Daily Development Workflows

About Pants

Since 22.09, we have migrated to Pants as our primary build system and dependency manager for the mono-repository of Python components.

Pants is a graph-based async-parallel task executor written in Rust and Python. It is tailored to building programs with explicit and auto-inferred dependency checks and aggressive caching.

Key concepts

  • The command pattern:

  • Goal: an action to execute

    • You may think this as the root node of the task graph executed by Pants.

  • Target: objectives for the action, usually expressed as path/to/dir:name

    • The targets are declared/defined by path/to/dir/BUILD files.

  • The global configuration is at pants.toml.

  • Recommended reading: https://www.pantsbuild.org/docs/concepts

Inspecting build configurations

  • Display all targets

    $ pants list ::
    • This list includes the full enumeration of individual targets auto-generated by collective targets (e.g., python_sources() generates multiple python_source() targets by globbing the sources pattern)

  • Display all dependencies of a specific target (i.e., all targets required to build this target)

    $ pants dependencies --transitive src/ai/backend/common:src
  • Display all dependees of a specific target (i.e., all targets affected when this target is changed)

    $ pants dependees --transitive src/ai/backend/common:src


Pants statically analyzes the source files to enumerate all its imports and determine the dependencies automatically. In most cases this works well, but sometimes you may need to manually declare explicit dependencies in BUILD files.

Running lint and check

Run lint/check for all targets:

$ pants lint ::
$ pants check ::

To run lint/check for a specific target or a set of targets:

$ pants lint src/ai/backend/common:: tests/common::
$ pants check src/ai/backend/manager::

Currently running mypy with pants is slow because mypy cannot utilize its own cache as pants invokes mypy per file due to its own dependency management scheme. (e.g., Checking all sources takes more than 1 minutes!) This performance issue is being tracked by pantsbuild/pants#10864. For now, try using a smaller target of files that you work on and use an option to select the targets only changed (--changed-since).

Running formatters

If you encounter failure from ruff, you may run the following to automatically fix the import ordering issues.

$ pants fix ::

If you encounter failure from black, you may run the following to automatically fix the code style issues.

$ pants fmt ::

Running unit tests

Here are various methods to run tests:

$ pants test ::
$ pants test tests/manager/test_scheduler.py::
$ pants test tests/manager/test_scheduler.py:: -- -k test_scheduler_configs
$ pants test tests/common::            # Run common/**/test_*.py
$ pants test tests/common:tests        # Run common/test_*.py
$ pants test tests/common/redis::      # Run common/redis/**/test_*.py
$ pants test tests/common/redis:tests  # Run common/redis/test_*.py

You may also try --changed-since option like lint and check.

To specify extra environment variables for tests, use the --test-extra-env-vars option:

$ pants test \
>   --test-extra-env-vars=MYVARIABLE=MYVALUE \
>   tests/common:tests

Running integration tests

$ ./backend.ai test run-cli user,admin

Building wheel packages

To build a specific package:

$ pants \
>   --tag="wheel" \
>   package \
>   src/ai/backend/common:dist
$ ls -l dist/*.whl

If the package content varies by the target platform, use:

$ pants \
>   --tag="wheel" \
>   --tag="+platform-specific" \
>   --platform-specific-resources-target=linux_arm64 \
>   package \
>   src/ai/backend/runner:dist
$ ls -l dist/*.whl

Using IDEs and editors

Pants has an export goal to auto-generate a virtualenv that contains all external dependencies installed in a single place. This is very useful when you use IDEs and editors.

To (re-)generate the virtualenv(s), run:

$ pants export --resolve=RESOLVE_NAME  # you may add multiple --resolve options

You may display the available resolve names by (the command works with Python 3.12 or later):

$ python -c 'import tomllib,pathlib;print("\n".join(tomllib.loads(pathlib.Path("pants.toml").read_text())["python"]["resolves"].keys()))'

Similarly, you can export all virtualenvs at once:

$ python -c 'import tomllib,pathlib;print("\n".join(tomllib.loads(pathlib.Path("pants.toml").read_text())["python"]["resolves"].keys()))' | sed 's/^/--resolve=/' | xargs ./pants export

Then configure your IDEs/editors to use dist/export/python/virtualenvs/python-default/PYTHON_VERSION/bin/python as the interpreter for your code, where PYTHON_VERSION is the interpreter version specified in pants.toml.

