Building Ray from Source#

To contribute to the Ray repository, follow the instructions below to build from the latest master branch.


If you are only editing Python files, follow instructions for Building Ray (Python Only) to avoid long build times.

If you already followed the instructions in Building Ray (Python Only) and want to switch to the Full build in this section, you will need to first uninstall.

Fork the Ray repository#

Forking an open source repository is a best practice when looking to contribute, as it allows you to make and test changes without affecting the original project, ensuring a clean and organized collaboration process. You can propose changes to the main project by submitting a pull request to the main project’s repository.

  1. Navigate to the Ray GitHub repository.

  2. Follow these GitHub instructions, and do the following:

    1. Fork the repo using your preferred method.

    2. Clone to your local machine.

    3. Connect your repo to the upstream (main project) Ray repo to sync changes.

Prepare a Python virtual environment#

Create a virtual environment to prevent version conflicts and to develop with an isolated, project-specific Python setup.

Set up a conda environment named myenv:

conda create -c conda-forge python=3.9 -n myenv

Activate your virtual environment to tell the shell/terminal to use this particular Python:

conda activate myenv

You need to activate the virtual environment every time you start a new shell/terminal to work on Ray.

Use Python’s integrated venv module to create a virtual environment called myenv in the current directory:

python -m venv myenv

This contains a directory with all the packages used by the local Python of your project. You only need to do this step once.

Activate your virtual environment to tell the shell/terminal to use this particular Python:

source myenv/bin/activate

You need to activate the virtual environment every time you start a new shell/terminal to work on Ray.

Creating a new virtual environment can come with older versions of pip and wheel. To avoid problems when you install packages, use the module pip to install the latest version of pip (itself) and wheel:

python -m pip install --upgrade pip wheel

Building Ray (Python Only)#


Unless otherwise stated, directory and file paths are relative to the project root directory.

RLlib, Tune, Autoscaler, and most Python files do not require you to build and compile Ray. Follow these instructions to develop Ray’s Python files locally without building Ray.

  1. Make sure you have a clone of Ray’s git repository as explained above.

  2. Make sure you activate the Python (virtual) environment as described above.

  3. Pip install the latest Ray wheels. See Daily Releases (Nightlies) for instructions.

# For example, for Python 3.8:
pip install -U
  1. Replace Python files in the installed package with your local editable copy. We provide a simple script to help you do this: python python/ray/ Running the script will remove the ray/tune, ray/rllib, ray/autoscaler dir (among other directories) bundled with the ray pip package, and replace them with links to your local code. This way, changing files in your git clone will directly affect the behavior of your installed Ray.

# This replaces `<package path>/site-packages/ray/<package>`
# with your local `ray/python/ray/<package>`.
python python/ray/


[Advanced] You can also optionally skip creating symbolic link for directories of your choice.

# This links all folders except "_private" and "dashboard" without user prompt.
python -y --skip _private dashboard


Do not run pip uninstall ray or pip install -U (for Ray or Ray wheels) if setting up your environment this way. To uninstall or upgrade, you must first rm -rf the pip-installation site (usually a directory at the site-packages/ray location), then do a pip reinstall (see the command above), and finally run the above script again.

# To uninstall, delete the symlinks first.
rm -rf <package path>/site-packages/ray # Path will be in the output of ``.
pip uninstall ray # or `pip install -U <wheel>`

Preparing to build Ray on Linux#


If you are only editing Tune/RLlib/Autoscaler files, follow instructions for Building Ray (Python Only) to avoid long build times.

To build Ray on Ubuntu, run the following commands:

sudo apt-get update
sudo apt-get install -y build-essential curl clang-12 pkg-config psmisc unzip

# Install Bazelisk.

# Install node version manager and node 14
$(curl -o-
nvm install 14
nvm use 14


The script installs bazelisk for building Ray. If you prefer to use bazel, only version 6.5.0 is currently supported.

For RHELv8 (Redhat EL 8.0-64 Minimal), run the following commands:

sudo yum groupinstall 'Development Tools'
sudo yum install psmisc

In RedHat, install Bazel manually from this link:

Preparing to build Ray on MacOS#


Assuming you already have Brew and Bazel installed on your mac and you also have grpc and protobuf installed on your mac consider removing those (grpc and protobuf) for smooth build through the commands brew uninstall grpc, brew uninstall protobuf. If you have built the source code earlier and it still fails with errors like No such file or directory:, try cleaning previous builds on your host by running the commands brew uninstall binutils and bazel clean --expunge.

