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From Gaurav Gupta <gau...@datatorrent.com>
Subject Re: Sequencing of operator calls
Date Wed, 11 Nov 2015 09:29:45 GMT
Chetan,

Your analysis is correct. 

I will again summarize how AsyncFSStorageAgent works. It involves two steps

1. Serializing the Operator state to local disk. 
2. Moving this serialized state to HDFS. 

These are how the above two steps work in different scenario

1. If PROCESSING is EXACTLY ONCE for an operator, then both the above steps happen in the
Operator thread. Once the above two steps are completed, checkpointed() call back to Operator
is made.
2. For other scenarios, the 1st step happens in the Operator thread and 2nd step is triggered
in different thread. During the reportStats call, if the 2nd step is complete checkpointed()
call back to Operator is made.

Thanks
- Gaurav

> On Nov 10, 2015, at 3:57 PM, Chetan Narsude <chetan@apache.org> wrote:
> 
> With async checkpointing the Node.reportStats is reporting it back and it
> looks like the code is checking for the top of the queue to see if it’s
> done (or not reporting at all). So I do not see a reason why they will be
> reported out of order.
> 
> Chandni, I read your subsequent responses. Valid point in the last email
> about documentation. It amazes me how much the documentation of our API
> can be improved (self guilty probably the most). One comment that I wanted
> to make even before that is that the semantics of copyToHdfs is that it
> either succeeds or throws exception in which case the recovery kicks in
> and nothing is reported as checkpointed.
> 
> Tim, we need the unit test, man!
> 
> —
> Chetan
> 
> 
> On 11/10/15, 3:05 PM, "Chetan Narsude (cnarsude)" <cnarsude@cisco.com>
> wrote:
> 
>> There are a lot of things which are different when it comes to async
>> checkpointing. I was evaluating it in the morning and expect that either I
>> am able to explain or open jira issues. With my partial observation is
>> that with Async checkpointing, checkpointed is not issued (chandni, the
>> last statement in the if block is ³return²). I am digging into it but feel
>> free to chime in if someone else is able to find that.
>> 
>> Also I realized that my morning email applies as it is to committed but
>> checkpointed has deviated a little bit from that. Will post the revised
>> response soon.
>> 
>> ‹
>> Chetan
>> 
>> 
>> 
>> 
>> On 11/10/15, 2:04 PM, "Chandni Singh" <chandni@datatorrent.com> wrote:
>> 
>>> Chetan,
>>> 
>>> Looking at the checkpoint(windowId) in Node.java, I don't think the steps
>>> you mentioned are followed.
>>> 
>>> *if (using AsyncFSStorageAgent)  {*
>>> *  asyncFSStorageAgent.copyToHdfs(...)*
>>> *}*
>>> *operator.checkpointed(windowId);*
>>> 
>>> This means even copyToHdfs fails the operator is notified that the window
>>> is check-pointed.
>>> 
>>> Are we saying that copyToHdfs will never fail with AsyncFSStorageAgent
>>> for
>>> a window since the operator is notified that the window is checkpointed?
>>> 
>>> Chandni
>>> 
>>> On Tue, Nov 10, 2015 at 11:33 AM, Timothy Farkas <tim@datatorrent.com>
>>> wrote:
>>> 
>>>> Will do
>>>> 
>>>> On Tue, Nov 10, 2015 at 11:01 AM, Pramod Immaneni
>>>> <pramod@datatorrent.com>
>>>> wrote:
>>>> 
>>>>> Is there a unit test covering it? Otherwise can you write one to test
>>>> the
>>>>> hypothesis.
>>>>> 
>>>>> On Tue, Nov 10, 2015 at 11:00 AM, Timothy Farkas
>>>> <tim@datatorrent.com>
>>>>> wrote:
>>>>> 
>>>>>> That is what it is looking like to me. The task is submitted
>>>>>> GenericNode#checkpoint line 504, then at the end of the
