IMO, I don’t 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 <gau...@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 <chan...@datatorrent.com >> <mailto:chan...@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 <chan...@datatorrent.com >> <mailto:chan...@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 <t...@datatorrent.com >>> <mailto:t...@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 <gau...@datatorrent.com >>>> <mailto:gau...@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 <tho...@datatorrent.com >>>>>> <mailto:tho...@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 < >>>> chan...@datatorrent.com <mailto:chan...@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 < >>>> gau...@datatorrent.com <mailto:gau...@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 < >>>> chan...@datatorrent.com <mailto:chan...@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. >>>>>>>> >>>>>>>> >>>>>>> >>>>> >>>>> >>>> >>> >>> >
