Contributions to the Acts project are very welcome and feedback on the documentation is greatly appreciated.
Bug reports and feature requests
To report an issue and before starting work, please create an issue in the GitHub issue tracker. A comprehensive explanation will help the development team to respond in a timely manner. Verbalising the issue before starting work allows the other contributors to chime in and avoids disagreements how to progress.
The title should summarise the issue
Describe the issue in as much detail as possible in the comment
GitHub does not allow editing labels, assignees or setting milestone to non-members of a project with at least “Triage” permission. These will have to be set by members with Triage permission after an issue/PR is created. Guidelines regarding labels, assignees and milestone therefore only concern members of acts-project with the necessary rights and can be ignored by others.
Assign to yourself or leave empty
Choose labels as appropriate
Type of issue
Which component is affected
Mention affected version(s)
A detailed description of the bug including a recipe on how to reproduce it and any hints which may help diagnosing the problem
A detailed description of the feature request including possible use cases and benefits for other users
Make a contribution
Anyone is welcome to contribute to Acts. Below is a short description how to contribute. If you have any questions, feel free to ask acts-developers@cern for help or guidance.
Please always fork the Acts repository you want to work on and create branches only in your own fork. Once you want to share your work, create a Pull Request (PR) (for gitlab users: equivalent to merge request) to the main branch of the upstream acts-project repository. If it is not yet ready to be merged in, please create a draft pull request (by clicking on the small arrow on the green “create pull request” button) to mark it work in progress. Once you want your branch to be merged in, request a review from the reviewers team. Once a draft merge request is reviewed, it can be merged in.
To get started with git, please refer to the short introduction as well as the full git documentation. Tutorials as well as explanations of concepts and workflows with git can also be found on Atlassian.
Checklist for pull requests
Your branch has been rebased on the target branch and can be integrated through a fast-forward merge.
A detailed description of the pull request is provided.
The issue the PR closes is linked.
All CI jobs pass.
All newly introduced functions and classes have been documented properly with doxygen.
Unit tests are provided for new functionalities.
For bugfixes: a test case has been added to avoid the re-appearance of this bug in the future.
In the following a few recommendations are outlined which should help you to get familiar with development process in the Acts project.
Each development in its own branch of your private fork!
Write access for developers has been disabled for developers on acts-project. Therefore, always start by creating your own fork and creating branches there. You should start a new branch for every development and all work which is logically/conceptually linked should happen in one branch. Try to keep your branches short. This helps immensely to understand the git history if you need to look at it in the future and helps reviewers of your code. If projects are complex (e.g. large code refactoring or complex new features), you may want to use sub-branches from the main development branch.
Never, ever directly work on any “official” branch!
Though not strictly necessary and in the end it is up to you, it is strongly recommended that you never commit directly on a branch which tracks an “official” branch. As all branches are equal in git, the definition of “official” branch is quite subjective. In the Acts project you should not work directly on branches which are protected in the repository. Usually, these are the main and release/X.Y.Z branches. The benefit of this strategy is that you will never have problems to update your fork. Any git merge in your local repository on such an “official” branch will always be a fast-forward merge.
Use atomic commits!
Similarly to the concept of branches, each commit should reflect a self-contained change. Try to avoid overly large commits (bad examples are for instance mixing logical change with code cleanup and typo fixes).
Write good commit messages!
Structure your commit messages into short title (max 50 characters) and longer description (max width 72 characters)! This is best achieved by avoiding the
commit -moption. Instead write the commit message in an editor/git tool/IDE…
Describe why you did the change (git diff already tells you what has changed)!
Mention any side effects/implications/consequences!
Prefer git pull –rebase!
If you work with a colleague on a new development, you may want to include his latest changes. This is usually done by calling
git pullwhich will synchronise your local working copy with the remote repository (which may have been updated by your colleague). By default, this action creates a merge commit if you have local commits which were not yet published to the remote repository. These merge commits are considered to contribute little information to the development process of the feature and they clutter the history (read more e.g. here or here). This problem can be avoided by using
git pull --rebasewhich replays your local (un-pushed) commits on the tip of the remote branch. You can make this the default behaviour by running
git config pull.rebase true. More about merging vs rebasing can be found here.
