Welcome! Thank you for interest in contributing to Akka!

We follow the standard GitHub fork & pull approach to pull requests. Just fork the official repo, develop in a branch, and submit a PR!

You're always welcome to submit your PR straight away and start the discussion (without reading the rest of this wonderful doc or the README.md). The goal of these notes is to make your experience contributing to Akka as smooth and pleasant as possible. We're happy to guide you through the process once you've submitted your PR.

The Akka Community

Reach out with questions via the forum at discuss.akka.io. You may also check out these other resources.

Navigating around the project & codebase

Branches summary

Depending on which version (or sometimes module) you want to work on, you should target a specific branch as explained below:

main – active development branch of Akka 2.9.x
release-2.8 – maintenance branch of Akka 2.8.x
similarly release-2.# branches contain legacy versions of Akka

Akka contributing guidelines

These guidelines apply to all Akka projects, by which we mean both the akka/akka repository, as well as any plugins or additional repositories located under the Akka GitHub organisation.

These guidelines are meant to be a living document that should be changed and adapted as needed. We encourage changes that make it easier to achieve our goals efficiently.

General workflow

The steps below describe how to get a patch into the main development branch (main). The steps are exactly the same for everyone involved in the project, including the core team and first-time contributors.

To avoid duplicated effort, it might be good to check the issue tracker and existing pull requests for existing work.
- If there is no ticket yet, feel free to create one to discuss the problem and the approach you want to take to solve it.
Fork the project on GitHub. You'll need to create a feature-branch for your work on your fork, as this way you'll be able to submit a pull request against the mainline Akka.
Create a branch on your fork and work on the feature. For example: git checkout -b custom-headers-akka-http
- Please make sure to follow the general quality guidelines (specified below) when developing your patch.
- Please write additional tests covering your feature and adjust existing ones if needed before submitting your pull request. The validatePullRequest sbt task (explained below) may come in handy to verify your changes are correct.
- Use the verifyCodeStyle sbt task to ensure your code is properly formatted and includes the proper copyright headers.
Once your feature is complete, prepare the commit following our Creating Commits And Writing Commit Messages. For example, a good commit message would be: feat: Adding compression support for Manifests #22222 (note the reference to the ticket it aimed to resolve).
If it's a new feature or a change of behavior, document it on the akka-docs. When the feature touches Scala and Java DSL, document both the Scala and Java APIs.
Now it's finally time to submit the pull request!
- Please make sure to include a reference to the issue you're solving in the comment for the Pull Request, as this will cause the PR to be linked properly with the issue. Examples of good phrases for this are: "Resolves #1234" or "Refs #1234".
If you have not already done so, you will be asked by our CLA bot to sign the Lightbend CLA online. CLA stands for Contributor License Agreement and protects intellectual property disputes from harming the project.
If you are a first time contributor, a core member must approve the CI to run for your pull request.
Now, both committers and interested people will review your code. This process ensures that the code we merge is of the best possible quality and that no silly mistakes slip through. You're expected to follow-up on these comments by adding new commits to the same branch. The commit messages of those commits can be more loose, for example: Removed debugging using printline, as they all will be squashed into one commit before merging into the main branch.
- The community and core team are really nice people, so don't be afraid to ask follow-up questions if you didn't understand some comment or would like clarification on how to continue with a given feature. We're here to help, so feel free to ask and discuss any questions you might have during the review process!
After the review, you should fix the issues as needed (pushing a new commit for a new review, etc.), iterating until the reviewers give their approval signaled by GitHub's pull-request approval feature. Usually, a reviewer will add an LGTM comment, which means "Looks Good To Me".
- In general, a PR is expected to get 2 approvals from the team before it is merged. If the PR is trivial or under exceptional circumstances (such as most of the core team being on vacation, a PR was very thoroughly reviewed/tested and surely is correct), a single LGTM may be fine as well.
If the code change needs to be applied to other branches as well (for example, a bugfix needing to be backported to a previous version), one of the team members will either ask you to submit a PR with the same commits to the old branch or will do this for you.
- Follow the backporting steps below.
Once everything is said and done, your pull request gets merged 🎉 Your feature will be available with the next "earliest" release milestone (i.e. if backported so that it will be in release x.y.z, find the relevant milestone for that release). Of course, you will be given credit for the fix in the release stats during the release's announcement. You've made it!

