On Wed, Mar 24, 2021 at 7:29 PM Vincent Marquez <vincent.marq...@gmail.com>
wrote:
>
> *~Vincent*
>
>
> On Wed, Mar 24, 2021 at 6:07 PM Kenneth Knowles <k...@apache.org> wrote:
>
>> The reason I was checking out the code is that sometimes a natural thing
>> to output would be a summary of what was written. So each chunk of writes
>> and the final chunk written in @FinishBundle. This is, for example, what
>> SQL engines do (output # of rows written).
>>
>> You could output both the summary and the full list of written elements
>> to different outputs, and users can choose. Outputs that are never consumed
>> should be very low or zero cost.n
>>
>>
> I like this approach. I would much prefer two outputs (one of which is
> all elements written) to returning an existential/wildcard PCollection.
>
+1, this would work well too. Returning a PCollectionTuple is extensible
too, as one could add more (or better) outputs in the future without
changing the signature.
>
>
>
>> Kenn
>>
>> On Wed, Mar 24, 2021 at 5:36 PM Robert Bradshaw <rober...@google.com>
>> wrote:
>>
>>> Yeah, the entire input is not always what is needed, and can generally
>>> be achieved via
>>>
>>> input -> wait(side input of write) -> do something with the input
>>>
>>> Of course one could also do
>>>
>>> entire_input_as_output_of_wait -> MapTo(KV.of(null, null)) ->
>>> CombineGlobally(TrivialCombineFn)
>>>
>>> to reduce this to a more minimal set with at least one element per
>>> Window.
>>>
>>> The file writing operations emit the actual files that were written,
>>> which can be handy. My suggestion of PCollection<?> was just so that we can
>>> emit something usable, and decide exactly what is the most useful is later.
>>>
>>>
>>> On Wed, Mar 24, 2021 at 5:30 PM Reuven Lax <re...@google.com> wrote:
>>>
>>>> I believe that the Wait transform turns this output into a side input,
>>>> so outputting the input PCollection might be problematic.
>>>>
>>>> On Wed, Mar 24, 2021 at 4:49 PM Kenneth Knowles <k...@apache.org>
>>>> wrote:
>>>>
>>>>> Alex's idea sounds good and like what Vincent maybe implemented. I am
>>>>> just reading really quickly so sorry if I missed something...
>>>>>
>>>>> Checking out the code for the WriteFn<T> I see a big problem:
>>>>>
>>>>> @Setup
>>>>> public void setup() {
>>>>> writer = new Mutator<>(spec, Mapper::saveAsync, "writes");
>>>>> }
>>>>>
>>>>> @ProcessElement
>>>>> public void processElement(ProcessContext c) throws
>>>>> ExecutionException, InterruptedException {
>>>>> writer.mutate(c.element());
>>>>> }
>>>>>
>>>>> @Teardown
>>>>> public void teardown() throws Exception {
>>>>> writer.close();
>>>>> writer = null;
>>>>> }
>>>>>
>>>>> It is only in writer.close() that all async writes are waited on. This
>>>>> needs to happen in @FinishBundle.
>>>>>
>>>>> Did you discover this when implementing your own Cassandra.Write?
>>>>>
>>>>> Until you have waited on the future, you should not output the element
>>>>> as "has been written". And you cannot output from the @TearDown method
>>>>> which is just for cleaning up resources.
>>>>>
>>>>> Am I reading this wrong?
>>>>>
>>>>> Kenn
>>>>>
>>>>> On Wed, Mar 24, 2021 at 4:35 PM Alex Amato <ajam...@google.com> wrote:
>>>>>
>>>>>> How about a PCollection containing every element which was
>>>>>> successfully written?
>>>>>> Basically the same things which were passed into it.
>>>>>>
>>>>>> Then you could act on every element after its been successfully
>>>>>> written to the sink.
>>>>>>
>>>>>> On Wed, Mar 24, 2021 at 3:16 PM Robert Bradshaw <rober...@google.com>
>>>>>> wrote:
>>>>>>
>>>>>>> On Wed, Mar 24, 2021 at 2:36 PM Ismaël Mejía <ieme...@gmail.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> +dev
>>>>>>>>
>>>>>>>> Since we all agree that we should return something different than
>>>>>>>> PDone the real question is what should we return.
>>>>>>>>
>>>>>>>
>>>>>>> My proposal is that one returns a PCollection<?> that consists,
>>>>>>> internally, of something contentless like nulls. This is future
>>>>>>> compatible
>>>>>>> with returning something more maningful based on the source source or
>>>>>>> write
>>>>>>> process itself, but at least this would be followable.
>>>>>>>
>>>>>>>
>>>>>>>> As a reminder we had a pretty interesting discussion about this
>>>>>>>> already in the past but uniformization of our return values has not
>>>>>>>> happened.
>>>>>>>> This thread is worth reading for Vincent or anyone who wants to
>>>>>>>> contribute Write transforms that return.
>>>>>>>>
>>>>>>>> https://lists.apache.org/thread.html/d1a4556a1e13a661cce19021926a5d0997fbbfde016d36989cf75a07%40%3Cdev.beam.apache.org%3E
>>>>>>>
>>>>>>>
>>>>>>> Yeah, we should go ahead and finally do something.
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> > Returning PDone is an anti-pattern that should be avoided, but
>>>>>>>> changing it now would be backwards incompatible.
>>>>>>>>
>>>>>>>> Periodic reminder most IOs are still Experimental so I suppose it is
>>>>>>>> worth to the maintainers to judge if the upgrade to return
>>>>>>>> someething
>>>>>>>> different of PDone is worth, in that case we can deprecate and
>>>>>>>> remove
>>>>>>>> the previous signature in short time (2 releases was the average for
>>>>>>>> previous cases).
