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From Kostas Kloudas <k.klou...@data-artisans.com>
Subject Re: ContinuousFileMonitoringFunction - deleting file after processing
Date Thu, 01 Dec 2016 09:35:59 GMT
Hi Maciek,

The first point seems interesting and we should definitely look into that, also for other
filesystems e.g. HDFS.
It would be nice if we could together find a more “one-size-fits-all” solution. 
Because local fs rounds up to a second but other filesystems may have different strategies.

Now for the second point, the problem with introducing another operator with parallelism of
1 would mean 
another point of congestion probably. But it is definitely a point worth discussing.

I have already started a discussion thread in the dev mailing list.
The title is: [DISCUSS] Handling event-time in continuous file processing.

I would suggest to move the discussion there so that also other people can participate and
with their own use cases and ideas.

When this is finalized, it would be more than appreciated if you could also help with the
If you still want to, of course ;)


> On Nov 26, 2016, at 9:40 PM, Maciek Próchniak <mpr@touk.pl> wrote:
> Hi Kostas,
> I didn't see any discussion on dev mailing list, so I'd like to share our problems/solutions
(we had a busy month...;)
> 1. we refactored ContinuousFileMonitoringFunction so that state includes not only lastModificationTime,
but also list of files that have exactly this modification time. This way we're sure that
we don't loose any files that appear later with same modification time. It turned out that
for local file system this is quite important, as modificationTime in java can have one second
resolution (see e.g. http://stackoverflow.com/questions/24804618/get-file-mtime-with-millisecond-resolution-from-java
- we learned it the hard way...)
> 2. we are able to safely delete files in following way:
>    - in ContinuousFileReaderOperator we emit additional marker event after end of split
>    - the split contains information how many splits are in the file
>    - we added additional operator of parallelism 1 after ContinuousFileReaderOperator
which tracks additional events so that it knows when all splits from file has been processed
and deletes finished files after appropriate checkpoints have been completed.
> If you & other committers find these ideas ok, I can prepare jiras and pull requests.
While the first point is pretty straightforward IMHO, I'd like to get some feedback one the
second one.
> thanks,
> maciek
> On 18/10/2016 11:52, Kostas Kloudas wrote:
>> Hi Maciek,
>> I agree with you that 1ms is often too long :P
>> This is the reason why I will open a discussion to have
>> all the ideas/ requirements / shortcomings in a single place.
>> This way the community can track and influence what
>> is coming next.
>> Hopefully I will do it in the afternoon and I will send you
>> the discussion thread.
>> Cheers,
>> Kostas
>>> On Oct 18, 2016, at 11:43 AM, Maciek Próchniak <mpr@touk.pl> wrote:
>>> Hi Kostas,
>>> thanks for quick answer.
>>> I wouldn't dare to delete files in InputFormat if they were splitted and processed
in parallel...
>>> As for using notifyCheckpointComplete - thanks for suggestion, it looks pretty
interesting, I'll try to try it out. Although I wonder a bit if relying only on modification
timestamp is enough - many things may happen in one ms :)
>>> thanks,
>>> macie
>>> On 18/10/2016 11:14, Kostas Kloudas wrote:
>>>> Hi Maciek,
>>>> Just a follow-up on the previous email, given that splits are read in parallel,
when the
>>>> ContinuousFileMonitoringFunction forwards the last split, it does not mean
that the
>>>> final splits is going to be processed last. If the node it gets assigned
is fast enough
>>>> then it may be processed faster than others.
>>>> This assumption only holds if you have a parallelism of 1.
>>>> Cheers,
>>>> Kostas
>>>>> On Oct 18, 2016, at 11:05 AM, Kostas Kloudas <k.kloudas@data-artisans.com>
>>>>> Hi Maciek,
>>>>> Currently this functionality is not supported but this seems like a good
>>>>> Actually, give that the feature is rather new, we were thinking of opening
a discussion
>>>>> in the dev mailing list in order to
>>>>> i) discuss some current limitations of the Continuous File Processing
>>>>> ii) see how people use it and adjust our features accordingly
>>>>> I will let you know as soon as I open this thread.
>>>>> By the way for your use-case, we should probably have a callback in the
>>>>> that will inform the source that a given checkpoint was successfully
performed and then
>>>>> we can purge the already processed files. This can be a good solution.
>>>>> Thanks,
>>>>> Kostas
>>>>>> On Oct 18, 2016, at 9:40 AM, Maciek Próchniak <mpr@touk.pl>
>>>>>> Hi,
>>>>>> we want to monitor hdfs (or local) directory, read csv files that
appear and after successful processing - delete them (mainly not to run out of disk space...)
>>>>>> I'm not quite sure how to achieve it with current implementation.
Previously, when we read binary data (unsplittable files) we made small hack and deleted them
>>>>>> in our FileInputFormat - but now we want to use splits and detecting
which split is 'the last one' is no longer so obvious - of course it's also problematic when
it comes to checkpointing...
>>>>>> So my question is - is there a idiomatic way of deleting processed
>>>>>> thanks,
>>>>>> maciek

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