As of Pants 2.16, you must export the virtualenvs by the individual lockfiles using the --resolve option, as all tools are unified to use the same custom resolve subsystem of Pants and the :: target no longer works properly, like:

$ pants export --resolve=python-default --resolve=mypy

To make LSP (language server protocol) services like PyLance to detect our source packages correctly, you should also configure PYTHONPATH to include the repository root’s src directory and plugins/*/ directories if you have added Backend.AI plugin checkouts.

For linters and formatters, configure the tool executable paths to indicate dist/export/python/virtualenvs/RESOLVE_NAME/PYTHON_VERSION/bin/EXECUTABLE. For example, ruff’s executable path is dist/export/python/virtualenvs/ruff/3.12.2/bin/ruff.

Currently we have the following Python tools to configure in this way:

  • ruff: Provides a fast linting (combining pylint, flake8, and isort) fixing (auto-fix for some linting rules and isort) and formatting (black)

  • mypy: Validates the type annotations and performs a static analysis


    For a long list of arguments or list/tuple items, you could explicitly add a trailing comma to force Ruff/Black to insert line-breaks after every item even when the line length does not exceed the limit (100 characters).


    You may disable auto-formatting on a specific region of code using # fmt: off and # fmt: on comments, though this is strongly discouraged except when manual formatting gives better readability, such as numpy matrix declarations.

  • pytest: The unit test runner framework.

  • coverage-py: Generates reports about which source lines were visited during execution of a pytest session.

  • towncrier: Generates the changelog from news fragments in the changes directory when making a new release.


Install the following extensions:

  • Python (ms-python.python)

  • Pylance (ms-python.vscode-pylance) (optional but recommended)

  • Mypy (ms-python.mypy-type-checker)

  • Ruff (charliermarsh.ruff)

  • For other standard Python extensions like Flake8, isort, and Black, disable them for the Backend.AI workspace only to prevent interference with Ruff’s own linting, fixing and formatting.

Set the workspace settings for the Python extension for code navigation and auto-completion:

Setting ID

Recommended value










{"source.fixAll": true}

Set the following keys in the workspace settings to configure Python tools:

Setting ID

Example value










Changed in July 2023

After applying the VSCode Python Tool migration, we no longer recommend to configure python.linting.*Path and python.formatting.*Path keys.


There are a large variety of plugins and usually heavy Vimmers should know what to do.

We recommend using ALE or CoC plugins to have automatic lint highlights, auto-formatting on save, and auto-completion support with code navigation via LSP backends.


Note that it is recommended to enable only one linter/formatter at a time (either ALE or CoC) with proper configurations, to avoid duplicate suggestions and error reports.

When using ALE, it is recommended to have a directory-local vimrc as follows. First, add set exrc in your user-level vimrc. Then put the followings in .vimrc (or .nvimrc for NeoVim) in the build root directory:

let s:cwd = getcwd()
let g:ale_python_mypy_executable = s:cwd . '/dist/export/python/virtualenvs/mypy/3.12.2/bin/mypy'
let g:ale_python_ruff_executable = s:cwd . '/dist/export/python/virtualenvs/ruff/3.12.2/bin/ruff'
let g:ale_linters = { "python": ['ruff', 'mypy'] }
let g:ale_fixers = {'python': ['ruff']}
let g:ale_fix_on_save = 1

When using CoC, run :CocInstall coc-pyright @yaegassy/coc-ruff and :CocLocalConfig after opening a file in the local working copy to initialize Pyright functionalities. In the local configuration file (.vim/coc-settings.json), you may put the linter/formatter configurations just like VSCode (see the official reference).

  "coc.preferences.formatOnType": false,
  "coc.preferences.willSaveHandlerTimeout": 5000,
  "ruff.enabled": true,
  "ruff.autoFixOnSave": true,
  "ruff.useDetectRuffCommand": false,
  "ruff.builtin.pythonPath": "dist/export/python/virtualenvs/ruff/3.12.2/bin/python",
  "ruff.serverPath": "dist/export/python/virtualenvs/ruff/3.12.2/bin/ruff-lsp",
  "python.pythonPath": "dist/export/python/virtualenvs/python-default/3.12.2/bin/python",
  "python.linting.mypyEnabled": true,
  "python.linting.mypyPath": "dist/export/python/virtualenvs/mypy/3.12.2/bin/mypy",

To activate Ruff (a Python linter and fixer), run :CocCommand ruff.builtin.installServer after opening any Python source file to install the ruff-lsp server.

Switching between branches

When each branch has different external package requirements, you should run pants export before running codes after git switch-ing between such branches.

Sometimes, you may experience bogus “glob” warning from pants because it sees a stale cache. In that case, run pgrep pantsd | xargs kill and it will be fine.

