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Contributing to Akka

In case of questions about the contribution process or for discussion of specific issues please visit the akka/dev gitter chat.

Infrastructure

Typesafe Project & Developer Guidelines

These guidelines are meant to be a living document that should be changed and adapted as needed. We encourage changes that makes it easier to achieve our goals in an efficient way.

These guidelines mainly applies to Typesafe’s “mature” projects - not necessarily to projects of the type ‘collection of scripts’ etc.

General Workflow

This is the process for committing code into master. There are of course exceptions to these rules, for example minor changes to comments and documentation, fixing a broken build etc.

  1. Make sure you have signed the Typesafe CLA, if not, sign it online.

  2. Before starting to work on a feature or a fix, make sure that:

    1. There is a ticket for your work in the project's issue tracker. If not, create it first.
    2. The ticket has been scheduled for the current milestone.
    3. The ticket is estimated by the team.
    4. The ticket have been discussed and prioritized by the team.

  3. You should always perform your work in a Git feature branch. The branch should be given a descriptive name that explains its intent. Some teams also like adding the ticket number and/or the GitHub user ID to the branch name, these details is up to each of the individual teams.

    Akka prefers the committer name as part of the branch name, the ticket number is optional.

  4. When the feature or fix is completed you should open a Pull Request on GitHub.

  5. The Pull Request should be reviewed by other maintainers (as many as feasible/practical). Note that the maintainers can consist of outside contributors, both within and outside Typesafe. Outside contributors (for example from EPFL or independent committers) are encouraged to participate in the review process, it is not a closed process.

  6. After the review you should fix the issues as needed (pushing a new commit for new review etc.), iterating until the reviewers give their thumbs up.

    When the branch conflicts with its merge target (either by way of git merge conflict or failing CI tests), do not merge the target branch into your feature branch. Instead rebase your branch onto the target branch. Merges complicate the git history, especially for the squashing which is necessary later (see below).

  7. Once the code has passed review the Pull Request can be merged into the master branch. For this purpose the commits which were added on the feature branch should be squashed into a single commit. This can be done using the command git rebase -i master (or the appropriate target branch), picking the first commit and squashing all following ones.

    Also make sure that the commit message conforms to the syntax specified below.

  8. If the code change needs to be applied to other branches as well, create pull requests against those branches which contain the change after rebasing it onto the respective branch and await successful verification by the continuous integration infrastructure; then merge those pull requests.

    Please mark these pull requests with (for validation) in the title to make the purpose clear in the pull request list.

  9. Once everything is said and done, associate the ticket with the “earliest” release milestone (i.e. if back-ported so that it will be in release x.y.z, find the relevant milestone for that release) and close it.

Pull Request Requirements

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

  1. Live up to the current code standard:

  2. Regardless if the code introduces new features or fixes bugs or regressions, it must have comprehensive tests.

  3. The code must be well documented in the Typesafe's standard documentation format (see the ‘Documentation’ section below).

  4. The commit messages must properly describe the changes, see further below.

  5. All Typesafe projects must include Typesafe copyright notices. Each project can choose between one of two approaches:

    1. All source files in the project must have a Typesafe copyright notice in the file header.
    2. The Notices file for the project includes the Typesafe copyright notice and no other files contain copyright notices. See http://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.

    Other guidelines to follow for copyright notices:

    • Use a form of Copyright (C) 2011-2015 Typesafe Inc. <http://www.typesafe.com>, where the start year is when the project or file was first created and the end year is the last time the project or file was modified.
    • Never delete or change existing copyright notices, just add additional info.
    • Do not use @author tags since it does not encourage Collective Code Ownership. However, each project should make sure that the contributors gets the credit they deserve—in a text file or page on the project website and in the release notes etc.

If these requirements are not met then the code should not be merged into master, or even reviewed - regardless of how good or important it is. No exceptions.

Whether or not a pull request (or parts of it) shall be back- or forward-ported will be discussed on the pull request discussion page, it shall therefore not be part of the commit messages. If desired the intent can be expressed in the pull request description.

Continuous Integration

Each project should be configured to use a continuous integration (CI) tool (i.e. a build server à la Jenkins). Typesafe has a Jenkins server farm that can be used. The CI tool should, on each push to master, build the full distribution and run all tests, and if something fails it should email out a notification with the failure report to the committer and the core team. The CI tool should also be used in conjunction with a Pull Request validator (discussed below).

