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Explain MiMa use for contributors #1751

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Jun 17, 2019
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Explain MiMa use for contributors #1751

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Explain MiMa use for contributors
ennru Jun 14, 2019
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Move link to MiMa
ennru Jun 17, 2019
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2 changes: 1 addition & 1 deletion .travis.yml
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Expand Up @@ -28,7 +28,7 @@ jobs:
- env: CMD="docs/paradox"
name: "Create site with Paradox"
- env: CMD="mimaReportBinaryIssues"
name: "Check binary compatibility"
name: "Check binary compatibility (MiMa)"

- stage: test
env:
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26 changes: 26 additions & 0 deletions CONTRIBUTING.md
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Expand Up @@ -39,6 +39,32 @@ In case of questions about the contribution process or for discussion of specifi
We've collected a few notes on how we would like Alpakka modules to be designed based on what has evolved so far.
Please have a look at our [contributor advice](contributor-advice.md).


## Binary compatibility (MiMa)

Binary compatibility rules and guarantees are described in depth in the [Binary Compatibility Rules
](https://doc.akka.io/docs/akka/snapshot/common/binary-compatibility-rules.html) section of the Akka documentation.

Akka projects use [MiMa](https://github.com/lightbend/mima) (which originally was called Lightbend Migration Manager) to
validate binary compatibility of incoming pull requests. In the Travis build step "Check binary compatibility", you may see
an error like this:

```
[info] akka-stream: found 1 potential binary incompatibilities while checking against com.typesafe.akka:akka-stream_2.12:2.4.2 (filtered 222)
[error] * method foldAsync(java.lang.Object,scala.Function2)akka.stream.scaladsl.FlowOps in trait akka.stream.scaladsl.FlowOps is present only in current version
[error] filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.stream.scaladsl.FlowOps.foldAsync")
```

In such situations it's good to consult with a core team member if the violation can be safely ignored (by adding the above snippet to `<module>/src/main/mima-filters/<latest-version>.backwards.excludes`), or if it would indeed break binary compatibility.

Situations when it may be fine to ignore a MiMa issued warning include:

- if it is concerning internal classes in package `impl` or `internal`
- add eg. `1.0.x.backwards.excludes` with `ProblemFilters.exclude[Problem]("akka.stream.alpakka.<module>.impl.*")` to ignore all internal changes
- if it is touching anything marked as `@InternalApi`

The binary compatibility of the current changes can be checked by running `mimaReportBinaryIssues` in sbt.

## Pull Request Requirements

For a Pull Request to be considered at all it has to meet these requirements:
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