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From Pramod Immaneni <pra...@datatorrent.com>
Subject Re: Why is Async checkpointing made default?
Date Mon, 23 Nov 2015 18:56:31 GMT
There is a problem I see with the older way in which checkpointed was
called as well which is relevant to this. It is called outside of window
boundary between end window and begin window which I don't think is the
correct as platform should not run operator business logic outside of
window boundaries. The operator could end up sending tuples accidentally in
this call for example.

I think we need to fix that and also not assume that checkpointed callback
will be called immediately after end window of the window being
checkpointed.

Thanks

On Mon, Nov 23, 2015 at 10:38 AM, Chandni Singh <chandni@datatorrent.com>
wrote:

> :-) I did not write it that semantics. Looks like someone made that
> assumption along the way. Fix the code which assumes that.
>
> What  do you mean?
> We go on fixing all the code which was written because the behavior with
> Synchronous checkpointing was
> beginWindow(x) -> endWindow -> checkpointWindow(x)   ( when checkpointing
> is aligned with application window boundary)
>
> This was broken recently with async checkpointing but instead of fixing
> that, we go on fixing all the existing solutions.
> I guess I find this ridiculous.
>
> And why should we make it harder and harder to write operators?
> I think majority of people who participated in this discussion do believe
> that we need to fix and I think this is critical.
>
> Chandni
>
>
>
> On Mon, Nov 23, 2015 at 10:16 AM, Chetan Narsude (cnarsude) <
> cnarsude@cisco.com> wrote:
>
> >
> >
> > On 11/23/15, 8:44 AM, "Chandni Singh" <chandni@datatorrent.com> wrote:
> >
> > >I think in the API, there is windowId in the checkpointed callback for
> > >cases when checkpointing could happen within application windows.
> > >
> > >IMHO backward compatibility is broken. For 3 years since the platform
> was
> > >created the semantics of checkpointed were
> > >beginWindow(x) -> endwindow -> checkpointWindow(x) when checkpointing
> was
> > >aligned with application window boundaries.
> > >
> >
> > :-) I did not write it that semantics. Looks like someone made that
> > assumption along the way. Fix the code which assumes that.
> >
> > ‹
> > Chetan
> >
> > >Please not that aligning Checkpointing with Application Window boundary
> is
> > >the DEFAUL behavior and I think most of us agree that aligning the
> > >checkpointing with application window boundaries is what we see in every
> > >use case.
> > >
> > >Code was written with that assumption by committers of Apex (not even by
> > >people who are new to Apex). It is broken by by a change introduced
> couple
> > >of months back (August 2015).
> > >
> > >Moreover, what do we achieve by NOT guaranteeing that semantics-
> delegate
> > >the complexity to operators to handle it. Even a simple scenario that I
> > >mentioned in my first mail, is very complicated.
> > >
> > >I think this is a big issue and we need to fix it.
> > >
> > >Chandni
> > >
> > >On Mon, Nov 23, 2015 at 7:12 AM, Munagala Ramanath <ram@datatorrent.com
> >
> > >wrote:
> > >
> > >> I too find Gaurav's argument cogent: endWindow() does not take a
> > >> windowId parameter
> > >> leading to a natural guarantee that it matches the immediately
> > >> preceding beginWindow().
> > >>
> > >> Both committed() and checkpointed() take that parameter which suggests
> > >>it
> > >> may
> > >> lag behind the current window. The comments on those methods say
> nothing
> > >> that can be be interpreted as a guarantee that the windowId will match
> > >>the
> > >> window just processed. So, from the viewpoint of "original intent" --
> > >> a phrase that
> > >> has a long and storied history in jurisprudence
> > >> (https://en.wikipedia.org/wiki/Original_intent) --
> > >> it seems that any code that assumed a guarantee when none existed must
> > >> be regarded
> > >> as erroneous.
> > >>
> > >> Having said that, we are all aware that in software it is not at all
> > >> unusual to preserve behavior
> > >> in the interests of backward compatibility, regardless of many other
> > >> reasons for that behavior
> > >> to be considered objectionable. So, if the cost of fixing all the code
> > >> that makes that
> > >> assumption is too high, we should seriously consider reverting it. In
> > >> this context, the
> > >> guarantee that checkpointed() simply means that the operator state has
> > >> been serialized
> > >> seems adequate.