>>>>>>>>
>>>>>>>>
>>>>>>>> On Wed, Mar 24, 2021 at 10:24 PM Alexey Romanenko
>>>>>>>> <aromanenko....@gmail.com> wrote:
>>>>>>>> >
>>>>>>>> > I thought that was said about returning a PCollection of write
>>>>>>>> results as it’s done in other IOs (as I mentioned as examples) that
>>>>>>>> have
>>>>>>>> _additional_ write methods, like “withWriteResults()” etc, that return
>>>>>>>> PTransform<…, PCollection<WriteResults>>.
>>>>>>>> > In this case, we keep backwards compatibility and just add new
>>>>>>>> funtionality. Though, we need to follow the same pattern for user API
>>>>>>>> and
>>>>>>>> maybe even naming for this feature across different IOs (like we have
>>>>>>>> for
>>>>>>>> "readAll()” methods).
>>>>>>>> >
>>>>>>>> > I agree that we have to avoid returning PDone for such cases.
>>>>>>>> >
>>>>>>>> > On 24 Mar 2021, at 20:05, Robert Bradshaw <rober...@google.com>
>>>>>>>> wrote:
>>>>>>>> >
>>>>>>>> > Returning PDone is an anti-pattern that should be avoided, but
>>>>>>>> changing it now would be backwards incompatible. PRs to add non-PDone
>>>>>>>> returning variants (probably as another option to the builders) that
>>>>>>>> compose well with Wait, etc. would be welcome.
>>>>>>>> >
>>>>>>>> > On Wed, Mar 24, 2021 at 11:14 AM Alexey Romanenko <
>>>>>>>> aromanenko....@gmail.com> wrote:
>>>>>>>> >>
>>>>>>>> >> In this way, I think “Wait” PTransform should work for you but,
>>>>>>>> as it was mentioned before, it doesn’t work with PDone, only with
>>>>>>>> PCollection as a signal.
>>>>>>>> >>
>>>>>>>> >> Since you already adjusted your own writer for that, it would be
>>>>>>>> great to contribute it back to Beam in the way as it was done for
>>>>>>>> other IOs
>>>>>>>> (for example, JdbcIO [1] or BigtableIO [2])
>>>>>>>> >>
>>>>>>>> >> In general, I think we need to have it for all IOs, at least to
>>>>>>>> use with “Wait” because this pattern it's quite often required.
>>>>>>>> >>
>>>>>>>> >> [1]
>>>>>>>> https://github.com/apache/beam/blob/ab1dfa13a983d41669e70e83b11f58a83015004c/sdks/java/io/jdbc/src/main/java/org/apache/beam/sdk/io/jdbc/JdbcIO.java#L1078
>>>>>>>> >> [2]
>>>>>>>> https://github.com/apache/beam/blob/ab1dfa13a983d41669e70e83b11f58a83015004c/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigtable/BigtableIO.java#L715
>>>>>>>> >>
>>>>>>>> >> On 24 Mar 2021, at 18:01, Vincent Marquez <
>>>>>>>> vincent.marq...@gmail.com> wrote:
>>>>>>>> >>
>>>>>>>> >> No, it only needs to ensure that one record seen on Pubsub has
>>>>>>>> successfully written to a database. So "record by record" is fine, or
>>>>>>>> even
>>>>>>>> "bundle".
>>>>>>>> >>
>>>>>>>> >> ~Vincent
>>>>>>>> >>
>>>>>>>> >>
>>>>>>>> >> On Wed, Mar 24, 2021 at 9:49 AM Alexey Romanenko <
>>>>>>>> aromanenko....@gmail.com> wrote:
>>>>>>>> >>>
>>>>>>>> >>> Do you want to wait for ALL records are written for Cassandra
>>>>>>>> and then write all successfully written records to PubSub or it should
>>>>>>>> be
>>>>>>>> performed "record by record"?
>>>>>>>> >>>
>>>>>>>> >>> On 24 Mar 2021, at 04:58, Vincent Marquez <
>>>>>>>> vincent.marq...@gmail.com> wrote:
>>>>>>>> >>>
>>>>>>>> >>> I have a common use case where my pipeline looks like this:
>>>>>>>> >>> CassandraIO.readAll -> Aggregate -> CassandraIO.write ->
>>>>>>>> PubSubIO.write
>>>>>>>> >>>
>>>>>>>> >>> I do NOT want my pipeline to look like the following:
>>>>>>>> >>>
>>>>>>>> >>> CassandraIO.readAll -> Aggregate -> CassandraIO.write
>>>>>>>> >>> |
>>>>>>>> >>> ->
>>>>>>>> PubsubIO.write
>>>>>>>> >>>
>>>>>>>> >>> Because I need to ensure that only items written to Pubsub have
>>>>>>>> successfully finished a (quorum) write.
>>>>>>>> >>>
>>>>>>>> >>> Since CassandraIO.write is a PTransform<A, PDone> I can't
>>>>>>>> actually use it here so I often roll my own 'writer', but maybe there
>>>>>>>> is a
>>>>>>>> recommended way of doing this?
>>>>>>>> >>>
>>>>>>>> >>> Thanks in advance for any help.
>>>>>>>> >>>
>>>>>>>> >>> ~Vincent
>>>>>>>> >>>
>>>>>>>> >>>
>>>>>>>> >>
>>>>>>>> >
>>>>>>>>
>>>>>>>
