We follow the standard GitHub [fork & pull](https://help.github.com/articles/using-pull-requests/#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.
- [good first issue](https://github.com/akka/akka/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22) - which identifies simple entry-level tickets, such as improvements of documentation or tests. If you're not sure how to solve a ticket but would like to work on it, feel free to ask in the issue about clarification or tips.
- [help wanted](https://github.com/akka/akka/labels/help%20wanted) - identifies issues that the core team will likely not have time to work on or that are nice entry-level tickets. If you're not sure how to solve a ticket but would like to work on it, feel free to ask in the issue about clarification or tips.
- [nice-to-have (low-priority)](https://github.com/akka/akka/labels/nice-to-have%20%28low-prio%29) - are tasks which make sense but are not a very high priority (in the face of other very high priority issues). If you see something interesting in this list, a contribution would be really wonderful!
- [0 - new](https://github.com/akka/akka/labels/0%20-%20new) - is assigned when an issue is unclear on its purpose or if it is valid or not. Sometimes the additional tag `discuss` is used if they propose large-scale changes and need more discussion before moving into triaged (or being closed as invalid).
- [1 - triaged](https://github.com/akka/akka/labels/1%20-%20triaged) - roughly speaking means "this issue makes sense". Triaged issues are safe to pick up for contributing in terms of the likeliness of a patch for it being accepted. It is not recommended to start working on an issue that is not triaged.
- [2 - pick next](https://github.com/akka/akka/labels/2%20-%20pick%20next) - used to mark issues that are next up in the queue to be worked on. Sometimes it's also used to mark which PRs are expected to be reviewed/merged for the next release. The tag is non-binding and mostly used as an organisational helper.
- [3 - in progress](https://github.com/akka/akka/labels/3%20-%20in%20progress) - means someone is working on this ticket. If you see an issue that has the tag but seems inactive, it could have been an omission with removing the tag. Feel free to ping the ticket then if it's still being worked on.
- [bug](https://github.com/akka/akka/labels/bug) indicates potential production issues. Bugs take priority in being fixed above features. The core team dedicates some days to work on bugs in each sprint. Bugs which have reproducers are also great for community contributions as they're well-isolated. Sometimes we're not as lucky to have reproducers, though, then a bugfix should also include a test reproducing the original error along with the fix.
- [failed](https://github.com/akka/akka/labels/failed) indicates a CI failure (for example, from a nightly build). These tickets usually include a stacktrace + link to the failed job, and we'll add a comment when we see the same problem again. Since these tickets can either indicate tests with incorrect assumptions, or legitimate issues in the production code, we look at them periodically. When the same problem isn't seen again over a period of 6 months we assume it to be a rare flaky test or a problem that might have since been fixed, so we close the issue until it pops up again.
1. To avoid duplicated effort, it might be good to check the [issue tracker](https://github.com/akka/akka/issues) and [existing pull requests](https://github.com/akka/akka/pulls) for existing work.
- If there is no ticket yet, feel free to [create one](https://github.com/akka/akka/issues/new) to discuss the problem and the approach you want to take to solve it.
1. [Fork the project](https://github.com/akka/akka#fork-destination-box) 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.
- Please write additional tests covering your feature and adjust existing ones if needed before submitting your pull request. The `validatePullRequest` sbt task ([explained below](#the-validatepullrequest-task)) may come in handy to verify your changes are correct.
1. Once your feature is complete, prepare the commit following our [Creating Commits And Writing Commit Messages](#creating-commits-and-writing-commit-messages). For example, a good commit message would be: `Adding compression support for Manifests #22222` (note the reference to the ticket it aimed to resolve).
1. If it's a new feature or a change of behavior, document it on the [akka-docs](https://github.com/akka/akka/tree/main/akka-docs). When the feature touches Scala and Java DSL, document both the Scala and Java APIs.
- 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".
1. If you have not already done so, you will be asked by our CLA bot to [sign the Lightbend CLA](https://www.lightbend.com/contribute/cla) online. CLA stands for Contributor License Agreement and protects intellectual property disputes from harming the project.
1. If you are a first time contributor, a core member must approve the CI to run for your pull request.
1. 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!
1. 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.
1. 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.
1. Once everything is said and done, your pull request gets merged :tada: 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!
> **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](https://docs.github.com/en/github/administering-a-repository/managing-repository-settings/disabling-or-limiting-github-actions-for-a-repository) 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
Akka is using the [sbt](https://github.com/sbt/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](https://www.scala-sbt.org/1.x/docs/Getting-Started.html) documentation.
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`
[error] * method foldAsync(java.lang.Object,scala.Function2)akka.stream.scaladsl.FlowOps in trait akka.stream.scaladsl.FlowOps is present only in current version
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
Generation depends on protoc `3.9.0` being on the path. See [protobuf project](https://github.com/protocolbuffers/protobuf#protocol-compiler-installation) for installation instructions, and
1. The code must be well documented in the Lightbend's standard documentation format (see the 'Documentation' section below).
1. The commit messages must properly describe the changes. See further below.
1. A pull request must be [linked to the issue](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-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.
1. 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.
- Do not use "@author "tags since it does not encourage [Collective Code Ownership](http://www.extremeprogramming.org/rules/collective.html).
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.
All documentation is preferred to be in Lightbend's standard documentation format [Paradox](https://github.com/lightbend/paradox), which among other things, allows all code in the documentation to be externalized into compiled files and imported into the 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](https://github.com/lightbend/sbt-paradox-apidoc#examples))
*`@scaladoc[Flow](akka.stream.scaladsl.Flow)` (see [Paradox docs](https://developer.lightbend.com/docs/paradox/current/directives/linking.html#scaladoc-directive))
*`@javadoc[Flow](akka.stream.javadsl.Flow)` (see [Paradox docs](https://developer.lightbend.com/docs/paradox/current/directives/linking.html#javadoc-directive))
The Scaladoc tool needs the `dot` command from the [Graphviz](https://graphviz.org/#download) 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).
Akka generates JavaDoc-style API documentation using the [genjavadoc](https://github.com/typesafehub/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.
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](https://www.apache.org/licenses/LICENSE-2.0).
Which licenses are compatible with Apache 2 are defined in [this doc](https://www.apache.org/legal/resolved.html#category-a), 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 "](https://www.apache.org/legal/resolved.html#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.
1. If your work spans multiple local commits (for example; if you do safe point commits while working in a feature branch or work in a branch for a long time doing merges/rebases etc.) then please do not commit it all but rewrite the history by squashing the commits 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](https://sandofsky.com/workflow/git-workflow/). Every commit should be able to be used in isolation, cherry picked etc.
2. The first line should be a descriptive sentence what the commit is doing, including the ticket number. 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](https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html)).
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.
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:
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.
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.
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](https://github.com/akka/akka/blob/main/akka-testkit/src/main/scala/akka/testkit/TestKit.scala).
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,
1. 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
