He-Pin opened a new pull request, #3127:
URL: https://github.com/apache/pekko/pull/3127
### Motivation
`alsoTo` uses `Broadcast[Out](2, eagerCancel = true)` internally. When the
side sink fails or cancels, the entire stream is terminated. Users cannot
isolate the side sink — for example, a fire-and-forget logging sink should not
kill the main business stream when the logging destination is temporarily
unavailable.
Fixes #3104.
### Modification
Add a new `alsoTo(sink, propagateCancellation: Boolean)` overload to both
Scala and Java DSLs:
- When `propagateCancellation = true` (default), behavior is identical to
existing `alsoTo`
- When `propagateCancellation = false`, a new `ResilientAlsoTo` GraphStage
is used that:
- Backpressures when either output backpressures (same contract as
`alsoTo`/`Broadcast`)
- When the side sink cancels or fails, logs a warning and continues
forwarding to main downstream only
- When the main downstream cancels, cancels the side sink normally
Changes:
- New `ResilientAlsoTo` GraphStage in `Graph.scala`
- `alsoTo(Graph, Boolean)` and `alsoToMat(Graph, Boolean)(matF)` overloads
in `FlowOps`/`FlowOpsMat`
- Java DSL overloads: `Flow`, `Source`, `SubFlow`, `SubSource`
- Java DSL `alsoToMat(Graph, Boolean, Function2)` overloads: `Flow`, `Source`
- `FlowWithContext`/`SourceWithContext` overrides
- `DefaultAttributes.resilientAlsoTo` for stage naming
### Result
Users can now use `alsoTo(sink, propagateCancellation = false)` to
fire-and-forget to a side sink without risking main stream termination. Default
behavior is unchanged and fully backwards-compatible.
### Tests
- `FlowAlsoToSpec`: 11/11 passed — covers both propagation modes, upstream
failure, backpressure, side cancellation scenarios
- `DslConsistencySpec`: 12/12 passed — Scala/Java DSL consistency verified
- `FlowAlsoToAllSpec`: 2/2 passed — no regression
```
sbt "stream-tests / Test / testOnly
org.apache.pekko.stream.scaladsl.FlowAlsoToSpec"
```
### References
Fixes #3104
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