Design Documents

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If you're planning to add, change, or remove a user-facing feature, or make a significant architectural change to Bazel, you must write a design document and have it reviewed before you can submit the change.

Here are some examples of significant changes:

  • Addition or deletion of native build rules
  • Breaking-changes to native rules
  • Changes to a native build rule semantics that affect the behavior of more than a single rule
  • Changes to Bazel's rule definition API
  • Changes to the APIs that Bazel uses to connect to other systems
  • Changes to the Starlark language, semantics, or APIs
  • Changes that could have a pervasive effect on Bazel performance or memory usage (for better or for worse)
  • Changes to widely used internal APIs
  • Changes to flags and command-line interface.

Reasons for design reviews

When you write a design document, you can coordinate with other Bazel developers and seek guidance from Bazel's core team. For example, when a proposal adds, removes, or modifies any function or object available in BUILD, WORKSPACE, or bzl files, add the Starlark team as reviewers. Design documents are reviewed before submission because:

  • Bazel is a very complex system; seemingly innocuous local changes can have significant global consequences.
  • The team gets many feature requests from users; such requests need to be evaluated not only for technical feasibility but importance with regards to other feature requests.
  • Bazel features are frequently implemented by people outside the core team; such contributors have widely varying levels of Bazel expertise.
  • The Bazel team itself has varying levels of expertise; no single team member has a complete understanding of every corner of Bazel.
  • Changes to Bazel must account for backward compatibility and avoid breaking changes.

Bazel's design review policy helps to maximize the likelihood that:

  • all feature requests get a baseline level of scrutiny.
  • the right people will weigh in on designs before we've invested in an implementation that may not work.

To help you get started, take a look at the design documents in the Bazel Proposals Repository. Designs are works in progress, so implementation details can change over time and with feedback. The published design documents capture the initial design, and not the ongoing changes as designs are implemented. Always go to the documentation for descriptions of current Bazel functionality.

Contributor Workflow

As a contributor, you can write a design document, send pull requests and request reviewers for your proposal.

Write the design document

All design documents must have a header that includes:

  • author
  • date of last major change
  • list of reviewers, including one (and only one) lead reviewer
  • current status (draft, in review, approved, rejected, being implemented, implemented)
  • link to discussion thread (to be added after the announcement)

The document can be written either as a world-readable Google Doc or using Markdown. Read below about for a Markdown / Google Docs comparison.

Proposals that have a user-visible impact must have a section documenting the impact on backward compatibility (and a rollout plan if needed).

Create a Pull Request

Share your design doc by creating a pull request (PR) to add the document to the design index. Add your markdown file or a document link to your PR.

When possible, choose a lead reviewer. and cc other reviewers. If you don't choose a lead reviewer, a Bazel maintainer will assign one to your PR.

After you create your PR, reviewers can make preliminary comments during the code review. For example, the lead reviewer can suggest extra reviewers, or point out missing information. The lead reviewer approves the PR when they believe the review process can start. This doesn't mean the proposal is perfect or will be approved; it means that the proposal contains enough information to start the discussion.

Announce the new proposal

Send an announcement to bazel-dev when the PR is submitted.

You may copy other groups (for example, bazel-discuss, to get feedback from Bazel end-users).

Iterate with reviewers

Anyone interested can comment on your proposal. Try to answer questions, clarify the proposal, and address concerns.

Discussion should happen on the announcement thread. If the proposal is in a Google Doc, comments may be used instead (Note that anonymous comments are allowed).

Update the status

Create a new PR to update the status of the proposal, when iteration is complete. Send the PR to the same lead reviewer and cc the other reviewers.

To officially accept the proposal, the lead reviewer approves the PR after ensuring that the other reviewers agree with the decision.

There must be at least 1 week between the first announcement and the approval of a proposal. This ensures that users had enough time to read the document and share their concerns.

Implementation can begin before the proposal is accepted, for example as a proof-of-concept or an experimentation. However, you cannot submit the change before the review is complete.

Choosing a lead reviewer

A lead reviewer should be a domain expert who is:

  • Knowledgeable of the relevant subsystems
  • Objective and capable of providing constructive feedback
  • Available for the entire review period to lead the process

Consider checking the contacts for various team labels.

Markdown vs Google Docs

Decide what works best for you, since both are accepted.

Benefits of using Google Docs:

  • Effective for brainstorming, since it is easy to get started with.
  • Collaborative editing.
  • Quick iteration.
  • Easy way to suggest edits.

Benefits of using Markdown files:

  • Clean URLs for linking.
  • Explicit record of revisions.
  • No forgetting to set up access rights before publicizing a link.
  • Easily searchable with search engines.
  • Future-proof: Plain text is not at the mercy of any specific tool and doesn't require an Internet connection.
  • It is possible to update them even if the author is not around anymore.
  • They can be processed automatically (update/detect dead links, fetch list of authors, etc.).

You can choose to first iterate on a Google Doc, and then convert it to Markdown for posterity.

Using Google Docs

For consistency, use the Bazel design doc template. It includes the necessary header and creates visual consistency with other Bazel related documents. To do that, click on File > Make a copy or click this link to make a copy of the design doc template.

To make your document readable to the world, click on Share > Advanced > Change…, and choose "On - Anyone with the link". If you allow comments on the document, anyone can comment anonymously, even without a Google account.

Using Markdown

Documents are stored on GitHub and use the GitHub flavor of Markdown (Specification).

Create a PR to update an existing document. Significant changes should be reviewed by the document reviewers. Trivial changes (such as typos, formatting) can be approved by anyone.

Reviewer workflow

A reviewer comments, reviews and approves design documents.

General reviewer responsibilities

You're responsible for reviewing design documents, asking for additional information if needed, and approving a design that passes the review process.

When you receive a new proposal

  1. Take a quick look at the document.
  2. Comment if critical information is missing, or if the design doesn't fit with the goals of the project.
  3. Suggest additional reviewers.
  4. Approve the PR when it is ready for review.

During the review process

  1. Engage in a dialogue with the design author about issues that are problematic or require clarification.
  2. If appropriate, invite comments from non-reviewers who should be aware of the design.
  3. Decide which comments must be addressed by the author as a prerequisite to approval.
  4. Write "LGTM" (Looks Good To Me) in the discussion thread when you are happy with the current state of the proposal.

Follow this process for all design review requests. Do not approve designs affecting Bazel if they are not in the design index.

Lead reviewer responsibilities

You're responsible for making the go / no-go decision on implementation of a pending design. If you're not able to do this, you should identify a suitable delegate (reassign the PR to the delegate), or reassign the bug to a Bazel manager for further disposition.

During the review process

  1. Ensure that the comment and design iteration process moves forward constructively.
  2. Prior to approval, ensure that concerns from other reviewers have been resolved.

After approval by all reviewers

  1. Make sure there has been at least 1 week since the announcement on the mailing list.
  2. Make sure the PR updates the status.
  3. Approve the PR sent by the proposal author.

Rejecting designs

  1. Make sure the PR author sends a PR; or send them a PR.
  2. The PR updates the status of the document.
  3. Add a comment to the document explaining why the design can't be approved in its current state, and outlining next steps, if any (such as "revisit invalid assumptions and resubmit").