In the POutput case (4), does that mean we will have to compute all those outputs in the transform even if they aren't used?
If yes, I prefer (6) because it allows for the transform structure to be modified to either produce these additional outputs only if they will be consumed instead of having them produced all the time. On Mon, Jul 15, 2019 at 11:20 AM Ismaël Mejía <ieme...@gmail.com> wrote: > Just wanted to bring back the conversation on this subject. A quick > abstract of the discussion so far: > > We are trying to agree in the best approach for return types in Write > transforms towards some sort of ‘homogenization’ in IOs. > At the moment we mostly agree that the best approach for return types > in Writes are: > > 4. Write returns ‘a class that implements POutput’ > 6. Write returns `PCollection<SourceSpecificWriteResult>` > > There are still some details to discuss: > > 1. Is (4) somehow less composable than (6) ? > 2. Could it make sense (in the API sense) that `SourceSpecificWriteResult` > contains a PCollection too as an attribute to cover the ‘tuple’ return > type issue? > > In its last email Reuven mentioned some extra points that could change > the direction towards one of the options. Anyone else has comments / > more ideas we may be missing? > > On Thu, Jun 27, 2019 at 6:32 PM Reuven Lax <re...@google.com> wrote: > > > > This is a good question, because many sinks are logically _not_ > windowed. They aren't producing aggregations, so logically they are often > treated as if they are in the global window (and many internal window into > the global window first thing). > > > > Wait is a nice transform that reuses existing windowing, but I wonder if > there's another way to model this without relying on windowing. Essentially > you want way to track element provenance - when all results from a single > element are flushed through another transform, then trigger a second > transform. Element provenance is interesting in many other use cases as > well (e.g. debugging: given an output element, what input elements caused > it?). Maybe there's a more direct way to model this problem without trying > to use windowing to track causality? > > > > Reuven > > > > On Thu, Jun 27, 2019 at 4:38 AM Reza Rokni <r...@google.com> wrote: > >> > >> The use case of a transform waiting for a SInk or Sinks to complete is > very interesting indeed! > >> > >> Curious, if a sink internally makes use of a Global Window with > processing time triggers to push its writes, what mechanism could be used > to release a transform waiting for a signal from the Sink(s) that all > processing is done and it can move forward? > >> > >> On Thu, 27 Jun 2019 at 03:58, Robert Bradshaw <rober...@google.com> > wrote: > >>> > >>> 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? > >> > >> > >> > >> -- > >> > >> This email may be confidential and privileged. If you received this > communication by mistake, please don't forward it to anyone else, please > erase all copies and attachments, and please let me know that it has gone > to the wrong person. > >> > >> The above terms reflect a potential business arrangement, are provided > solely as a basis for further discussion, and are not intended to be and do > not constitute a legally binding obligation. No legally binding obligations > will be created, implied, or inferred until an agreement in final form is > executed in writing by all parties involved. >