The TL;DR; of the above very precise workflow version is:

Fork Akka
Hack and test on your feature (on a branch)
Document it
Submit a PR
Sign the CLA if necessary
Keep polishing it until getting the required number of approvals
Profit!

Note: Github Actions runs all the workflows for the forked project. We have filters to ensure that each action efectively runs only for the akka/akka repository, but you may also want to disable Github Actions entirely in your fork.

Backporting

Backport pull requests such as these are marked using the phrase for validation in the title to make the purpose clear in the pull request list. They can be merged once validation passes without additional review (if there are no conflicts). Using, for example: current.version 2.5.22, previous.version 2.5, milestone.version 2.6.0-M1

Label this PR with to-be-backported
Mark this PR with Milestone ${milestone.version}
Mark the issue with Milestone ${current.version}
git checkout release-${previous.version}
git pull
Create wip branch
git cherry-pick <commit>
Open PR, target release-${previous.version}
Label that PR with backport
Merge backport PR after validation (no need for full PR reviews)
Close issue.

Getting started with sbt

Akka is using the sbt build system. So the first thing you have to do is to download and install sbt. You can read more about how to do that in the sbt setup documentation.

To compile all the Akka core modules, use the compile command:

sbt compile

You can run all tests with the test command:

sbt test

If you want to deploy the artifacts to your local Ivy repository (for example, to use from an sbt project) use the publishLocal command:

sbt publishLocal

Note that in the examples above we are calling sbt compile and sbt test and so on, but sbt also has an interactive mode. If you just run sbt, you start the interactive sbt shell and enter the commands directly. This saves starting up a new JVM instance for each command and can be much faster and more convenient.

For example, building Akka as above is more commonly done like this:

% sbt
[info] Set current project to default (in build file:/.../akka/project/plugins/)
[info] Set current project to akka (in build file:/.../akka/)
> compile
...
> test
...

To run a single multi-jvm test:

sbt
project akka-cluster
MultiJvm/testOnly akka.cluster.SunnyWeather

To format the Scala source code:

sbt
akka-cluster/scalafmtAll
akka-persistence/scalafmtAll

To format the Java source code:

sbt
project akka-actor
javafmtAll

To keep the imports sorted with:

sbt
project akka-actor
sortImports

To verify code style with:

sbt
verifyCodeStyle

To apply code style with:

sbt
applyCodeStyle

Do not use `-optimize` Scala compiler flag

Akka has not been compiled or tested with -optimize Scala compiler flag. (In sbt, you can specify compiler options in the scalacOptions key.) Strange behavior has been reported by users that have tried it.

Compiling with Graal JIT

Akka, like most Scala projects, compiles faster with the Graal JIT enabled. The easiest way to use it for compiling Akka is to:

Use a JDK > 10
Use the following JVM options for SBT e.g. by adding them to the SBT_OPTS environment variable: -XX:+UnlockExperimentalVMOptions -XX:+EnableJVMCI -XX:+UseJVMCICompiler

The `validatePullRequest` task

The Akka build includes a special task called validatePullRequest, which investigates the changes made as well as dirty (uncommitted changes) in your local working directory and figures out which projects are impacted by those changes, then running tests only on those projects.

For example, changing something in akka-actor would cause tests to be run in all projects which depend on it (e.g. akka-actor-tests, akka-stream, akka-docs etc.).

