Recommendations on Revision Control

Phabricator Flavor Text (Revision Control and Code Review)

Project recommendations on how to organize revision control.

This document is purely advisory. Phabricator works with a variety of revision control strategies, and diverging from the recommendations in this document will not impact your ability to use it for code review and source management.

This is my (epriestley's) personal take on the issue and not necessarily representative of the views of the Phabricator team as a whole.

Overview

There are a few ways to use SVN, a few ways to use Mercurial, and many many many ways to use Git. Particularly with Git, every project does things differently, and all these approaches are valid for small projects. When projects scale, strategies which enforce one idea is one commit are better.

One Idea is One Commit

Choose a strategy where one idea is one commit in the authoritative master/remote version of the repository. Specifically, this means that an entire conceptual changeset ("add a foo widget") is represented in the remote as exactly one commit (in some form), not a sequence of checkpoint commits.

• In SVN, this means don't commit until after an idea has been completely written. All reasonable SVN workflows naturally enforce this.
• In Git, this means squashing checkpoint commits as you go (with git commit --amend) or before pushing (with git rebase -i or git merge --squash), or having a strict policy where your master/trunk contains only merge commits and each is a merge between the old master and a branch which represents a single idea. Although this preserves the checkpoint commits along the branches, you can view master alone as a series of single-idea commits.
• In Mercurial, you can use the "queues" extension before 2.2 or --amend after Mercurial 2.2, or wait to commit until a change is complete (like SVN), although the latter is not recommended. Without extensions, older versions of Mercurial do not support liberal mutability doctrines (so you can't ever combine checkpoint commits) and do not let you build a default out of only merge commits, so it is not possible to have an authoritative repository where one commit represents one idea in any real sense.

Why This Matters

A strategy where one idea is one commit has no real advantage over any other strategy until your repository hits a velocity where it becomes critical. In particular:

• Essentially all operations against the master/remote repository are about ideas, not commits. When one idea is many commits, everything you do is more complicated because you need to figure out which commits represent an idea ("the foo widget is broken, what do I need to revert?") or what idea is ultimately represented by a commit ("commit af3291029 makes no sense, what goal is this change trying to accomplish?").
• Release engineering is greatly simplified. Release engineers can pick or drop ideas easily when each idea corresponds to one commit. When an idea is several commits, it becomes easier to accidentally pick or drop half of an idea and end up in a state which is virtually guaranteed to be wrong.
• Automated testing is greatly simplified. If each idea is one commit, you can run automated tests against every commit and test failures indicate a serious problem. If each idea is many commits, most of those commits represent a known broken state of the codebase (e.g., a checkpoint with a syntax error which was fixed in the next checkpoint, or with a half-implemented idea).
• Understanding changes is greatly simplified. You can bisect to a break and identify the entire idea trivially, without fishing forward and backward in the log to identify the extents of the idea. And you can be confident in what you need to revert to remove the entire idea.
• There is no clear value in having checkpoint commits (some of which are guaranteed to be known broken versions of the repository) persist into the remote. Consider a theoretical VCS which automatically creates a checkpoint commit for every keystroke. This VCS would obviously be unusable. But many checkpoint commits aren't much different, and conceptually represent some relatively arbitrary point in the sequence of keystrokes that went into writing a larger idea. Get rid of them or create an abstraction layer (merge commits) which allows you to ignore them when you are trying to understand the repository in terms of ideas (which is almost always).

All of these become problems only at scale. Facebook pushes dozens of ideas every day and thousands on a weekly basis, and could not do this (at least, not without more people or more errors) without choosing a repository strategy where one idea is one commit.

Next Steps

Continue by:

• reading recommendations on structuring branches with Recommendations on Branching; or
• reading recommendations on structuring changes with Writing Reviewable Code.