ADR 72: Restore Requests for Comments

Changelog

20-Aug-2021: Initial draft (@creachadair)

Status

Implemented

Context

In the past, we kept a collection of Request for Comments (RFC) documents in docs/rfc. Prior to the creation of the ADR process, these documents were used to document design and implementation decisions about Tendermint Core. The RFC directory was removed in favor of ADRs, in commit 3761aa69 (PR #6345).

For issues where an explicit design decision or implementation change is required, an ADR is generally preferable to an open-ended RFC: An ADR is relatively narrowly-focused, identifies a specific design or implementation question, and documents the consensus answer to that question.

Some discussions are more open-ended, however, or don't require a specific decision to be made (yet). Such conversations are still valuable to document, and several members of the Tendermint team have been doing so by writing gists or Google docs to share them around. That works well enough in the moment, but gists do not support any kind of collaborative editing, and both gists and docs are hard to discover after the fact. Google docs have much better collaborative editing, but are worse for discoverability, especially when contributors span different Google accounts.

Discoverability is important, because these kinds of open-ended discussions are useful to people who come later -- either as new team members or as outside contributors seeking to use and understand the thoughts behind our designs and the architectural decisions that arose from those discussion.

With these in mind, I propose that:

We re-create a new, initially empty docs/rfc directory in the repository, and use it to capture these kinds of open-ended discussions in supplement to ADRs.
Unlike in the previous RFC scheme, documents in this new directory will not be used directly for decision-making. This is the key difference between an RFC and an ADR.

Instead, an RFC will exist to document background, articulate general principles, and serve as a historical record of discussion and motivation.

In this system, an RFC may only result in a decision indirectly, via ADR documents created in response to the RFC.

In short: If a decision is required, write an ADR; otherwise if a sufficiently broad discussion is needed, write an RFC.

Just so that there is a consistent format, I also propose that:

RFC files are named rfc-XXX-title.{md,rst,txt} and are written in plain text, Markdown, or ReStructured Text.
Like an ADR, an RFC should include a high-level change log at the top of the document, and sections for:
- Abstract: A brief, high-level synopsis of the topic.
- Background: Any background necessary to understand the topic.
- Discussion: Detailed discussion of the issue being considered.
Unlike an ADR, an RFC does not include sections for Decisions, Detailed Design, or evaluation of proposed solutions. If an RFC leads to a proposal for an actual architectural change, that must be recorded in an ADR in the usual way, and may refer back to the RFC in its References section.

Alternative Approaches

Leaving aside implementation details, the main alternative to this proposal is to leave things as they are now, with ADRs as the only log of record and other discussions being held informally in whatever medium is convenient at the time.

Decision

(pending)

Detailed Design

Create a new docs/rfc directory in the tendermint repository. Note that this proposal intentionally does not pull back the previous contents of that path from Git history, as those documents were appropriately merged into the ADR process.
Create a README.md for RFCs that explains the rules and their relationship to ADRs.
Create an rfc-template.md file for RFC files.

Consequences

Positive

We will have a more discoverable place to record open-ended discussions that do not immediately result in a design change.

Negative

Potentially some people could be confused about the RFC/ADR distinction.

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