To use the task, simply type validatePullRequest, and the output should include entries like shown below:

> validatePullRequest
[info] Diffing [HEAD] to determine changed modules in PR...
[info] Detected uncomitted changes in directories (including in dependency analysis): [akka-protobuf,project]
[info] Detected changes in directories: [akka-actor-tests, project, akka-stream, akka-docs, akka-persistence]

By default, changes are diffed with the main branch when working locally. If you want to validate against a different target PR branch, you can do so by setting the PR_TARGET_BRANCH environment variable for sbt:

PR_TARGET_BRANCH=origin/example sbt validatePullRequest

If you already ran all tests and just need to check formatting and MiMa, there is a set of all* command aliases that run Test/compile (also formats), mimaReportBinaryIssues, and validateCompile (compiles multi-jvm if enabled for that project). See build.sbt or use completion to find the most appropriate one e.g. allCluster, allTyped.

Binary compatibility

Binary compatibility rules and guarantees are described in depth in the Binary Compatibility Rules section of the documentation.

Akka uses MiMa to validate the binary compatibility of incoming pull requests. If your PR fails due to binary compatibility issues, 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 whether the violation can be safely ignored or if it would indeed break binary compatibility. If the violation can be ignored add exclude statements from the mima output to a new file named <module>/src/main/mima-filters/<last-version>.backwards.excludes/<pr-or-issue>-<issue-number>-<description>.excludes, e.g. akka-actor/src/main/mima-filters/2.6.0.backwards.excludes/pr-12345-rename-internal-classes.excludes. Make sure to add a comment in the file that describes briefly why the incompatibility can be ignored.

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

if it is touching any class marked as private[akka], /** INTERNAL API*/ or similar markers
if it is concerning internal classes (often recognisable by package names like dungeon, impl, internal etc.)
if it is adding API to classes / traits which are only meant for extension by Akka itself, i.e. should not be extended by end-users
other tricky situations

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

Wire compatibility

Changes to the binary protocol of remoting, cluster and the cluster tools require great care so that it is possible to do rolling upgrades. Note that this may include older nodes communicating with a newer node, so compatibility may have to be both ways.

Since during a rolling upgrade nodes producing the 'new' format and nodes producing the 'old' format coexist, a change can require a two-release process: the first change is to add a new binary format but still use the old one. A second step then starts actually emitting the new wire format. This ensures users can complete a rolling upgrade first to the intermediate version and then another rolling upgrade to the next version.

All wire protocol changes that may concern rolling upgrades should be documented in the Rolling Update Changelog (found in akka-docs/src/main/paradox/project/rolling-update.md)

Protobuf

Akka includes a shaded version of protobuf 3 that is used for internal communication. To generate files, run protobufGenerate. The generated files are put in each project's src/main/java and need to be committed. The generated files are automatically transformed to use the shaded version of protobuf.

Generation depends on protoc 3.11.4 being on the path. See protobuf project for installation instructions, and Protobuf.scala for details of how to override the settings for generation.

Pull request requirements

For a pull request to be considered at all, it has to meet these requirements:

Regardless if the code introduces new features or fixes bugs or regressions, it must have comprehensive tests.
The code must be well documented in the Lightbend's standard documentation format (see the 'Documentation' section below).
The commit messages must properly describe the changes. See further below.
A pull request must be linked to the issue it aims to resolve in the PR's description (or comments). This can be achieved by writing "Fixes #1234" or similar in PR description.
All Lightbend projects must include Lightbend copyright notices. Each project can choose between one of two approaches:
1. All source files in the project must have a Lightbend copyright notice in the file header.
2. The Notices file for the project includes the Lightbend copyright notice and no other files contain copyright notices. See https://www.apache.org/legal/src-headers.html for instructions for managing this approach for copyrights.
Akka uses the first choice, having copyright notices in every file header. When absent, these are added automatically during sbt compile.

Additional guidelines

Some additional guidelines regarding source code are:

Keep the code DRY.
Apply the Boy Scout Rule whenever you have the chance to.
Never delete or change existing copyright notices, just add additional info.
Do not use "@author "tags since it does not encourage Collective Code Ownership. Contributors, each project should ensure that the contributors get the credit they deserve—in a text file or page on the project website and in the release notes, etc.

