Thanks Fabian for such detailed explanation.
I am using a datset in between so i guess csv is read once. Now to my real
issue i have 6 task managers each having 4 cores and i have 2 slots per
task manager.
Now my csv file is jus 1 gb and i create table and transform to dataset and
then run 15 different filters and extra processing which all run in almost
parallel.
However it fails with error no space left on device on one of the task
manager. Space on each task manager is 32 gb in /tmp. So i am not sure why
it is running out of space. I do use some joins with othrr tables but those
are few megabytes.
So i was assuming that somehow all parallel executions were storing data in
/tmp and were filling it.
So i would like to know wht could be filling space.
Thanks
On 19 Feb 2018 10:10 am, "Fabian Hueske" <fhue...@gmail.com> wrote:
Hi,
this works as follows.
- Table API and SQL queries are translated into regular DataSet jobs
(assuming you are running in a batch ExecutionEnvironment).
- A query is translated into a sequence of DataSet operators when you 1)
transform the Table into a DataSet or 2) write it to a TableSink. In both
cases, the optimizer is invoked and recursively goes back from the
converted/emitted Table back to its roots, i.e., a TableSource or a
DataSet.
This means, that if you create a Table from a TableSource and apply
multiple filters on it and write each filter to a TableSink, the CSV file
will be read 10 times, filtered 10 times and written 10 times. This is not
efficient, because, you could also just read the file once and apply all
filters in parallel.
You can do this by converting the Table that you read with a TableSource
into a DataSet and register the DataSet again as a Table. In that case, the
translations of all TableSinks will stop at the DataSet and not include the
TableSource which reads the file.
The following figures illustrate the difference:
1) Without DataSet in the middle:
TableSource -> Filter1 -> TableSink1
TableSource -> Filter2 -> TableSink2
TableSource -> Filter3 -> TableSink3
2) With DataSet in the middle:
/-> Filter1 -> TableSink1
TableSource -<-> Filter2 -> TableSink2
\-> Filter3 -> TableSink3
I'll likely add a feature to internally translate an intermediate Table to
make this a bit easier.
The underlying problem is that the SQL optimizer cannot translate queries
with multiple sinks.
Instead, each sink is individually translated and the optimizer does not
know that common execution paths could be shared.
Best,
Fabian
2018-02-19 2:19 GMT+01:00 Darshan Singh <darshan.m...@gmail.com>:
> Thanks for reply.
>
> I guess I am not looking for alternate. I am trying to understand what
> flink does in this scenario and if 10 tasks ar egoing in parallel I am sure
> they will be reading csv as there is no other way.
>
> Thanks
>
> On Mon, Feb 19, 2018 at 12:48 AM, Niclas Hedhman <nic...@hedhman.org>
> wrote:
>
>>
>> Do you really need the large single table created in step 2?
>>
>> If not, what you typically do is that the Csv source first do the common
>> transformations. Then depending on whether the 10 outputs have different
>> processing paths or not, you either do a split() to do individual
>> processing depending on some criteria, or you just have the sink put each
>> record in separate tables.
>> You have full control, at each step along the transformation path whether
>> it can be parallelized or not, and if there are no sequential constraints
>> on your model, then you can easily fill all cores on all hosts quite easily.
>>
>> Even if you need the step 2 table, I would still just treat that as a
>> split(), a branch ending in a Sink that does the storage there. No need to
>> read records from file over and over again, nor to store them first in step
>> 2 table and read them out again.
>>
>> Don't ask *me* about what happens in failure scenarios... I have myself
>> not figured that out yet.
>>
>> HTH
>> Niclas
>>
>> On Mon, Feb 19, 2018 at 3:11 AM, Darshan Singh <darshan.m...@gmail.com>
>> wrote:
>>
>>> Hi I would like to understand the execution model.
>>>
>>> 1. I have a csv files which is say 10 GB.
>>> 2. I created a table from this file.
>>>
>>> 3. Now I have created filtered tables on this say 10 of these.
>>> 4. Now I created a writetosink for all these 10 filtered tables.
>>>
>>> Now my question is that are these 10 filetered tables be written in
>>> parallel (suppose i have 40 cores and set up parallelism to say 40 as well.
>>>
>>> Next question I have is that the table which I created form the csv file
>>> which is common wont be persisted by flink internally rather for all 10
>>> filtered tables it will read csv files and then apply the filter and write
>>> to sink.
>>>
>>> I think that for all 10 filtered tables it will read csv again and again
>>> in this case it will be read 10 times. Is my understanding correct or I am
>>> missing something.
>>>
>>> What if I step 2 I change table to dataset and back?
>>>
>>> Thanks
>>>
>>
>>
>>
>> --
>> Niclas Hedhman, Software Developer
>> http://polygene.apache.org - New Energy for Java
>>
>
>
