*~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.
> 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
>>>>>>> >>>
>>>>>>> >>>
>>>>>>> >>
>>>>>>> >
>>>>>>>
>>>>>>