> > >>
> > >> We have a roughly analogous situation with respect to OS write()
> calls:
> > >> When the call returns, we _do not_ have an assurance that the data has
> > >> gone to disk.
> > >> All we know is that the OS has copied the data to its buffers for
> > >> flushing to disk. If we
> > >> want to know when the actual write to disk is done, we need to use a
> > >> different call -- fsync().
> > >>
> > >> Ram
> > >>
> > >> On Mon, Nov 23, 2015 at 6:19 AM, Pramod Immaneni
> > >><pramod@datatorrent.com>
> > >> wrote:
> > >> > I think the original intent for checkpointed callback was that it
> can
> > >>be
> > >> > called anytime after checkpoint and not necessarily immediately
> after
> > >>the
> > >> > window prior of checkpoint. As Gaurav mentioned the API bears it
> out.
> > >> > Furthermore the callback has to be called within the lifecycle
> > >>methods of
> > >> > the operator so it will be called inside a window so you anyway have
> > >>to
> > >> > deal with new data anyway before the callback is called. Even though
> > >> > checkpointed is not committed shouldn't it at least guarantee that
> the
> > >> > operator is recoverable to that state in which case it should be
> > >>called
> > >> > after the save to HDFS actually finishes.
> > >> >
> > >> > Thanks
> > >> >
> > >> > On Mon, Nov 23, 2015 at 5:38 AM, Gaurav Gupta <
> gaurav@datatorrent.com
> > >
> > >> > wrote:
> > >> >
> > >> >> IMO, I don¹t think there is any backward incompatibility wrt
> > >> Checkpointing
> > >> >> call back semantics because
> > >> >>
> > >> >> 1. The checkpointed call is only made once the operator state
is
> > >> preserved.
> > >> >> 2. The window ids being passed to checkpointed are in increasing
> > >>order.
> > >> >> 3. The window ids being passed are still the same ids as were
> passed
> > >> >> earlier.
> > >> >> 4. The sequence is still
> begingWindow()->endWindow()->checkpointed().
> > >> >>
> > >> >> Thanks
> > >> >> - Gaurav
> > >> >>
> > >> >> > On Nov 23, 2015, at 1:03 AM, Gaurav Gupta <
> gaurav@datatorrent.com>
> > >> >> wrote:
> > >> >> >
> > >> >> > If the requirement is that the order is always
> > >> >> begingWindow()->endWindow()->checkpointed(), why to pass
windowId
> in
> > >>the
> > >> >> checkpointed() call back?
> > >> >> >
> > >> >> > Thanks
> > >> >> > - Gaurav
> > >> >> >
> > >> >> >> On Nov 22, 2015, at 11:22 PM, Chandni Singh
> > >><chandni@datatorrent.com
> > >> >> <mailto:chandni@datatorrent.com>> wrote:
> > >> >> >>
> > >> >> >> FYI,
> > >> >> >>
> > >> >> >> HDHTWriter implementation is dependent on the older semantics
> and
> > >> seems
> > >> >> to
> > >> >> >> be broken now.
> > >> >> >> startWindow(x) -> endWindow(x) -> checkpointed(x)
> > >> >> >> In the checkpointed implementation, it copies certain
state
> > >> (transient)
> > >> >> and
> > >> >> >> transfers it to a checkpointedWriteCache with respect
to window
> > >>'x'.
> > >> >> >>
> > >> >> >> With Async checkpointing it, the state that is transferred
is
> much
> > >> more
> > >> >> >> recent than window 'x'.
> > >> >> >>
> > >> >> >> Chandni
> > >> >> >>
> > >> >> >>
> > >> >> >> On Sun, Nov 22, 2015 at 11:04 PM, Chandni Singh <
> > >> >> chandni@datatorrent.com <mailto:chandni@datatorrent.com>>
> > >> >> >> wrote:
> > >> >> >>
> > >> >> >>> Agreed. Thomas's solution fixes the backward incompatibility.
I
> > >> think
> > >> >> we
> > >> >> >>> really need to fix this.