>>>>>> GenericNode#checkpoint line 531 checkpointed is called. I am likely
>>>>> missing
>>>>>> something, just would like to know what :)
>>>>>> 
>>>>>> Tim
>>>>>> 
>>>>>> On Tue, Nov 10, 2015 at 10:51 AM, Pramod Immaneni <
>>>>> pramod@datatorrent.com>
>>>>>> wrote:
>>>>>> 
>>>>>>> Tim,
>>>>>>> 
>>>>>>> Are you suggesting that checkpointed is called before the
>>>> checkpoint
>>>> is
>>>>>>> completely persisted in the storage.
>>>>>>> 
>>>>>>> Thanks
>>>>>>> 
>>>>>>> On Tue, Nov 10, 2015 at 10:49 AM, Timothy Farkas <
>>>> tim@datatorrent.com>
>>>>>>> wrote:
>>>>>>> 
>>>>>>>> Chetan,
>>>>>>>> 
>>>>>>>> I do not see the process of reporting the checkpoint to stram,
>>>>>> receiving
>>>>>>>> the ack, and then calling checkpointed. The logic I'm seeing
in
>>>>>>> GenericNode
>>>>>>>> line 484 is that the checkpoint method is called, it spawns
>>>> another
>>>>>>> thread
>>>>>>>> that writes to hdfs, and then checkpointed is immediately
>>>> called
>>>>>>>> afterwards. I am missing something, can you give me some
>>>> pointers
>>>> so
>>>>>>> that I
>>>>>>>> can better understand the flow?
>>>>>>>> 
>>>>>>>> Tim
>>>>>>>> 
>>>>>>>> On Tue, Nov 10, 2015 at 10:33 AM, Munagala Ramanath <
>>>>>> ram@datatorrent.com
>>>>>>>> 
>>>>>>>> wrote:
>>>>>>>> 
>>>>>>>>> Chetan's answer provides a good explanation as well as
>>>> clarifying
>>>>>> that
>>>>>>>>> the difference can be more than 1.
>>>>>>>>> 
>>>>>>>>> Since checkpointing (i.e. "commit notification" as Thomas
>>>> refers
>>>> to
>>>>>>>>> it) is asynchronous, I'm curious
>>>>>>>>> about whether the window ids in the checkpointed call
are
>>>>> guaranteed
>>>>>>>>> to be sequential or if they could
>>>>>>>>> be out of order, i.e. can the checkpointed call see window
id
>>>> 101
>>>>>>>>> before it sees 100 ?
>>>>>>>>> 
>>>>>>>>> Ram
>>>>>>>>> 
>>>>>>>>> On Tue, Nov 10, 2015 at 10:27 AM, Bhupesh Chawda
>>>>>>>>> <bhupesh@datatorrent.com> wrote:
>>>>>>>>>> Hi Tim,
>>>>>>>>>> Thanks for the detailed explanation.
>>>>>>>>>> I understand that the sequence would be
>>>>>>>>>> beginWindow  (x) -> endWindow (x) -> checkpointed
(x)  ->
>>>>>> beginWindow
>>>>>>>>>> (x+1)
>>>>>>>>>> 
>>>>>>>>>> However when I try to print out the window ids in
>>>> beginWindow,
>>>>>>>> endWindow
>>>>>>>>>> and checkpointed calls,  I see x and x-1 respectively.
>>>>>>>>>> I.e. If the window just before checkpoint is 100,
I see
>>>> that
>>>> the
>>>>>>>>>> checkpointed call had window id 99.
>>>>>>>>>> 
>>>>>>>>>> Note: This is observed in the local mode of Apex.
>>>>>>>>>> 
>>>>>>>>>> Thanks
>>>>>>>>>> -Bhupesh
>>>>>>>>>> On 10-Nov-2015 11:25 pm, "Timothy Farkas"
>>>> <tim@datatorrent.com
>>>>> 
>>>>>>> wrote:
>>>>>>>>>> 
>>>>>>>>>>> Hi Bhupesh,
>>>>>>>>>>> 
>>>>>>>>>>> The sequencing of checkpoint called in relation
to
>>>> beginWindow
>>>>> and
>>>>>>>>>>> endWindow depends on how your APPLICATION_WINDOW_COUNT
and
>>>>>>>>>>> CHECKPOINT_WINDOW_COUNT are configured. If the
two
>>>> WINDOW_COUNTs
>>>>>> are
>>>>>>>> not
>>>>>>>>>>> configured to be the same then it is possible
that
>>>> checkpointed
>>>>> is
>>>>>>>>> called
>>>>>>>>>>> within an application window. So the sequence
of events
>>>> would
>>>> be
>>>>>>> this:
>>>>>>>>>>> 
>>>>>>>>>>> beginWindow -> checkpointed -> endWindow
>>>>>>>>>>> 
>>>>>>>>>>> If the APPLICATION_WINDOW_COUNT and
>>>> CHECKPOINT_WINDOW_COUNT
>>>> are
>>>>>> the
>>>>>>>> same
>>>>>>>>>>> (default). Then the sequence of calls would be
this:
>>>>>>>>>>> 
>>>>>>>>>>> beginWindow  (100) -> endWindow (100) ->
checkpointed
>>>> (100)
>>>> ->
>>>>>>>>> beginWindow
>>>>>>>>>>> (101)
>>>>>>>>>>> 
>>>>>>>>>>> The numbers in the sequence represent possible
