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). 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?