To build Ray on MacOS, first install these dependencies:

brew update
brew install wget

# Install Bazel.

Building Ray on Linux & MacOS (full)#

Make sure you have a local clone of Ray’s git repository as explained above. You will also need to install NodeJS to build the dashboard.

Enter into the project directory, for example:

cd ray

Now you can build the dashboard. From inside of your local Ray project directory enter into the dashboard client directory:

cd python/ray/dashboard/client

Then you can install the dependencies and build the dashboard:

npm ci
npm run build

After that, you can now move back to the top level Ray directory:

cd ../..

Now let’s build Ray for Python. Make sure you activate any Python virtual (or conda) environment you could be using as described above.

Enter into the python/ directory inside of the Ray project directory and install the project with pip:

# Install Ray.
cd python/
# Install required dependencies.
pip install -r requirements.txt
# You may need to set the following two env vars if you have a macOS ARM64(M1) platform.
# See for more details.
pip install -e . --verbose  # Add --user if you see a permission denied error.

The -e means “editable”, so changes you make to files in the Ray directory will take effect without reinstalling the package.


if you run python install, files will be copied from the Ray directory to a directory of Python packages (/lib/python3.6/site-packages/ray). This means that changes you make to files in the Ray directory will not have any effect.


If your machine is running out of memory during the build or the build is causing other programs to crash, try adding the following line to ~/.bazelrc:

build --local_ram_resources=HOST_RAM*.5 --local_cpu_resources=4

The build --disk_cache=~/bazel-cache option can be useful to speed up repeated builds too.


Warning: If you run into an error building protobuf, switching from miniconda to anaconda might help.

Building Ray on Windows (full)#


The following links were correct during the writing of this section. In case the URLs changed, search at the organizations’ sites.

You can also use the included script to install Bazel:

# Install Bazel.
# (Windows users: please manually place Bazel in your PATH, and point
# BAZEL_SH to MSYS2's Bash: ``set BAZEL_SH=C:\Program Files\Git\bin\bash.exe``)


  1. Enable Developer mode on Windows 10 systems. This is necessary so git can create symlinks.

    1. Open Settings app;

    2. Go to “Update & Security”;

    3. Go to “For Developers” on the left pane;

    4. Turn on “Developer mode”.

  2. Add the following Miniconda subdirectories to PATH. If Miniconda was installed for all users, the following paths are correct. If Miniconda is installed for a single user, adjust the paths accordingly.

    • C:\ProgramData\Miniconda3

    • C:\ProgramData\Miniconda3\Scripts

    • C:\ProgramData\Miniconda3\Library\bin

  3. Define an environment variable BAZEL_SH to point to bash.exe. If git for Windows was installed for all users, bash’s path should be C:\Program Files\Git\bin\bash.exe. If git was installed for a single user, adjust the path accordingly.

4. Bazel 6.5.0 installation. Go to Bazel 6.5.0 release web page and download bazel-4.2.1-windows-x86_64.exe. Copy the exe into the directory of your choice. Define an environment variable BAZEL_PATH to full exe path (example: set BAZEL_PATH=C:\bazel\bazel.exe). Also add the Bazel directory to the PATH (example: set PATH=%PATH%;C:\bazel)

  1. Download ray source code and build it.

# cd to the directory under which the ray source tree will be downloaded.
git clone -c core.symlinks=true
cd ray\python
pip install -e . --verbose

Environment variables that influence builds#

You can tweak the build with the following environment variables (when running pip install -e . or python install):

  • RAY_INSTALL_JAVA: If set and equal to 1, extra build steps will be executed to build java portions of the codebase

  • RAY_INSTALL_CPP: If set and equal to 1, ray-cpp will be installed

  • RAY_DISABLE_EXTRA_CPP: If set and equal to 1, a regular (non - cpp) build will not provide some cpp interfaces

  • SKIP_BAZEL_BUILD: If set and equal to 1, no Bazel build steps will be executed

  • SKIP_THIRDPARTY_INSTALL: If set will skip installation of third-party python packages

  • RAY_DEBUG_BUILD: Can be set to debug, asan, or tsan. Any other value will be ignored

  • BAZEL_ARGS: If set, pass a space-separated set of arguments to Bazel. This can be useful for restricting resource usage during builds, for example. See for more information about valid arguments.

