Yes, a PTansform can have no sub-transforms, as long as it only returns its inputs. Updating the docs would be a good idea.
On Tue, Jan 12, 2021 at 1:04 PM Brian Hulette <bhule...@google.com> wrote: > A recent bug with SqlTransform on Dataflow Runner V2 [1] revealed an > interesting ambiguity in the Beam model: it's not clear if a composite > transform is allowed to have zero sub-transforms [2]. This may sound like > an academic concern, but it can happen if a PTransform returns its own > input, making it a no-op. > > I tend to agree with Kenn's comment in the jira that we should allow it. > If we don't this puts a burden on SDKs, they would need to either > a) detect when a PTransform returns one of its inputs and raise an error, > or > b) find and replace any such no-ops before generating a portable pipeline > graph > > If there aren't any objections to allowing "empty" composites I'll send a > PR to clarify this in beam_runner_api.proto > > Brian > > [1] https://issues.apache.org/jira/browse/BEAM-11614 > [2] > https://github.com/apache/beam/blob/05c8471b27e03e5611a2a13137c4a785f2d17fc9/model/pipeline/src/main/proto/beam_runner_api.proto#L152-L155 >