Running entrypoints

To run a Python program within the unified virtualenv, use the ./py helper script. It automatically passes additional arguments transparently to the Python executable of the unified virtualenv.

./backend.ai is an alias of ./py -m ai.backend.cli.


$ ./py -m ai.backend.storage.server
$ ./backend.ai mgr start-server
$ ./backend.ai ps

Working with plugins

To develop Backend.AI plugins together, the repository offers a special location ./plugins where you can clone plugin repositories and a shortcut script scripts/install-plugin.sh that does this for you.

$ scripts/install-plugin.sh lablup/backend.ai-accelerator-cuda-mock

This is equivalent to:

$ git clone \
>   https://github.com/lablup/backend.ai-accelerator-cuda-mock \
>   plugins/backend.ai-accelerator-cuda-mock

These plugins are auto-detected by scanning setup.cfg of plugin subdirectories by the ai.backend.plugin.entrypoint module, even without explicit editable installations.

Writing test cases

Mostly it is just same as before: use the standard pytest practices. Though, there are a few key differences:

  • Tests are executed in parallel in the unit of test modules.

  • Therefore, session-level fixtures may be executed multiple times during a single run of pants test.


If you interrupt (Ctrl+C, SIGINT) a run of pants test, it will immediately kill all pytest processes without fixture cleanup. This may accumulate unused Docker containers in your system, so it is a good practice to run docker ps -a periodically and clean up dangling containers.

To interactively run tests, see Debugging test cases (or interactively running test cases).

Here are considerations for writing Pants-friendly tests:

  • Ensure that it runs in an isolated/mocked environment and minimize external dependency.

  • If required, use the environment variable BACKEND_TEST_EXEC_SLOT (an integer value) to uniquely define TCP port numbers and other resource identifiers to allow parallel execution. Refer the Pants docs.

  • Use ai.backend.testutils.bootstrap to populate a single-node Redis/etcd/Postgres container as fixtures of your test cases. Import the fixture and use it like a plain pytest fixture.

    • These fixtures create those containers with OS-assigned public port numbers and give you a tuple of container ID and a ai.backend.common.types.HostPortPair for use in test codes. In manager and agent tests, you could just refer local_config to get a pre-populated local configurations with those port numbers.

    • In this case, you may encounter flake8 complaining about unused imports and redefinition. Use # noqa: F401 and # noqa: F811 respectively for now.


About using /tmp in tests

If your Docker service is installed using Snap (e.g., Ubuntu 20.04 or later), it cannot access the system /tmp directory because Snap applies a private “virtualized” tmp directory to the Docker service.

You should use other locations under the user’s home directory (or preferably .tmp in the working copy directory) to avoid mount failures for the developers/users in such platforms.

It is okay to use the system /tmp directory if they are not mounted inside any containers.

Writing documentation

  • Create a new pyenv virtualenv based on Python 3.10.

    $ pyenv virtualenv 3.10.9 venv-bai-docs
  • Activate the virtualenv and run:

    $ pyenv activate venv-bai-docs
    $ pip install -U pip setuptools wheel
    $ pip install -U -r docs/requirements.txt
  • You can build the docs as follows:

    $ cd docs
    $ pyenv activate venv-bai-docs
    $ make html
  • To locally serve the docs:

    $ cd docs
    $ python -m http.server --directory=_build/html

(TODO: Use Pants’ own Sphinx support when pantsbuild/pants#15512 is released.)

Advanced Topics

Adding new external dependencies

  • Add the package version requirements to the unified requirements file (./requirements.txt).

  • Update the module_mapping field in the root build configuration (./BUILD) if the package name and its import name differs.

  • Update the type_stubs_module_mapping field in the root build configuration if the package provides a type stubs package separately.

  • Run:

    $ pants generate-lockfiles
    $ pants export

Merging lockfile conflicts

When you work on a branch that adds a new external dependency and the main branch has also another external dependency addition, merging the main branch into your branch is likely to make a merge conflict on python.lock file.

In this case, you can just do the followings since we can just regenerate the lockfile after merging requirements.txt and BUILD files.

$ git merge main
... it says a conflict on python.lock ...
$ git checkout --theirs python.lock
$ pants generate-lockfiles --resolve=python-default
$ git add python.lock
$ git commit

Resetting Pants

If Pants behaves strangely, you could simply reset all its runtime-generated files by:

$ pgrep pantsd | xargs -r kill
$ rm -r /tmp/*-pants/ .pants.d .pids ~/.cache/pants

After this, re-running any Pants command will automatically reinitialize itself and all cached data as necessary.

Note that you may find out the concrete path inside /tmp from .pants.rc’s local_execution_root_dir option set by install-dev.sh.