  • IS_AUTOMATED_BUILD: Used in CI to tweak the build for the CI machines

  • SRC_DIR: Can be set to the root of the source checkout, defaults to None which is cwd()

  • BAZEL_SH: used on Windows to find a bash.exe, see below

  • BAZEL_PATH: used on Windows to find bazel.exe, see below

  • MINGW_DIR: used on Windows to find bazel.exe if not found in BAZEL_PATH

Installing additional dependencies for development#

Dependencies for the linter (scripts/ can be installed with:

pip install -c python/requirements_compiled.txt -r python/requirements/lint-requirements.txt

Dependencies for running Ray unit tests under python/ray/tests can be installed with:

pip install -c python/requirements_compiled.txt -r python/requirements/test-requirements.txt

Requirement files for running Ray Data / ML library tests are under python/requirements/.

Pre-commit Hooks#

Ray is planning to replace the pre-push hooks that are invoked from scripts/ with pre-commit hooks using the pre-commit python package in the future. At the moment, we have configured a .pre-commit-config.yaml which runs all the same checks done by scripts/ along with a few additional ones too. Currently this developer tooling is opt-in, with any formatting changes made by scripts/ expected to be caught by pre-commit as well. To start using pre-commit:

pip install pre-commit
pre-commit install

This will install pre-commit into the current environment, and enable pre-commit checks every time you commit new code changes with git. To temporarily skip pre-commit checks, use the -n or --no-verify flag when committing:

git commit -n

If you find that scripts/ makes a change that is different from what pre-commit does, please report an issue on the Ray github page.

Fast, Debug, and Optimized Builds#

Currently, Ray is built with optimizations, which can take a long time and interfere with debugging. To perform fast, debug, or optimized builds, you can run the following (via -c fastbuild/dbg/opt, respectively):

bazel build -c fastbuild //:ray_pkg

This will rebuild Ray with the appropriate options (which may take a while). If you need to build all targets, you can use "//:all" instead of //:ray_pkg.

To make this change permanent, you can add an option such as the following line to your user-level ~/.bazelrc file (not to be confused with the workspace-level .bazelrc file):

build --compilation_mode=fastbuild

If you do so, remember to revert this change, unless you want it to affect all of your development in the future.

Using dbg instead of fastbuild generates more debug information, which can make it easier to debug with a debugger like gdb.

Building the Docs#

To learn more about building the docs refer to Contributing to the Ray Documentation.

Using a local repository for dependencies#

If you’d like to build Ray with custom dependencies (for example, with a different version of Cython), you can modify your .bzl file as follows:

  name = "cython",
) if False else native.new_local_repository(
  name = "cython",
  build_file = "bazel/BUILD.cython",
  path = "../cython",

This replaces the existing http_archive rule with one that references a sibling of your Ray directory (named cython) using the build file provided in the Ray repository (bazel/BUILD.cython). If the dependency already has a Bazel build file in it, you can use native.local_repository instead, and omit build_file.

To test switching back to the original rule, change False to True.


If importing Ray (python3 -c "import ray") in your development clone results in this error:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File ".../ray/python/ray/", line 63, in <module>
    import ray._raylet  # noqa: E402
  File "python/ray/_raylet.pyx", line 98, in init ray._raylet
    import ray.memory_monitor as memory_monitor
  File ".../ray/python/ray/", line 9, in <module>
    import psutil  # noqa E402
  File ".../ray/python/ray/thirdparty_files/psutil/", line 159, in <module>
    from . import _psosx as _psplatform
  File ".../ray/python/ray/thirdparty_files/psutil/", line 15, in <module>
    from . import _psutil_osx as cext
ImportError: cannot import name '_psutil_osx' from partially initialized module 'psutil' (most likely due to a circular import) (.../ray/python/ray/thirdparty_files/psutil/

Then you should run the following commands:

rm -rf python/ray/thirdparty_files/
python3 -m pip install setproctitle