Documentation

All documentation is preferred to be in Lightbend's standard documentation format Paradox, which among other things, allows all code in the documentation to be externalized into compiled files and imported into the documentation.

To build the documentation locally:

sbt
akka-docs/paradox

The generated HTML documentation is in akka-docs/target/paradox/site/main/index.html.

Alternatively, use akka-docs/paradoxBrowse to open the generated docs in your default web browser.

Links to API documentation

Akka Paradox supports directives to link to the Scaladoc- and Javadoc-generated API documentation:

@apidoc[Flow] searches for the class name and creates links to Scaladoc and Javadoc (see variants in sbt-paradox-apidoc)
@scaladoc[Flow](akka.stream.scaladsl.Flow) (see Paradox docs)
@javadoc[Flow](akka.stream.javadsl.Flow) (see Paradox docs)

Scaladoc

Akka generates class diagrams for the API documentation using ScalaDoc.

Links to methods in ScalaDoc comments should be formatted [[Like#this]], because [[this]] does not work with genjavadoc, and IntelliJ warns about [[#this]]. For further hints on how to disambiguate links in ScalaDoc comments see this StackOverflow answer, though note that this syntax may not correctly render as Javadoc.

The Scaladoc tool needs the dot command from the Graphviz software package to be installed to avoid errors. You can disable the diagram generation by adding the flag -Dakka.scaladoc.diagrams=false. After installing Graphviz, make sure you add the toolset to the PATH (definitely on Windows).

JavaDoc

Akka generates JavaDoc-style API documentation using the genjavadoc sbt plugin, since the sources are written mostly in Scala.

Generating JavaDoc is not enabled by default, as it's not needed on day-to-day development as it's expected to just work. If you'd like to check if your links and formatting look good in JavaDoc (and not only in ScalaDoc), you can generate it by running:

sbt -Dakka.genjavadoc.enabled=true Javaunidoc/doc

Which will generate JavaDoc style docs in ./target/javaunidoc/index.html. This requires a JDK version 11 or later.

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.

This must be ensured by manually verifying the license for all the dependencies for the project:

Whenever a committer to the project changes a version of a dependency (including Scala) in the build file.
Whenever a committer to the project adds a new dependency.
Whenever a new release is cut (public or private for a customer).

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 "are automatically compatible with Apache 2, while the ones listed under "Category B " need additional action:

Each license in this section requires some degree of reciprocity. This may require additional action to minimize the chance that a user of an Apache product will create a derivative work of a differently-licensed portion of an Apache product without being aware of the applicable requirements.

Each project must also create and maintain a list of all dependencies and their licenses, including all their transitive dependencies. This can be done either in the documentation or in the build file next to each dependency.

Creating commits and writing commit messages

Follow these guidelines when creating public commits and writing commit messages.

While working in a feature branch or a PR branch for a long time it's useful that the work spans multiple commits, for safe points and simplify reviewing partial changes. When a PR is done it should be squashed into a single big commit which you write a good commit message for (like discussed in the following sections). For more info read this article: Git Workflow. Every commit should be able to be used in isolation, cherry picked etc.
The first line should be a descriptive sentence what the commit is doing, including the ticket number (if any). It should be possible to fully understand what the commit does—but not necessarily how it does it—by just reading this single line. We follow the "imperative present tense" style for commit messages (more info here). It's encouraged to use Conventional Commits prefixes, such as feat:, fix:, perf:, build:, chore:, docs:, but no need to use the scope syntax.

It is not ok to only list the ticket number, type "minor fix" or similar. If the commit is a small fix, then you are done. If not, go to 3.
Following the single line description should be a blank line followed by an enumerated list with the details of the commit.

Example:

feat: Adding compression support for Manifests #22222

* Details 1
* Details 2
* Details 3

Pull request validation workflow details

Akka uses GitHub Actions to validate pull requests, which involves checking code style, run tests, check binary compatibility, etc.