If you have run pants or the installation script with sudo, some of the above directories may be owned by root and running pants as the user privilege would not work. In such cases, remove the directories with sudo and retry.

Resolve the error message ‘Pants is not abailable for your platform’, When installing Backend.AI with pants

When installing Backend.AI, you may find the following error message saying ‘Pants is not available for your platform’ if you have installed Pants 2.17 or older with prior versions of Backend.AI.

[INFO] Bootstrapping the Pants build system...
Pants system command is already installed.
Failed to fetch https://binaries.pantsbuild.org/tags/pantsbuild.pants/release_2.19.0: [22] HTTP response code said error (The requested URL returned error: 404)
Bootstrapping Pants 2.19.0 using cpython 3.9.15
Installing pantsbuild.pants==2.19.0 into a virtual environment at /home/aaa/.cache/nce/bad1ad5b44f41a6ca9c99a135f9af8849a3b93ec5a018c7b2d13acaf0a969e3a/bindings/venvs/2.19.0
    ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 65.4/65.4 KB 3.3 MB/s eta 0:00:00
ERROR: Could not find a version that satisfies the requirement pantsbuild.pants==2.19.0 (from versions: 0.0.17, 0.0.18, 0.0.20, 0.0.21, 0.0.22, ... (a long list of versions) ..., 2.17.0,
2.17.1rc0, 2.17.1rc1, 2.17.1rc2, 2.17.1rc3, 2.17.1, 2.18.0.dev0, 2.18.0.dev1, 2.18.0.dev3, 2.18.0.dev4, 2.18.0.dev5, 2.18.0.dev6, 2.18.0.dev7, 2.18.0a0)
ERROR: No matching distribution found for pantsbuild.pants==2.19.0
Install failed: Command '['/home/aaa/.cache/nce/bad1ad5b44f41a6ca9c99a135f9af8849a3b93ec5a018c7b2d13acaf0a969e3a/bindings/venvs/2.19.0/bin/python', '-sE', '-m', 'pip', '--disable-pip-versi
on-check', '--no-python-version-warning', '--log', PosixPath('/home/aaa/.cache/nce/bad1ad5b44f41a6ca9c99a135f9af8849a3b93ec5a018c7b2d13acaf0a969e3a/bindings/venvs/2.19.0/pants-install.log'
), 'install', '--quiet', '--find-links', 'file:///home/aaa/.cache/nce/bad1ad5b44f41a6ca9c99a135f9af8849a3b93ec5a018c7b2d13acaf0a969e3a/bindings/find_links/2.19.0/e430175b/index.html', '--p
rogress-bar', 'off', 'pantsbuild.pants==2.19.0']' returned non-zero exit status 1.
More information can be found in the log at: /home/aaa/.cache/nce/bad1ad5b44f41a6ca9c99a135f9af8849a3b93ec5a018c7b2d13acaf0a969e3a/bindings/logs/install.log

Error: Isolates your Pants from the elements.

Please select from the following boot commands:

<default>: Detects the current Pants installation and launches it.
bootstrap-tools: Introspection tools for the Pants bootstrap process.
pants: Runs a hermetic Pants installation.
pants-debug: Runs a hermetic Pants installation with a debug server for debugging Pants code.
update: Update scie-pants.

You can select a boot command by passing it as the 1st argument or else by setting the SCIE_BOOT environment variable.

ERROR: Failed to establish atomic directory /home/aaa/.cache/nce/bad1ad5b44f41a6ca9c99a135f9af8849a3b93ec5a018c7b2d13acaf0a969e3a/locks/install-a4f15e2d2c97473883ec33b4ee0f9d11f99dcf5bee63
8b1cc7a0270d55d0ec8d. Population of work directory failed: Boot binding command failed: exit status: 1

[ERROR] Cannot proceed the installation because Pants is not available for your platform!

To resolve this error, reinstall or upgrade Pants. As of the Pants 2.18.0 release, they no longer use the Python Package Index but GitHub releases to distribute the binary builds.

Resolving missing directories error when running Pants

ValueError: Failed to create temporary directory for immutable inputs: No such file or directory (os error 2) at path "/tmp/bai-dev-PN4fpRLB2u2xL.j6-pants/immutable_inputsvIpaoN"

If you encounter errors like above when running daily Pants commands like lint, you may manually create the directory one step higher. For the above example, run:

mkdir -p /tmp/bai-dev-PN4fpRLB2u2xL.j6-pants/

If this workaround does not work, backup your current working files and reinstall by running scripts/delete-dev.sh and scripts/install-dev.sh serially.

