Contributing guide

Thank you for considering to contribute to Rex! As a free and open source project, it is developed by volunteers all around the world like you.

The guidelines collected here are aimed at helping communication around contributions in order to make an efficient use of one of our most important resource: time.

Since most contributions we receive is about code, this guide also focuses a lot on them, but it's far from being the only way you can participate. Improving documentation, submitting and triaging bug reports, or spreading the word via blogs, talks and social media are all equally encouraged and welcome. Even starring the project on GitHub, or adding it to your favorites on MetaCPAN helps :)

Project guidelines

Over the course of years, many decisions were taken around Rex. We found some of the ideas coming up more often than others, so we collected them here (in no particular order).

Getting help

For both community and commercial support, please see our support page.

Please don't use the issue tracker for support questions (“how do I…?”, “why does it…?”, etc.).

GitHub issues and pull requests

The first step of any change proposal is to open an issue about it. This gives a chance to discuss the details, and to design potential solutions before spending effort on the implementation in a follow-up pull request.

To cover the vast majority of typical discussion points in advance, there are predefined templates for issues and pull requests. Please make sure to use them in order to streamline the workflow.

If something comes up that is not a good fit for the templates, that's probably already an early indicator that it should be discussed more closely. In this case please contact us first, or at least provide a reasoning about why the template had to be ignored in that specific case.

Cross platform support

Rex is expected to be able to run wherever Perl can run. This includes Linux, BSDs, Mac OS X, Windows and possibly others. Patches and guides about how to run Rex on even more platforms are more than welcome! As a general rule, running Rex is only supported on platforms which are actively maintained by their respective upstream teams.

Supported OSes

Rex is expected to be able to manage machines running various operating systems. This mainly includes Unix-like systems. Patches and guides about how to manage other operating systems are more than welcome! As a general rule, managing a machine with Rex is only supported for OSes that are actively maintained by their respective upstream teams.

Supported Perl versions

The minimum version of Perl that is supported by Rex is determined by matching the oldest version of Perl 5 that is supplied by the platforms where Rex is supported to run. Up until the retirement date of RHEL/CentOS 5 on 2017-03-31, this meant 5.8.8. Currently it is 5.10.1.

On top of the supported minimum version of Perl, the goal is to support the latest versions of all minor Perl 5 releases. That makes the full list the following:

  • 5.10.1
  • 5.12.5
  • 5.14.4
  • 5.16.3
  • 5.18.4
  • 5.20.3
  • 5.22.4
  • 5.24.4
  • 5.26.3
  • 5.28.3
  • 5.30.3
  • 5.32.0

Backwards compatibility

The goal is to remain backwards compatible within major versions of Rex (which is also implied by following Dotted Semantic Versioning rules).

To still be able to introduce new features while staying backwards compatible, Rex uses feature flags. This makes it possible to selectively opt in or out of new features, depending on the needs of the given use case. The collection of preferred settings for a specific version of Rex can also be enabled via feature flags named after the minor releases. The goal is to have pairs of feature flags for opting in and out.

Deprecation policy

Features and code paths may be dropped by following a planned deprecation procedure. In order to provide a transition period for the users, there should be warnings about the deprecation before the behavior is changed.

If an OS gets deprecated that is supported by Rex either to run on or to manage, we may try to keep it supported until it doesn't cause any problems. As soon a retired OS starts to cause bugs, or gets in the way of progress, it is a candidate to be dropped from support. In other words, we may choose to be lazy to keep the support around, but it's probably not worth putting effort actively into something that is not supported anymore even by their own creators.

In case you depend on a deprecated feature, or must use or manage a retired OS, you might be interested to get community or commercial support.

Code layout

Rex uses Perl::Tidy to format its codebase according to the rules described in .perltidyrc. All contributions are expected to be formatted using the same rules. To avoid unnecessary "tidy only" commits, we recommend to integrate formatting directly into your workflow, for example via a git pre-commit hook, or via your editor as a shortcut or automatic action.

It is important to note that the emphasis is not on the formatting rules themselves, but on having a consistent layout throughout the codebase. Since .perltidyrc is part of the repo, it can also be the subject of contributions.

Code quality

Rex uses Perl::Critic to make sure the codebase follows best practices, and conforms to the code quality rules described in .perlcriticrc.

Since .perlcriticrc is part of the repo, it can also be the subject of contributions. In fact, improving the rules and the codebase in this regard is highly welcome.

Tests should pass

Rex has two major test suites:

  • the unit tests included with the code, which are exercising various modules and features
  • the functional tests in the RexOps/rex-build repo, which are making sure Rex can manage actual VMs running various OSes

In general, when adding a new feature or when changing behavior, tests should be added too, and all previous tests should still pass.

In order to make it easier to follow the guidelines listed here, these should also have corresponding tests as well.

Have fun

It is highly important to have fun while using Rex or contributing to it. If it is not the case, then that's probably a bug somewhere, so please let us know.

Development tools

The Rex project uses Git for version control of the source code. The repository is hosted on GitHub.

It is recommended (but optional) to use a separate Perl environment for development, like the ones you can manage with Perlbrew.

The code and examples use cpanm to install modules from CPAN.

Rex uses Dist::Zilla as an authoring tool. With that, installing dependencies can be done by the following commands after cloning the source code:

dzil authordeps | cpanm
dzil listdeps | cpanm

Some of the optional dependencies might not be available on all platforms, but to install them as well, use this command:

dzil listdeps --suggests | cpanm

To install remaining OS-specific dependencies and Rex itself, run:

dzil install

To install the OS-specific dependencies only, run one of these commands depending on your OS:

  • Windows: cpanm Net::SSH2
  • non-Windows: cpanm Net::OpenSSH Net::SFTP::Foreign IO::Pty

Perltidy takes care of maintaining a consistent source code formatting.

Perlcritic makes sure the codebase keeps following best practices and minimum quality requirements.


The test suite included with the source code of Rex can be executed with prove:

prove --lib --recurse t/

Extended, author and release tests may need further dependencies, before being executed with dzil:

dzil listdeps --author --missing | cpanm
dzil test --all

Git workflow

The preferred way for sending contributions is to fork the repository on GitHub, and send pull requests against the default branch of the repository.

It is recommended to use feature branches when working on contributions. This makes it easy to separate the commits related to a specific changeset from the main line of development, while still keeping them together in one place.

Ideally, a single commit represents a single logical change, has a readable commit message, and passes tests in itself. There are many articles written on the topic, but this is a good example about how to write a git commit message.

It is generally recommended to:

  • add new breaking tests on a first commit before changing the code to fix them on a follow up commit
  • use multiple commits in a single pull request to separate logical steps, and to help understanding the changes as long as the history is still easy to follow and read
  • open draft pull requests to share the idea and ask for feedback early
  • rebase your feature branch on top of the default branch if there are new commits since the feature branch has been created
  • use follow up/clean up commits on the same PR, but then please also squash related commits together in the feature branch before merging in order to keep a tidy history (in other words, no "tidy only" or "fix typo" commits are necessary)

Contribute to this guide

If you think some of the information here is outdated, not clear enough, or have bugs, feel free to contribute to it too!

Useful resources