For existing contributors, Github Actions will run without requiring any manual intervention from a core team member.

For first time contributors, the workflow will be run after an approval from a core team member. After that, whenever new commits are pushed to the pull request, a validation job will be automatically started.

To speed up PR validation times the Akka build contains a special sbt task called validatePullRequest, which is smart enough to figure out which projects should be built if a PR only has changes in some parts of the project. For example, if your PR only touches akka-persistence, no akka-remote tests need to be run, however the task will validate all projects that depend on akka-persistence (including samples). Also, tests tagged as PerformanceTest, TimingTest, LongRunningTest, and all multi-node tests are excluded from PR validation.

You can exclude the same kind of tests in your local build by starting sbt with:

sbt -Dakka.test.tags.exclude=performance,timing,long-running -Dakka.test.multi-in-test=false

It is also possible to exclude groups of test by their names. For example:

sbt -Dakka.test.names.exclude=akka.cluster.Stress

Will exclude any tests that have names containing akka.cluster.Stress.

Source style

Sometimes it is convenient to place 'internal' classes in their own package. In such situations, we prefer 'internal' over 'impl' as a package name.

Scala style

Akka uses Scalafmt to enforce some of the code style rules.

It's recommended to enable Scalafmt formatting in IntelliJ. Use version 2019.1 or later. In Preferences > Editor > Code Style > Scala, select Scalafmt as formatter and enable "Reformat on file save". IntelliJ will then use the same settings and version as defined in .scalafmt.conf file. Then it's not needed to use sbt scalafmtAll when editing with IntelliJ.

PR validation includes checking that the Scala sources are formatted and will fail if they are not.

Akka prefers flattened imports rather than grouped, which helps reduce merge conflicts. If you are using IntelliJ IDEA, you can disable it by unchecking: Preferences -> Code Style -> Scala -> Imports -> Merge imports with the same prefix into one statement.

It's recommended to run sbt +sortImports to keep the imports sorted.

Java style

Akka uses the sbt Java Formatter plugin to format Java sources.

PR validation includes checking that the Java sources are formatted and will fail if they are not.

Code discipline opt out

In addition to formatting, the Akka build enforces code discipline through a set of compiler flags. While exploring ideas, the discipline may be more of a hindrance than a help. Therefore, it is possible to disable it by setting the system property akka.no.discipline to any non-empty string value when starting up sbt:

sbt -Dakka.no.discipline=youbet

PR validation includes the discipline flags and hence may fail if the flags were disabled during development. Make sure you compile your code at least once with discipline enabled before sending a PR.

Preferred ways to use timeouts in tests

Avoid short test timeouts since Github Actions runners may GC heavily, causing spurious test failures. GC pause or other hiccups of 2 seconds are common in our CI environment. Please note that usually giving a larger timeout does not slow down the tests, as in an expectMessage call for example it usually will complete quickly.

There are a number of ways timeouts can be defined in Akka tests. The following ways to use timeouts are recommended (in order of preference):

remaining is the first choice (requires within block)
remainingOrDefault is the second choice
3.seconds is the third choice if not using testkit
lower timeouts must come with a very good reason (e.g. Awaiting on a known to be "already completed" Future)

Special care should be given to expectNoMessage calls, which indeed will wait for the entire timeout before continuing. Therefore a shorter timeout should be used in those, for example 200.millis or 300.millis. Prefer the method without timeout parameter, which will use the configured expect-no-message-default timeout.

You can read up on remaining and friends in TestKit.scala.

Contributing modules

For external contributions of entire features, the normal way is to establish it as a stand-alone project first, to show that there is a need for the feature. If there is enough interested, the next step would be to add it to Akka as an "may change"-feature (possibly in a new subproject) and marking it's public api with the ApiMayChange annotation, then when the feature is hardened, well documented and tested it becomes an officially supported Akka feature.

