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Writing Reviewable Code Article

Project recommendations on how to structure changes.

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.

Overview

This document describes a strategy for structuring changes used successfully at Facebook and in Phabricator. In essence:

  • Each commit should be as small as possible, but no smaller.
  • The smallest a commit can be is a single cohesive idea: don't make commits so small that they are meaningless on their own.
  • There should be a one-to-one mapping between ideas and commits: each commit should build one idea, and each idea should be implemented by one commit.
  • Turn large commits into small commits by dividing large problems into smaller problems and solving the small problems one at a time.
  • Write sensible commit messages.

Many Small Commits

Small, simple commits are generally better than large, complex commits. They are easier to understand, easier to test, and easier to review. The complexity of understanding, testing and reviewing a change often increases faster than its size: ten 200-line changes each doing one thing are often far easier to understand than one 2,000 line change doing ten things. Splitting a change which does many things into smaller changes which each do only one thing can decrease the total complexity associated with accomplishing the same goal.

Each commit should do one thing. Generally, this means that you should separate distinct changes into different commits when developing. For example, if you're developing a feature and run into a preexisting bug, stash or checkpoint your change, check out a clean HEAD/tip, fix the bug in one change, and then merge/rebase your new feature on top of your bugfix so that you have two changes, each with one idea ("add feature x", "fix a bug in y"), not one change with two ideas ("add feature x and fix a bug in y").

(In Git, you can do this easily with local feature branches and commands like git rebase, git rebase -i, and git stash, or with merges. In Mercurial, you can use bookmarks or the queues extension. In SVN, there are few builtin tools, but you can use multiple working copies or treat Differential like a stash you access with arc patch.)

Even changes like fixing style problems should ideally be separated: they're accomplishing a different goal. And it is far easier to review one 300-line change which "converts tabs to spaces" plus one 30-line change which "implements feature z" than one 330-line change which "implements feature z and also converts a bunch of tabs to spaces".

Similarly, break related but complex changes into smaller, simpler components. Here's a ridiculous analogy: if you're adding a new house, don't make one 5,000-line change which adds the whole house in one fell sweep. Split it apart into smaller steps which are each easy to understand: start with the foundation, then build the frame, etc. If you decided to dig the foundation with a shovel or build the frame out of cardboard, it's both easier to miss and harder to correct if the decisions are buried in 5,000 lines of interior design and landscaping. Do it one piece at a time, providing enough context that the larger problem can be understood but accomplishing no more with each step than you need to in order for it to stand on its own.

The minimum size of a change should be a complete implementation of the simplest subproblem which works on its own and expresses an entire idea, not just part of an idea. You could mechanically split a 1,000-line change into ten 100-line changes by choosing lines at random, but none of the individual changes would make any sense and you would increase the collective complexity. The real goal is for each change to have minimal complexity, line size is just a proxy that is often well-correlated with complexity.

We generally follow these practices in Phabricator. The median change size for Phabricator is 35 lines.

See Differential User Guide: Large Changes for information about reviewing big checkins.

Write Sensible Commit Messages

There are lots of resources for this on the internet. All of them say pretty much the same thing; this one does too.

The single most important thing is: commit messages should explain why you are making the change.

Differential attempts to encourage the construction of sensible commit messages, but can only enforce structure, not content. Structurally, commit messages should probably:

  • Have a title, briefly describing the change in one line.
  • Have a summary, describing the change in more detail.
  • Maybe have some other fields.

The content is far more important than the structure. In particular, the summary should explain why you're making the change and why you're choosing the implementation you're choosing. The what of the change is generally well-explained by the change itself. For example, this is obviously an awful commit message:

fix a bug

But this one is almost as bad:

Allow dots in usernames

Change the regexps so usernames can have dots in them.

This is better than nothing but just summarizes information which can be inferred from the text of the diff. Instead, you should provide context and explain why you're making the change you're making, and why it's the right one:

Allow dots in usernames to support Google and LDAP auth

To prevent nonsense, usernames are currently restricted to A-Z0-9. Now that
we have Google and LDAP auth, a couple of installs want to allow "." too
since they have schemes like "abraham.lincoln@mycompany.com" (see Tnnn). There
are no technical reasons not to do this, so I opened up the regexps a bit.

We could mostly open this up more but I figured I'd wait until someone asks
before allowing "ke$ha", etc., because I personally find such names
distasteful and offensive.

This information can not be extracted from the change itself, and is much more useful for the reviewer and for anyone trying to understand the change after the fact.

An easy way to explain why is to reference other objects (bugs/issues/revisions) which motivate the change.

Differential also includes a "Test Plan" field which is required by default. There is a detailed description of this field in Differential User Guide: Test Plans. You can make it optional or disable it in the configuration, but consider adopting it. Having this information can be particularly helpful for reviewers.

Some things that people sometimes feel strongly about but which are probably not really all that important in commit messages include:

  • If/where text is wrapped.
  • Maximum length of the title.
  • Whether there should be a period or not in the title.
  • Use of voice/tense, e.g. "fix"/"add" vs "fixes"/"adds".
  • Other sorts of pedantry not related to getting the context and reasons why a change is happening into the commit message.
  • Although maybe the spelling and grammar shouldn't be egregiously bad?

Phabricator does not have guidelines for this stuff. You can obviously set guidelines at your organization if you prefer, but getting the why into the message is the most important part.

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