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Some advice for Alpakka contributors

Reference connector

To inspect how all of the below listed guidelines are to be implemented in practice, take a look at the reference connector. Feel free to use the reference connector as a starting point for new connectors.

API design

Public factory methods

Depending on the technology you integrate with Akka Streams and Alpakka you'll create Sources, Flows and Sinks. Regardless on how they are implemented make sure that you create the relevant Sources, Sinks and Flows APIs so they are simple and easy to use.


Reference connector model classes

When designing Flows, consider adding an extra field to the in- and out-messages which is passed through. A common use case we see, is committing a Kafka offset after passing data to another system.

Implementing the Java API in Scala

Reference connector Java API factory methods

Alpakka, same as Akka, aims to keep 100% feature parity between the various language DSLs. Implementing even the API for Java in Scala has proven the most viable way to do it, as long as you keep the following in mind:

  1. Keep entry points separated in javadsl and scaladsl

  2. Provide factory methods for Sources, Flows and Sinks in the javadsl package wrapping all the methods in the Scala API. The Akka Stream Scala instances have a .asJava method to convert to the counterparts.

  3. When using Scala object instances, offer a getInstance() method and add a sealed abstract class (to support 2.11) to get the return type.

  4. When the Scala API contains an apply method, use create for Java users.

  5. Do not nest Scala objects more than two levels (as access from Java becomes weird)

  6. Be careful to convert values within data structures (eg. for scala.Long vs. java.lang.Long, use

  7. When compiling with both Scala 2.11 and 2.12, some methods considered overloads in 2.11, become ambiguous in 2.12 as both may be functional interfaces.

  8. Complement any methods with Scala collections with a Java collection version

  9. Use the akka.japi.Pair class to return tuples

  10. If the underlying Scala code requires an ExecutionContext, make the Java API take an Executor and use ExecutionContext.fromExecutor(executor) for conversion.

  11. Make use of scala-java8-compat conversions, see GitHub (eg. scala.compat.java8.FutureConverters to translate Futures to CompletionStages).

Overview of Scala types and their Java counterparts

Scala Java
scala.Option[T] java.util.Optional<T> (OptionalDouble, ...)
scala.collection.immutable.Seq[T] java.util.List<T>
scala.concurrent.Future[T] java.util.concurrent.CompletionStage<T>
scala.concurrent.Promise[T] java.util.concurrent.CompletableFuture<T>
scala.concurrent.duration.FiniteDuration java.time.Duration

Functions, if you want to allow the function to throw checked exceptions (like Scala functions - the infrastructure should recover from those gracefully) or leverage variance in the parameter or return types:

Scala Java
T => Unit akka.japi.function.Procedure<T>
() => R (scala.Function0[R]) akka.japi.function.Creator<R>
T => R (scala.Function1[T, R]) akka.japi.function.Function<T, R>

Functions, if you don't want to allow the function to throw checked exceptions (unlike Scala functions):

Scala Java
T => Unit java.util.function.Consumer<T>
() => R (scala.Function0[R]) java.util.function.Supplier<R>
T => R (scala.Function1[T, R]) java.util.function.Function<T, R>


Reference connector settings classes

Most technologies will have a couple of configuration settings that will be needed for several Sinks, Flows, or Sinks. Create case classes collecting these settings instead of passing them in every method.

Create a default instance in the companion object with good defaults which can be updated via withXxxx methods.

Add withXxxx methods to specify certain fields in the settings instance.

In case you see the need to support reading the settings from Config, offer a method taking the Config instance so that the user can apply a proper namespace. Refrain from using as Config prefix, prefer alpakka as root namespace.

Implementation details

Evolving APIs with binary compatibility

All Akka APIs aim to evolve in a binary compatible way within minor versions.

  1. Do not use any default arguments

  2. Do not use case classes (as the public copy method relies on default arguments)

  3. To generate a case class replacement, consider using Kaze Class

See Binary Compatibilty Rules in the Akka documentation.

See Binary Compatibility for library authors

Use MigrationManager (MiMa) to validate, if versions are binary compatible. See Binary compatibility (MiMa) in for details.

External Dependencies

All the external runtime dependencies for the project, including transitive dependencies, must have an open source license that is equal to, or compatible with, Apache 2.

Which licenses are compatible with Apache 2 are defined in this doc, where you can see that the licenses that are listed under Category A automatically compatible with Apache 2, while the ones listed under Category B needs additional action:

Each license in this category requires some degree of reciprocity; therefore, additional action must be taken in order to minimize the chance that a user of an Apache product will create a derivative work of a reciprocally-licensed portion of an Apache product without being aware of the applicable requirements.

Dependency licenses will be checked automatically by the sbt Whitesource plug-in.

Packages & Scoping

Use private, private[connector] and final extensively to limit the API surface.

Package Purpose Java-only part of the API, normally factories for Sources, Flows and Sinks Scala-only part of the API, normally factories for Sources, Flows and Sinks Shared API, eg. settings classes Internal implementation in separate package

Graph stage checklist

Reference connector operator implementations

  • Keep mutable state within the GraphStageLogic only
  • Open connections in preStart
  • Release resources in postStop
  • Fail early on configuration errors
  • Make sure the code is thread-safe; if in doubt, please ask!
  • No Blocking At Any Time -- in other words, avoid blocking whenever possible and replace it with asynchronous programming (async callbacks, stage actors)

Use of blocking APIs

Many technologies come with client libraries that only support blocking calls. Akka Stream stages that use blocking APIs should preferably be run on Akka's IODispatcher. (In rare cases you might want to allow the users to configure another dispatcher to run the blocking operations on.)

To select Akka's IODispatcher for a stage use

override protected def initialAttributes: Attributes = Attributes(ActorAttributes.IODispatcher)

When the IODispatcher is selected, you do NOT need to wrap the blocking calls in Futures or blocking.

(Issue akka/akka#25540 requests better support for selecting the correct execution context.)

Keep the code DRY

Avoid duplication of code between different Sources, Sinks and Flows. Extract the common logic to a common abstract base GraphStageLogic that is inherited by the GraphStages.

Sometimes it may be useful to provide a Sink or Source for a connector, even if the main concept is implemented as a Flow. This can be easily done by reusing the Flow implementation:

  • Source: Source.maybe.viaMat(MyFlow)(Keep.right)
  • Sink: MyFlow.toMat(Sink.ignore)(Keep.right)

You do not need to expose every configuration a Flow offers this way -- Focus on the most important ones.


Reference connector Scala and Java tests

Write tests for all public methods and possible settings.

Use Docker containers for any testing needed by your application. The container's configuration can be added to the docker-compose.yml file. Please ensure that you limit the amount of resources used by the containers.


Reference connector paradox documentation

Using Paradox syntax (which is very close to markdown), create or complement the documentation in the docs module. Prepare code snippets to be integrated by Paradox in the tests. Such example should be part of real tests and not in unused methods.

Use ScalaDoc if you see the need to describe the API usage better than the naming does. The @apidoc Paradox directive automatically creates links to the corresponding Scaladoc page for both scaladsl and javadsl. Be sure to add a $ at the end of the name if you point to an object. @apidoc[AmqpSink$] will link to akka/stream/alpakka/amqp/scaladsl/AmqpSink$.html when viewing "Scala" and akka/stream/alpakka/amqp/javadsl/AmqpSink$.html for "Java".

Run `sbt docs/previewSite` to generate reference and API docs, start an embedded web-server, and open a tab to the generated documentation while developing.