List of Akka features marked as may change

Java APIs in Akka

Akka aims to keep 100% feature parity between Java and Scala. 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:

Keep entry points separated in javadsl and scaladsl unless changing existing APIs which for historical and binary compatibility reasons do not have this subdivision.
Have methods in the javadsl package delegate to the methods in the Scala API, or the common internal implementation. For example, the Akka Stream Scala instances have a .asJava method to convert to the akka.stream.javadsl counterparts.
When using Scala object instances, offer a getInstance() method. See akka.Done for an example.
When the Scala API contains an apply method, use create or of for Java users.
Do not nest Scala objects to more than two levels.
Do not define traits nested in other classes or objects deeper than one level.
Be careful to convert values within data structures (eg. for scala.Long vs. java.lang.Long, use scala.Long.box(value))
Complement any methods with Scala collections with a Java collection version
Use the akka.japi.Pair class to return tuples
If the underlying Scala code requires an ExecutionContext, make the Java API take an Executor and use ExecutionContext.fromExecutor(executor) for conversion.
Make use of scala-java8-compat conversions, see GitHub (eg. scala.compat.java8.FutureConverters to translate Futures to CompletionStages). Note that we cannot upgrade to a newer version scala-java8-compat because of binary compatibility issues.
Make sure there are Java tests or sample code touching all parts of the API
Do not use lower type bounds: trait[T] { def method[U >: Something]: U } as they do not work with Java
Provide getX style accessors for values in the Java APIs
Place classes not part of the public APIs in a shared internal package. This package can contain implementations of both Java and Scala APIs. Make such classes private[akka] and also, since that becomes public from Java's point of view, annotate with @InternalApi and add a scaladoc saying INTERNAL API
Traits that are part of the Java API should only be used to define pure interfaces, as soon as there are implementations of methods, prefer abstract class.
Any method definition in a class that will be part of the Java API should not use any default parameters, as they will show up ugly when using them from Java. Use plain old method overloading instead.

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` (use `akka.util.JavaDurationConverters`)
`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>`

Contributing new Akka Streams operators

Documentation of Akka Streams operators is automatically enforced. If a method exists on Source / Sink / Flow, or any other class listed in project/StreamOperatorsIndexGenerator.scala, it must also have a corresponding documentation page under akka-docs/src/main/paradox/streams/operators/....

Akka Streams operators' consistency is enforced by ConsistencySpec, normally an operator should exist on both Source / SubSource, Flow / SubFlow, Sink / SubSink.

The pages structure is well-defined and must be the same on all documentation pages. Please refer to any neighbouring docs pages in there to see the pattern in action. In general the page must consist of:

the title, including where the operator is defined (e.g. ActorFlow.ask or Source.map)
a short explanation of what this operator does, 1 sentence is optimal
an image explaining the operator more visually (whenever possible)
a link to the operators' "category" (these are listed in akka-docs/src/main/paradox/categories)
the method signature snippet (use the built in directives to generate it)
a longer explanation about the operator and its exact semantics (when it pulls, cancels, signals elements)
at least one usage example

By using this structure, the surrounding infrastructure will generate the index pages, so you do not need to maintain the index or category pages manually.

Adding new top-level objects/classes containing operators

In case you are adding not only a new operator but also a new class/object, you need to add it to the project/StreamOperatorsIndexGenerator.scala so it can be included in the automatic docs generation and enforcing the existence of those docs.

Supporting infrastructure

Reporting security issues

If you have found an issue in an Akka project that might have security implications, you can report it to security@lightbend.com. We will make sure those will get handled with priority. Thank you for your responsible disclosure!

Continuous integration

Akka currently uses Github Actions to run continuous integration. There are workflows for different purposes, such as:

Validating pull requests
Nightly builds
Run a larger group of tests when pushing code to main branch.

Anyone can propose new changes to our CI workflows, and we will gladly review them as we do for regular pull-requests.

Files

CONTRIBUTING.md

Latest commit

History