> > >> >> >>>
> > >> >> >>> On Sun, Nov 22, 2015 at 10:23 PM, Timothy Farkas
<
> > >> tim@datatorrent.com
> > >> >> <mailto:tim@datatorrent.com>>
> > >> >> >>> wrote:
> > >> >> >>>
> > >> >> >>>> Gaurav,
> > >> >> >>>>
> > >> >> >>>> I think if the state copy fails then STRAM should
roll back
> the
> > >> >> operator
> > >> >> >>>> to
> > >> >> >>>> a checkpoint that is further back than the last
checkpoint. If
> > >>you
> > >> are
> > >> >> >>>> saying that you want to preserve the semantic
that
> checkpointed
> > >>is
> > >> >> only
> > >> >> >>>> called after a checkpoint is completed, I would
argue that
> that
> > >> >> guarantee
> > >> >> >>>> is already pointless in the current implementation
since it is
> > >> always
> > >> >> >>>> possible for an operator to be rolled back to
a checkpoint
> > >>before
> > >> it's
> > >> >> >>>> last
> > >> >> >>>> completed checkpoint. So, it is already currently
possible for
> > >>some
> > >> >> >>>> database or file operation performed after a
completed
> > >>checkpoint
> > >> to
> > >> >> be
> > >> >> >>>> redone after a failure. Because of this I think
Thomas's
> > >>solution
> > >> >> makes
> > >> >> >>>> the
> > >> >> >>>> most sense. Thomas's solution would also address
Chandni's
> > >>original
> > >> >> point
> > >> >> >>>> that the semantics for the checkpointed call
back have been
> > >> violated.
> > >> >> >>>> There
> > >> >> >>>> are operators in our libraries that have depended
on the
> > >> >> beginWindow(x),
> > >> >> >>>> endWindow(x), and checkpointed(x) call sequence,
which is now
> > >> broken.
> > >> >> We
> > >> >> >>>> should probably fix that.
> > >> >> >>>>
> > >> >> >>>> Tim
> > >> >> >>>>
> > >> >> >>>> On Sun, Nov 22, 2015 at 10:02 PM, Gaurav Gupta
<
> > >> >> gaurav@datatorrent.com <mailto:gaurav@datatorrent.com>>
> > >> >> >>>> wrote:
> > >> >> >>>>
> > >> >> >>>>> Thomas,
> > >> >> >>>>>
> > >> >> >>>>> This was done to preserve checkpointing semantics
that is to
> > >>tell
> > >> the
> > >> >> >>>>> operator that its state is preserved. Say
if database is
> > >>updated
> > >> or
> > >> >> >>>> files
> > >> >> >>>>> are moved in checkpointed call but the state
copy fails, how
> to
> > >> >> address
> > >> >> >>>>> such scenarios?
> > >> >> >>>>>
> > >> >> >>>>> Thanks
> > >> >> >>>>> - Gaurav
> > >> >> >>>>>
> > >> >> >>>>>> On Nov 22, 2015, at 9:44 PM, Thomas Weise
<
> > >> thomas@datatorrent.com
> > >> >> <mailto:thomas@datatorrent.com>>
> > >> >> >>>>> wrote:
> > >> >> >>>>>>
> > >> >> >>>>>> Alternatively I would ask why the checkpointed
callback
> needs
> > >>to
> > >> >> wait
> > >> >> >>>>> until
> > >> >> >>>>>> the data was copied to HDFS instead upon
completion of the
> > >>state
> > >> >> >>>>>> serialization.
> > >> >> >>>>>>
> > >> >> >>>>>> Thomas
> > >> >> >>>>>>
> > >> >> >>>>>>
> > >> >> >>>>>> On Sun, Nov 22, 2015 at 9:41 PM, Chandni
Singh <
> > >> >> >>>> chandni@datatorrent.com <mailto:chandni@datatorrent.com>>
> > >> >> >>>>>> wrote:
> > >> >> >>>>>>
> > >> >> >>>>>>> Gaurav,
> > >> >> >>>>>>>
> > >> >> >>>>>>> My question is about why Async was
made the default when it
> > >> changed
> > >> >> >>>> the
> > >> >> >>>>>>> semantics of operator callbacks.
Your response doesn't
> answer
> > >> that.
> > >> >> >>>>>>>
> > >> >> >>>>>>> In a way we broke backward compatibility.
