mechanism for the tuples.  Chandni already
> > has
> > > something that fits this usage case in Apex Malhar.  The class is
> > > IdempotentStorageManager.  In order for this to be used in Apex core,
> we
> > > need to deprecate the class in Apex Malhar and move it to Apex Core.
> > >
> > > A JIRA ticket has been created for this particular work:
> > >
> > > https://malhar.atlassian.net/browse/APEX-128
> > >
> > > Some of the above has been discussed among Thomas, Chetan, Chandni, and
> > > myself.
> > >
> > > For partitioning, we have not started any discussion or brainstorming.
> > We
> > > appreciate any feedback on this and any other aspect related to
> > supporting
> > > iterations in general.
> > >
> > > Thanks!
> > >
> > > David
> > >
> >
>

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