Update the documentation!
Make sure that the documentation is still valid after your changes. Perform updates where needed and ensure integrity between the code and its documentation.
Coding style and guidelines
The Acts project uses
formatting its source code. A
.clang-format configuration file comes
with the project and should be used to automatically format the code.
Developers can use the provided Docker image to format their project or
install clang-format locally. Developers should be aware that
clang-format will behave differently for different versions, so
installing the same clang version as used in the
is recommended. There are several instructions available on how to
integrate clang-format with your favourite IDE (e.g. Xcode,
The Acts CI system will automatically check code formatting using the
provided clang-format configuration and will notify incompatible formatting.
In addition, some conventions are used in Acts code, details can be found here. For Doxygen documentation, please follow these recommendations:
Put all documentation in the header files.
///as block comment (instead of
/* ... */).
Doxygen documentation goes in front of the documented entity (class, function, (member) variable).
Document all (template) parameters using @(t)param and explain the return value for non-void functions. Mention important conditions which may affect the return value.
@remarkto specify pre-conditions.
@noteto provide additional information.
Link other related entities (e.g. functions) using
Example: Make a bugfix while working on a feature
During the development of a new feature you discover a bug which needs to be fixed. In order to not mix bugfix and feature development, the bugfix should happen in a different branch. The recommended procedure for handling this situation is the following:
Get into a clean state of your working directory on your feature branch (either by commiting open changes or by stashing them).
Checkout the branch the bugfix should be merged into (either main or release/X.Y.Z) and get the most recent version.
Create a new branch for the bugfix.
Fix the bug, write a test, update documentation etc.
Open a pull request for the bug fix.
Switch back to your feature branch.
Merge your local bugfix branch into the feature branch. Continue your feature development.
Eventually, the bugfix will be merged into main. Then, you can rebase your feature branch on main which will remove all duplicate commits related to the bugfix.
Example: Backporting a feature or bugfix
Suppose you have a bugfix or feature branch that is eventually going to
be merged in
main. You might want to have the feature/bugfix
avilable in a patch (say
0.25.1) tag. To to that, find the
corresponding release branch, for this example that would be
release/v0.25.X. You must create a dedicated branch that only
contains the commits that relate to your feature/bugfix, otherwise the
PR will contain all other commits that were merged into main since the
release was branched off. With that branch, open a PR to that branch,
and make it clear that it is a backport, also linking to a potential
equivalent PR that targets
Tips for users migrating from GitLab
The most obvious difference first: What is called Merge Request in GitLab is called Pull Request (PR) in GitHub.
Once your PR is ready to be merged, request a review by the users in the reviewers team
As Acts started enforcing using your own fork with the switch to GitHub, developers no longer have write access to the upstream repository.
The CI will fail if a PR does not yet have the required approvals.
Review other contributions
Acts requires that every pull request receives at least one review by a member of the reviewers team before being merged but anyone is welcome to contribute by commenting on code changes. You can help reviewing proposed contributions by going to the “pull requests” section of the Acts (core) GitHub repository.
As some of the guidelines recommended here require rights granted to the reviewers team, this guide specifically addresses the people in this team. The present contribution guide should serve as a good indication of what we expect from code submissions.
Approving a pull request
Does its title and description reflect its contents?
Do the automated continuous integration tests pass without problems?
Have all the comments raised by previous reviewers been addressed?
If you are confident that a pull request is ready for integration, please make it known by clicking the “Approve pull request” button of the GitHub interface. This notifies other members of the Acts team of your decision, and marks the pull request as ready to be merged.
Merging a pull request
If you have been granted write access on the Acts repository, you can merge a pull request into the Acts main branch after it has been approved.
GitHub may warn you that a “Fast-forward merge is not possible”. This warning means that the pull request has fallen behind the current Acts main branch, and should be updated through a rebase. Please notify the pull request author in order to make sure that the latest main changes do not affect the pull request, and to have it updated as appropriate.
For a PR that is behind main, a button “Update branch” may appear. This should NOT be used as it merges instead of rebasing, which is not our workflow.