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Contributing code to Synapse

Everyone is welcome to contribute code to projects, provided that they are willing to license their contributions under the same license as the project itself. We follow a simple 'inbound=outbound' model for contributions: the act of submitting an 'inbound' contribution means that the contributor agrees to license the code under the same terms as the project's overall 'outbound' license - in our case, this is almost always Apache Software License v2 (see LICENSE).

How to contribute

The preferred and easiest way to contribute changes is to fork the relevant project on github, and then create a pull request to ask us to pull your changes into our repo.

Some other points to follow:

Code style

Synapse's code style is documented here. Please follow it, including the conventions for the sample configuration file.

Many of the conventions are enforced by scripts which are run as part of the continuous integration system. To help check if you have followed the code style, you can run scripts-dev/ locally. You'll need python 3.6 or later, and to install a number of tools:

# Install the dependencies
pip install -U black flake8 flake8-comprehensions isort

# Run the linter script

Note that the script does not just test/check, but also reformats code, so you may wish to ensure any new code is committed first.

By default, this script checks all files and can take some time; if you alter only certain files, you might wish to specify paths as arguments to reduce the run-time:

./scripts-dev/ path/to/ path/to/ path/to/folder

Before pushing new changes, ensure they don't produce linting errors. Commit any files that were corrected.

Please ensure your changes match the cosmetic style of the existing project, and never mix cosmetic and functional changes in the same commit, as it makes it horribly hard to review otherwise.


All changes, even minor ones, need a corresponding changelog / newsfragment entry. These are managed by Towncrier.

To create a changelog entry, make a new file in the changelog.d directory named in the format of PRnumber.type. The type can be one of the following:

  • feature
  • bugfix
  • docker (for updates to the Docker image)
  • doc (for updates to the documentation)
  • removal (also used for deprecations)
  • misc (for internal-only changes)

This file will become part of our changelog at the next release, so the content of the file should be a short description of your change in the same style as the rest of the changelog. The file can contain Markdown formatting, and should end with a full stop (.) or an exclamation mark (!) for consistency.

Adding credits to the changelog is encouraged, we value your contributions and would like to have you shouted out in the release notes!

For example, a fix in PR #1234 would have its changelog entry in changelog.d/1234.bugfix, and contain content like:

The security levels of Florbs are now validated when received via the /federation/florb endpoint. Contributed by Jane Matrix.

If there are multiple pull requests involved in a single bugfix/feature/etc, then the content for each changelog.d file should be the same. Towncrier will merge the matching files together into a single changelog entry when we come to release.

How do I know what to call the changelog file before I create the PR?

Obviously, you don't know if you should call your newsfile 1234.bugfix or 5678.bugfix until you create the PR, which leads to a chicken-and-egg problem.

There are two options for solving this:

  1. Open the PR without a changelog file, see what number you got, and then add the changelog file to your branch (see Updating your pull request), or:

  2. Look at the list of all issues/PRs, add one to the highest number you see, and quickly open the PR before somebody else claims your number.

    This script might be helpful if you find yourself doing this a lot.

Sorry, we know it's a bit fiddly, but it's really helpful for us when we come to put together a release!

Debian changelog

Changes which affect the debian packaging files (in debian) are an exception to the rule that all changes require a changelog.d file.

In this case, you will need to add an entry to the debian changelog for the next release. For this, run the following command:


This will make up a new version number (if there isn't already an unreleased version in flight), and open an editor where you can add a new changelog entry. (Our release process will ensure that the version number and maintainer name is corrected for the release.)

If your change affects both the debian packaging and files outside the debian directory, you will need both a regular newsfragment and an entry in the debian changelog. (Though typically such changes should be submitted as two separate pull requests.)

Sign off

In order to have a concrete record that your contribution is intentional and you agree to license it under the same terms as the project's license, we've adopted the same lightweight approach that the Linux Kernel submitting patches process, Docker, and many other projects use: the DCO (Developer Certificate of Origin: This is a simple declaration that you wrote the contribution or otherwise have the right to contribute it to Matrix:

Developer Certificate of Origin
Version 1.1

Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
660 York Street, Suite 102,
San Francisco, CA 94110 USA

Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.

Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

(a) The contribution was created in whole or in part by me and I
    have the right to submit it under the open source license
    indicated in the file; or

(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
    license and I have the right under that license to submit that
    work with modifications, whether created in whole or in part
    by me, under the same open source license (unless I am
    permitted to submit under a different license), as indicated
    in the file; or

(c) The contribution was provided directly to me by some other
    person who certified (a), (b) or (c) and I have not modified

(d) I understand and agree that this project and the contribution
    are public and that a record of the contribution (including all
    personal information I submit with it, including my sign-off) is
    maintained indefinitely and may be redistributed consistent with
    this project or the open source license(s) involved.

If you agree to this for your contribution, then all that's needed is to include the line in your commit or pull request comment:

Signed-off-by: Your Name <>

We accept contributions under a legally identifiable name, such as your name on government documentation or common-law names (names claimed by legitimate usage or repute). Unfortunately, we cannot accept anonymous contributions at this time.

Git allows you to add this signoff automatically when using the -s flag to git commit, which uses the name and email set in your and git configs.

Continuous integration and testing

Buildkite will automatically run a series of checks and tests against any PR which is opened against the project; if your change breaks the build, this will be shown in GitHub, with links to the build results. If your build fails, please try to fix the errors and update your branch.

To run unit tests in a local development environment, you can use:

  • tox -e py35 (requires tox to be installed by pip install tox) for SQLite-backed Synapse on Python 3.5.
  • tox -e py36 for SQLite-backed Synapse on Python 3.6.
  • tox -e py36-postgres for PostgreSQL-backed Synapse on Python 3.6 (requires a running local PostgreSQL with access to create databases).
  • ./ for PostgreSQL-backed Synapse on Python 3.5 (requires Docker). Entirely self-contained, recommended if you don't want to set up PostgreSQL yourself.

Docker images are available for running the integration tests (SyTest) locally, see the documentation in the SyTest repo for more information.

Updating your pull request

If you decide to make changes to your pull request - perhaps to address issues raised in a review, or to fix problems highlighted by continuous integration - just add new commits to your branch, and push to GitHub. The pull request will automatically be updated.

Please avoid rebasing your branch, especially once the PR has been reviewed: doing so makes it very difficult for a reviewer to see what has changed since a previous review.

Notes for maintainers on merging PRs etc

There are some notes for those with commit access to the project on how we manage git here.


That's it! Matrix is a very open and collaborative project as you might expect given our obsession with open communication. If we're going to successfully matrix together all the fragmented communication technologies out there we are reliant on contributions and collaboration from the community to do so. So please get involved - and we hope you have as much fun hacking on Matrix as we do!