Changing or updating the Python runtime for Pants

When you run scripts/install-dev.sh, it automatically creates .pants.bootstrap to explicitly set a specific pyenv Python version to run Pants.

If you have removed/upgraded this specific Python version from pyenv, you also need to update .pants.bootstrap accordingly.

Debugging test cases (or interactively running test cases)

When your tests hang, you can try adding the --debug flag to the pants test command:

$ pants test --debug ...

so that Pants runs the designated test targets serially and interactively. This means that you can directly observe the console output and Ctrl+C to gracefully shutdown the tests with fixture cleanup. You can also apply additional pytest options such as --fulltrace, -s, etc. by passing them after target arguments and -- when executing pants test command.

Installing a subset of mono-repo packages in the editable mode for other projects

Sometimes, you need to editable-install a subset of packages into other project’s directories. For instance you could mount the client SDK and its internal dependencies for a Docker container for development.

In this case, we recommend to do it as follows:

  1. Run the following command to build a wheel from the current mono-repo source:

    $ pants --tag=wheel package src/ai/backend/client:dist

    This will generate dist/backend.ai_client-{VERSION}-py3-none-any.whl.

  2. Run pip install -U {MONOREPO_PATH}/dist/{WHEEL_FILE} in the target environment.

    This will populate the package metadata and install its external dependencies. The target environment may be one of a separate virtualenv or a container being built. For container builds, you need to first COPY the wheel file and install it.

  3. Check the internal dependency directories to link by running the following command:

    $ pants dependencies --transitive src/ai/backend/client:src \
    >   | grep src/ai/backend | grep -v ':version' | cut -d/ -f4 | uniq
  4. Link these directories in the target environment.

    For example, if it is a Docker container, you could add -v {MONOREPO_PATH}/src/ai/backend/{COMPONENT}:/usr/local/lib/python3.10/site-packages/ai/backend/{COMPONENT} to the docker create or docker run commands for all the component directories found in the previous step.

    If it is a local checkout with a pyenv-based virtualenv, you could replace $(pyenv prefix)/lib/python3.10/site-packages/ai/backend/{COMPONENT} directories with symbolic links to the mono-repo’s component source directories.

Boosting the performance of Pants commands

Since Pants uses temporary directories for aggressive caching, you could make the .tmp directory under the working copy root a tmpfs partition:

$ sudo mount -t tmpfs -o size=4G tmpfs .tmp
  • To make this persistent across reboots, add the following line to /etc/fstab:

    tmpfs /path/to/dir/.tmp tmpfs defaults,size=4G 0 0
  • The size should be more than 3GB. (Running pants test :: consumes about 2GB.)

  • To change the size at runtime, you could simply remount it with a new size option:

    $ sudo mount -t tmpfs -o remount,size=8G tmpfs .tmp

Making a new release

  • Update ./VERSION file to set a new version number. (Remove the ending new line, e.g., using set noeol in Vim. This is also configured in ./editorconfig)

  • Run LOCKSET=tools/towncrier ./py -m towncrier to auto-generate the changelog.

    • You may append --draft to see a preview of the changelog update without actually modifying the filesystem.

    • (WIP: lablup/backend.ai#427).

  • Make a new git commit with the commit message: “release: <version>”.

  • Make an annotated tag to the commit with the message: “Release v<version>” or “Pre-release v<version>” depending on the release version.

  • Push the commit and tag. The GitHub Actions workflow will build the packages and publish them to PyPI.

  • When making a new major release, snapshot of prior release’s final DB migration history should be dumped. This will later help to fill out missing gaps of DB revisions when upgrading outdated cluster. The output then should be committed to next major release.

    $ ./backend.ai mgr schema dump-history > src/ai/backend/manager/models/alembic/revision_history/<version>.json

    Suppose you are trying to create both fresh baked 24.09.0 and good old 24.03.10 releases. In such cases you should first make a release of version 24.03.10, move back to latest branch, and then execute code snippet above with <version> set as 24.03.10, and release 24.09.0 including the dump.

    To make workflow above effective, be aware that backporting DB revisions to older major releases will no longer be permitted after major release version is switched.

Backporting to legacy per-pkg repositories

  • Use git diff and git apply instead of git cherry-pick.

    • To perform a three-way merge for conflicts, add -3 option to the git apply command.

    • You may need to rewrite some codes as the package structure differs. (The new mono repository has more fine-grained first party packages divided from the backend.ai-common package.)

  • When referring the PR/issue numbers in the commit for per-pkg repositories, update them like lablup/backend.ai#NNN instead of #NNN.