streaming
>>>> window
>>>>>> Ids
>>>>>>>> that
>>>>>>>>>>> each call would be associated with.
>>>>>>>>>>> 
>>>>>>>>>>> The StateMachine which calls these callbacks
for non-input
>>>>>> operators
>>>>>>>> is
>>>>>>>>> in
>>>>>>>>>>> GenericNode.java.
>>>>>>>>>>> 
>>>>>>>>>>> Tim
>>>>>>>>>>> 
>>>>>>>>>>> On Tue, Nov 10, 2015 at 3:36 AM, Bhupesh Chawda
<
>>>>>>>>> bhupesh@datatorrent.com>
>>>>>>>>>>> wrote:
>>>>>>>>>>> 
>>>>>>>>>>>> Hi Chetan / Community,
>>>>>>>>>>>> 
>>>>>>>>>>>> Can someone please elaborate on why the window
id
>>>> supplied
>>>> to
>>>>>>>>>>>> CheckpointListener and the Operator would
differ.
>>>>>>>>>>>> I tried looking at the window ids of checkpointed()
and
>>>> the
>>>>>>>>> beginWindow()
>>>>>>>>>>>> calls and they differ by 1. Don't know why
this should
>>>> be
>>>> the
>>>>>>> case.
>>>>>>>>>>>> 
>>>>>>>>>>>> Thanks.
>>>>>>>>>>>> -Bhupesh
>>>>>>>>>>>> 
>>>>>>>>>>>> On Thu, Sep 17, 2015 at 5:56 AM, Chetan Narsude
<
>>>>>>>>> chetan@datatorrent.com>
>>>>>>>>>>>> wrote:
>>>>>>>>>>>> 
>>>>>>>>>>>>> Short answer is yes.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> All the control tuples are scheduled
to be delivered
>>>> outside
>>>>>> of
>>>>>>>> the
>>>>>>>>>>>> window.
>>>>>>>>>>>>> As checkpointed callback is triggered
because of
>>>> CHECKPOINT
>>>>>>>> control
>>>>>>>>>>>> tuple,
>>>>>>>>>>>>> it will happen after endWindow and before
the next
>>>>>> beginWindow.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> The windowId supplied to CheckpointListener
and the
>>>> one
>>>>>> provided
>>>>>>>> to
>>>>>>>>>>>>> Operator need not match even though the
sequence is
>>>> defined.
>>>>>> So
>>>>>>> I
>>>>>>>> am
>>>>>>>>>>>>> curious how you intend to use this knowledge.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> --
>>>>>>>>>>>>> Chetan
>>>>>>>>>>>>> 
>>>>>>>>>>>>> 
>>>>>>>>>>>>> On Tue, Sep 15, 2015 at 8:31 AM, Thomas
Weise <
>>>>>>>>> thomas@datatorrent.com>
>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>> 
>>>>>>>>>>>>>> It has not changed the operator execution
model.
>>>> State
>>>>>>>>> serialization
>>>>>>>>>>> is
>>>>>>>>>>>>>> still synchronous, write to HDFS
is taken out of the
>>>>>> operator
>>>>>>>>> thread.
>>>>>>>>>>>>>> 
>>>>>>>>>>>>>> On Tue, Sep 15, 2015 at 8:18 AM,
Amol Kekre <
>>>>>>>> amol@datatorrent.com
>>>>>>>>>> 
>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>> Sent too soon. Has asynchronous
checkpointing
>>>> changed
>>>>>> this?
>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>> Amol
>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>> Sent from my iPhone
>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>>> On Sep 15, 2015, at 12:38
AM, Bhupesh Chawda <
>>>>>>>>>>>>> bhupesh@datatorrent.com>
>>>>>>>>>>>>>>> wrote:
>>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>>> Hi All,
>>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>>> Is it safe to assume that
the checkpointed() and
>>>> the
>>>>>>>>>>> beginWindow()
>>>>>>>>>>>>>> calls
>>>>>>>>>>>>>>>> are sequenced?
>>>>>>>>>>>>>>>> In other words, are these
calls part of the same
>>>>> thread
>>>>>>> and
>>>>>>>>> may
>>>>>>>>>>> not
>>>>>>>>>>>>> run
>>>>>>>>>>>>>>> in
>>>>>>>>>>>>>>>> parallel?
>>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>>> Thanks.
>>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>>>> --
>>>>>>>>>>>>>>>> -Bhupesh
>>>>>>>>>>>>>>> 
>>>>>>>>>>>>>> 
>>>>>>>>>>>>> 
>>>>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>> 
>>>>>>>> 
>>>>>>> 
>>>>>> 
>>>>> 
>>>> 
>> 
> 
> 


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