vgough opened a new issue, #39909:
URL: https://github.com/apache/beam/issues/39909
### What would you like to happen?
I'd like to port the Wait.On functionality of Java to the Go SDK, so that
runtime ordering constraints can be added between transforms without requiring
modification of the transforms.
I often have operations which rely on side-effects (eg database writes
having completed, GCS storage, BigTable updates, etc). In order to sequence
these operation in Go, I end up wiring in completion signals manually into
producers and then side-inputs into consumers. This is effectively a manual
implementation of `wait.On` which reduces composability of the operations since
it means the shape of the operation has to be modified for the particular
situation.
The goal would be to provide a transforms/wait package containing an `On`
method:
```go
// To finish writing each window to one database before writing it
// to another:
firstWriteResults := beam.ParDo(s, writeToFirstDB, data)
delayed := wait.On(s, data, firstWriteResults)
beam.ParDo0(s, writeToSecondDB, delayed)
```
Where `On` would look like this:
```go
// On returns col unchanged, but delays each main window until its mapped
window
// has closed in every signal.
//
// With no signals, On validates s and col and returns col directly.
Otherwise,
// it panics at pipeline construction time for an invalid scope or
PCollection,
// CoGBK inputs, or session-windowed signals.
func On(s beam.Scope, col beam.PCollection, signals ...beam.PCollection)
beam.PCollection {
```
### Issue Priority
Priority: 2 (default / most feature requests should be filed as P2)
### Issue Components
- [ ] Component: Python SDK
- [ ] Component: Java SDK
- [x] Component: Go SDK
- [ ] Component: Typescript SDK
- [ ] Component: IO connector
- [ ] Component: Beam YAML
- [ ] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [ ] Component: Website
- [ ] Component: Infrastructure
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Prism Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [ ] Component: Google Cloud Dataflow Runner
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