Regarding Python, yes and no. Python doesn't distinguish at compile time between (1), (2), and (6), but that doesn't mean it isn't part of the public API and people might start counting on it, so it's in some sense worse. We can also do (3) (which is less cumbersome in Python, either returning a tuple or a dict) or (4).
Good point about providing a simple solution (something that can be waited on at least) and allowing for with* modifiers to return more. On Wed, Jun 26, 2019 at 7:08 PM Chamikara Jayalath <chamik...@google.com> wrote: > > BTW regarding Python SDK, I think the answer to this question is simpler for > Python SDK due to the lack of types. Most examples I know just return a > PCollection from the Write transform which may or may not be ignored by > users. If the PCollection is used, the user should be aware of the element > type of the returned PCollection and should use it accordingly in subsequent > transforms. > > Thanks, > Cham > > On Wed, Jun 26, 2019 at 9:57 AM Chamikara Jayalath <chamik...@google.com> > wrote: >> >> >> >> On Wed, Jun 26, 2019 at 5:46 AM Robert Bradshaw <rober...@google.com> wrote: >>> >>> Good question. >>> >>> I'm not sure what could be done with (5) if it contains no deferred >>> objects (e.g there's nothing to wait on). >>> >>> There is also (6) return PCollection<SourceSpecificWriteResult>. The >>> advantage of (2) is that one can migrate to (1) or (6) without >>> changing the public API, while giving something to wait on without >>> promising anything about its contents. >>> >>> >>> I would probably lean towards (4) for anything that would want to >>> return multiple signals/outputs (e.g. successful vs. failed writes) >>> and view (3) as being a "cheap" but more cumbersome for the user way >>> of writing (4). In both cases, more information can be added in a >>> forward-compatible way. Technically (4) could extend (3) if one wants >>> to migrate from (3) to (4) to provide a nicer API in the future. (As >>> an aside, it would be interesting if any of the schema work that lets >>> us get rid of tuple tags for elements (e.g. join operations) could let >>> us get rid of tuple tags for PCollectionTuples (e.g. letting a POJO >>> with PCollection members be as powerful as a PCollectionTuple). >>> >>> On Wed, Jun 26, 2019 at 2:23 PM Ismaël Mejía <ieme...@gmail.com> wrote: >>> > >>> > Beam introduced in version 2.4.0 the Wait transform to delay >>> > processing of each window in a PCollection until signaled. This opened >>> > new interesting patterns for example writing to a database and when >>> > ‘fully’ done write to another database. >>> > >>> > To support this pattern an IO connector Write transform must return a >>> > type different from PDone to signal the processing of the next step. >>> > Some IOs have already started to implement this return type, but each >>> > returned type has different pros and cons so I wanted to open the >>> > discussion on this to see if we could somehow find a common pattern to >>> > suggest IO authors to follow (Note: It may be the case that there is >>> > not a pattern that fits certain use cases). >>> > >>> > So far the approaches in our code base are: >>> > >>> > 1. Write returns ‘PCollection<Void>’ >>> > >>> > This is the simplest case but if subsequent transforms require more >>> > data that could have been produced during the write it gets ‘lost’. >>> > Used by JdbcIO and DynamoDBIO. >>> > >>> > 2. Write returns ‘PCollection<?>’ >>> > >>> > We can return whatever we want but the return type is uncertain for >>> > the user in case he wants to use information from it. This is less >>> > user friendly but has the maintenance advantage of not changing >>> > signatures if we want to change the return type in the future. Used by >>> > RabbitMQIO. >>> > >>> > 3. Write returns a `PCollectionTuple` >>> > >>> > It is like (2) but with the advantage of returning an untyped tuple of >>> > PCollections so we can return more things. Used by SnsIO. >>> > >>> > 4. Write returns ‘a class that implements POutput’ >>> > >>> > This class wraps inside of the PCollections that were part of the >>> > write, e.g. SpannerWriteResult. This is useful because we can be >>> > interested on saving inside a PCollection of failed mutations apart of >>> > the ‘done’ signal. Used by BigQueryIO and SpannerIO. A generics case >>> > of this one is used by FileIO for Destinations via: >>> > ‘WriteFilesResult<DestinationT>’. >>> > >>> > 5. Write returns ‘a class that implements POutput’ with specific data >>> > (no PCollections) >>> > >>> > This is similar to (4) but with the difference that the returned type >>> > contains the specific data that may be needed next, for example not a >>> > PCollection but values like the number of rows written. Used by >>> > BigtableIO (PR in review at the moment). (This can be seen as a >>> > simpler version of 4). >> >> >> Thanks Ismaël for detailing various approaches with examples. >> >> I think current PR for BigTable returns a PCollection<BigTableWriteResult> >> from a PTransform 'WithWriteResults' that can be optionally invoked through >> a BigTableIO.Write.withWriteResults(). So this is more closer to (6) Robert >> mentioned. But (1) was also discussed as an option. PR is >> https://github.com/apache/beam/pull/7805 for anybody interested. >> >> I think (6) is less cumbersome to implement/use and allows us to easily >> extend the transform through more chaining or by changing the return >> transform through additional "with*" methods to the FooIO.Write class. >> >> Thanks, >> Cham >> >>> > >>> > I would like to have your opinions on which approach you think it is >>> > better or worse and arguments if you see other >>> > advantages/disadvantages. I am probably more in the (4) camp but I >>> > feel somehow attracted by the flexibility that the lack of strict >>> > typing brings in (2, 3) in case of changes to the public IO API (of >>> > course this can be contested too). >>> > >>> > Any other ideas, preferences, issues we may be missing?