Documentation

All documentation should be generated using the sbt-site-plugin, or publish artifacts to a repository that can be consumed by the Typesafe stack.

All documentation must abide by the following maxims:

  • Example code should be run as part of an automated test suite.
  • Version should be programmatically specifiable to the build.
  • Generation should be completely automated and available for scripting.
  • Artifacts that must be included in the Typesafe stack should be published to a maven “documentation” repository as documentation artifacts.

All documentation is preferred to be in Typesafe's standard documentation format reStructuredText compiled using Typesafe's customized Sphinx based documentation generation system, which among other things allows all code in the documentation to be externalized into compiled files and imported into the documentation.

For more info, or for a starting point for new projects, look at the Typesafe Documentation Template project.

For larger projects that have invested a lot of time and resources into their current documentation and samples scheme (like for example Play), it is understandable that it will take some time to migrate to this new model. In these cases someone from the project needs to take the responsibility of manual QA and verifier for the documentation and samples.

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:

  1. Whenever a committer to the project changes a version of a dependency (including Scala) in the build file.
  2. Whenever a committer to the project adds a new dependency.
  3. 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 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.

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

Work In Progress

It is ok to work on a public feature branch in the GitHub repository. Something that can sometimes be useful for early feedback etc. If so then it is preferable to name the branch accordingly. This can be done by either prefix the name with wip- as in ‘Work In Progress’, or use hierarchical names like wip/.., feature/.. or topic/... Either way is fine as long as it is clear that it is work in progress and not ready for merge. This work can temporarily have a lower standard. However, to be merged into master it will have to go through the regular process outlined above, with Pull Request, review etc..

Also, to facilitate both well-formed commits and working together, the wip and feature/topic identifiers also have special meaning. Any branch labelled with wip is considered “git-unstable” and may be rebased and have its history rewritten. Any branch with feature/topic in the name is considered “stable” enough for others to depend on when a group is working on a feature.

Creating Commits And Writing Commit Messages

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

  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 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. Every commit should be able to be used in isolation, cherry picked etc.

  2. First line should be a descriptive sentence what the commit is doing. 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 is not ok to only list the ticket number, type "minor fix" or similar. In order to help with automatic filtering of the commit history (generating ChangeLogs, writing the migration guide, code archaeology) we use the following encoding:

    • the first character is either '!' (for breaking API changes), '+' (for non-breaking API additions) or '=' (for API-neutral commits)
    • then follows a comma-separated list of module abbreviations, formed by using the first three letters of the module name (the “akka-” prefix being stripped off), e.g. act, clu, doc; it is intentional that akka-actor-tests receives the same abbreviation as akka-actor. For commits modifying the project itself (sbt files or anything in project/) please use pro.
    • then follows a space character, a hash sign '#' and the ticket number
    • the rest of the line (usually there are 64 characters left) makes up the textual summary

    If the commit is a small fix, then you are done. If not, go to 3.

  3. Following the single line description should be a blank line followed by an enumerated list with the details of the commit.

  4. Add keywords for your commit (depending on the degree of automation we reach, the list may change over time):

    • Review by @gituser - if you want to notify someone on the team. The others can, and are encouraged to participate.

Example:

+act #2731 Adding monadic API to Future

* Details 1
* Details 2
* Details 3

How To Enforce These Guidelines?

Make Use of Pull Request Validator

Akka uses Jenkins GitHub pull request builder plugin that automatically merges the code, builds it, runs the tests and comments on the Pull Request in GitHub.

Upon a submission of a Pull Request the Github pull request builder plugin will post a following comment:

Can one of the repo owners verify this patch?

This requires a member from a core team to start Pull Request validation process by posting comment consisting only of OK TO TEST. From now on, whenever new commits are pushed to the Pull Request, a validation job will be automaticaly started and the results of the validation posted to the Pull Request.

A Pull Request validation job can be started manually by posting PLS BUILD comment on the Pull Request.

In order 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 and the likes of it are excluded from PR validation.

In order to force the validatePullRequest task to build the entire project, regardless of dependency analysis of a PRs changes one can use the special PLS BUILD ALL command (typed in a comment on github, on the Pull Request), which will cause the validator to test all projects.

Source style

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

Contributing Modules

For external contributions of entire features, the normal way is to establish it as a stand-alone feature first, to show that there is a need for the feature. The next step would be to add it to Akka as an "experimental feature" (in the akka-contrib subproject), then when the feature is hardened, well documented and tested it becomes an officially supported Akka feature.

List of experimental Akka features