History and Motivation

West was added to the Zephyr project to fulfill two fundamental requirements:

  • The ability to work with multiple Git repositories
  • The ability to provide an extensible and user-friendly command-line interface for basic Zephyr workflows

During the development of west, a set of Design Constraints were identified to avoid the common pitfalls of tools of this kind.

Requirements

Although the motivation behind splitting the Zephyr codebase into multiple repositories is outside of the scope of this page, the fundamental requirements, along with a clear justification of the choice not to use existing tools and instead develop a new one, do belong here.

The basic requirements are:

  • R1: Keep externally maintained code in separately maintained repositories outside of the main zephyr repository, without requiring users to manually clone each of the external repositories
  • R2: Provide a tool that both Zephyr users and distributors can make use of to benefit from and extend
  • R3: Allow users and downstream distributions to override or remove repositories without having to make changes to the zephyr repository
  • R4: Support both continuous tracking and commit-based (bisectable) project updating

Topologies supported

The requirements above lead to three source code topologies supported by west:

  • T1: Star topology with zephyr as the manifest repository:

    • The zephyr repository acts as the central repository and includes a complete list of projects used upstream
    • Default (upstream) configuration
    • Analogy with existing mechanisms: Git submodules with zephyr as the superproject
    • See Introduction for how mainline Zephyr is an example of this topology
  • T2: Star topology with an application repository as the manifest repository:

    • A repository containing a Zephyr application acts as the central repository and includes a complete list of other projects, including the zephyr repository, required to build it

    • Useful for those focused on a single application

    • Analogy with existing mechanisms: Git submodules with the application as the superproject, zephyr and other projects as submodules

    • An installation using this topology could look like this:

      app-manifest-installation
      ├── application
      │   ├── CMakeLists.txt
      │   ├── prj.conf
      │   ├── src
      │   │   └── main.c
      │   └── west.yml
      ├── modules
      │   └── lib
      │       └── tinycbor
      └── zephyr
      
  • T3: Forest topology:

    • A dedicated manifest repository which contains no Zephyr source code, and specifies a list of projects all at the same “level”

    • Useful for downstream distributions with no “central” repository

    • Analogy with existing mechanisms: Google repo-based source distribution

    • An installation using this topology could look like this:

      forest
      ├── app1
      │   ├── CMakeLists.txt
      │   ├── prj.conf
      │   └── src
      │       └── main.c
      ├── app2
      │   ├── CMakeLists.txt
      │   ├── prj.conf
      │   └── src
      │       └── main.c
      ├── manifest-repo
      │   └── west.yml
      ├── modules
      │   └── lib
      │       └── tinycbor
      └── zephyr
      

Rationale for a custom tool

Existing tools were considered during west’s initial design and development. None were found suitable for Zephyr’s requirements. In particular, these were examined in detail:

  • Google repo
    • Does not cleanly support using zephyr as the manifest repository (R4)
    • Python 2 only
    • Does not play well with Windows
    • Assumes Gerrit is used for code review
  • Git submodules
    • Does not fully support R1, since the externally maintained repositories would still need to be inside the main zephyr Git tree
    • Does not support R3, since downstream copies would need to either delete or replace submodule definitions
    • Does not support continuous tracking of the latest HEAD in external repositories (R4)
    • Requires hardcoding of the paths/locations of the external repositories

Multiple Git Repositories

Zephyr intends to provide all required building blocks needed to deploy complex IoT applications. This in turn means that the Zephyr project is much more than an RTOS kernel, and is instead a collection of components that work together. In this context, there are a few reasons to work with multiple Git repositories in a standardized manner within the project:

  • Clean separation of Zephyr original code and imported projects and libraries
  • Avoidance of license incompatibilities between original and imported code
  • Reduction in size and scope of the core Zephyr codebase, with additional repositories containing optional components instead of being imported directly into the tree
  • Safety and security certifications
  • Enforcement of modularization of the components
  • Out-of-tree development based on subsets of the supported boards and SoCs

See Multiple Repository Management for a detailed explanation of west’s handling of multiple repository management.

Design Constraints

West is:

  • Optional: it is always possible to drop back to “raw” command-line tools, i.e. use Zephyr without using west (although west itself might need to be installed and accessible to the build system). It may not always be convenient to do so, however. (If all of west’s features were already conveniently available, there would be no reason to develop it.)
  • Compatible with CMake: building, flashing and debugging, and emulator support will always remain compatible with direct use of CMake.
  • Cross-platform: West is written in Python 3, and works on all platforms supported by Zephyr.
  • Usable as a Library: whenever possible, west features are implemented as libraries that can be used standalone in other programs, along with separate command line interfaces that wrap them. West itself is a Python package named west; its libraries are implemented as subpackages.
  • Conservative about features: no features will be accepted without strong and compelling motivation.
  • Clearly specified: West’s behavior in cases where it wraps other commands is clearly specified and documented. This enables interoperability with third party tools, and means Zephyr developers can always find out what is happening “under the hood” when using west.

See Zephyr issue #6205 and for more details and discussion.