> > >> >> >>>>>>>
> > >> >> >>>>>>> Chandni
> > >> >> >>>>>>>
> > >> >> >>>>>>> On Sun, Nov 22, 2015 at 9:22 PM,
Gaurav Gupta <
> > >> >> >>>> gaurav@datatorrent.com <mailto:gaurav@datatorrent.com>>
> > >> >> >>>>>>> wrote:
> > >> >> >>>>>>>
> > >> >> >>>>>>>> The idea behind Async checkpointing
is to unblock operator
> > >> while
> > >> >> the
> > >> >> >>>>>>> state
> > >> >> >>>>>>>> is getting transferred to HDFS.
> > >> >> >>>>>>>> Just to clarify that this beginWindow
(x) -> endWindow(x)
> ->
> > >> >> >>>>> checkpointed
> > >> >> >>>>>>>> (x-1 ) should be an ideal sequence,
but if the HDFS is
> slow
> > >>or
> > >> for
> > >> >> >>>> some
> > >> >> >>>>>>>> other reason transferring the
state to HDFS is slow this
> > >> sequence
> > >> >> >>>> may
> > >> >> >>>>> not
> > >> >> >>>>>>>> hold true.
> > >> >> >>>>>>>>
> > >> >> >>>>>>>> Can your use case be addressed
by
> > >> >> >>>>>>>> https://malhar.atlassian.net/browse/APEX-78
<
> > >> >> https://malhar.atlassian.net/browse/APEX-78> <
> > >> >> >>>>>>>> https://malhar.atlassian.net/browse/APEX-78
<
> > >> >> https://malhar.atlassian.net/browse/APEX-78>>?
> > >> >> >>>>>>>>
> > >> >> >>>>>>>> Thanks
> > >> >> >>>>>>>> - Gaurav
> > >> >> >>>>>>>>
> > >> >> >>>>>>>>> On Nov 22, 2015, at 3:56
PM, Chandni Singh <
> > >> >> >>>> chandni@datatorrent.com <mailto:chandni@datatorrent.com>>
> > >> >> >>>>>>>> wrote:
> > >> >> >>>>>>>>>
> > >> >> >>>>>>>>> With Async checkpointing
the checkpoint callback in
> > >> >> CheckpointPoint
> > >> >> >>>>>>>>> listener is called for a
previous window, that is,
> > >> >> >>>>>>>>> beginWindow (x) -> endWindow(x)
-> checkpointed (x-1 )
> > >> >> >>>>>>>>>
> > >> >> >>>>>>>>> This feature was newly introduced.
With synchronous
> > >> >> checkpointing,
> > >> >> >>>> the
> > >> >> >>>>>>>>> behavior was always
> > >> >> >>>>>>>>> beginWindow(x) -> endWindow(x)
-> checkpointed (x)
> > >> >> >>>>>>>>>
> > >> >> >>>>>>>>> A lot of operators were written
before asynchronous
> > >> checkpointing
> > >> >> >>>> was
> > >> >> >>>>>>>>> introduced and few of them
can rely on the sequencing
> > >> guaranteed
> > >> >> by
> > >> >> >>>>>>>>> synchronous checkpointing.
> > >> >> >>>>>>>>>
> > >> >> >>>>>>>>> So why was Async Checkpointed
made default?
> > >> >> >>>>>>>>>
> > >> >> >>>>>>>>> With how Async checkpoint
is today, the complexity to
> > >>handle
> > >> >> >>>> transient
> > >> >> >>>>>>>>> state in checkpointed callback
falls on every operator.
> For
> > >> eg,
> > >> >> >>>> lets
> > >> >> >>>>>>> say
> > >> >> >>>>>>>>> earlier I had a transient
map which I cleared every time
> > >>the
> > >> >> >>>>>>> checkpointed
> > >> >> >>>>>>>>> was called, with async checkpointing
this simple task
> will
> > >>be
> > >> a
> > >> >> lot
> > >> >> >>>>>>> more
> > >> >> >>>>>>>>> complicated.
> > >> >> >>>>>>>>>
> > >> >> >>>>>>>>> I think Async checkpointing
broke the semantics of
> operator
> > >> >> >>>> callbacks
> > >> >> >>>>>>> and
> > >> >> >>>>>>>>> should NOT be the default.
> > >> >> >>>>>>>>
> > >> >> >>>>>>>>
> > >> >> >>>>>>>
> > >> >> >>>>>
> > >> >> >>>>>
> > >> >> >>>>
> > >> >> >>>
> > >> >> >>>
> > >> >> >
> > >> >>
> > >> >>
> > >>
> >
> >
>

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