Effective Code Review

by alex_gaynor

Maybe you practice code review, either as a part of your open source project or as a part of your team at work, maybe you don't yet. But if you're working on a software project with more than one person it is, in my view, a necessary piece of a healthy workflow. The purpose of this piece is to try to convince you its valuable, and show you how to do it effectively.

This is based on my experience doing code review both as a part of my job at several different companies, as well as in various open source projects.

What

It seems only seems fair that before I try to convince you to make code review an integral part of your workflow, I precisely define what it is.

Code review is the process of having another human being read over a diff. It's exactly like what you might do to review someone's blog post or essay, except it's applied to code. It's important to note that code review is about code. Code review doesn't mean an architecture review, a system design review, or anything like that.

Why

Why should you do code review? It's got a few benefits:

  • It raises the bus factor. By forcing someone else to have the familiarity to review a piece of code you guarantee that at least two people understand it.
  • It ensures readability. By getting someone else to provide feedback based on reading, rather than writing, the code you verify that the code is readable, and give an opportunity for someone with fresh eyes to suggest improvements.
  • It catches bugs. By getting more eyes on a piece of code, you increase the chances that someone will notice a bug before it manifests itself in production. This is in keeping with Eric Raymond's maxim that, "given enough eyeballs, all bugs are shallow".
  • It encourages a healthy engineering culture. Feedback is important for engineers to grow in their jobs. By having a culture of "everyone's code gets reviewed" you promote a culture of positive, constructive feedback. In teams without review processes, or where reviews are infrequent, code review tends to be a tool for criticism, rather than learning and growth.

How

So now that I've, hopefully, convinced you to make code review a part of your workflow how do you put it into practice?

First, a few ground rules:

  • Don't use humans to check for things a machine can. This means that code review isn't a process of running your tests, or looking for style guide violations. Get a CI server to check for those, and have it run automatically. This is for two reasons: first, if a human has to do it, they'll do it wrong (this is true of everything), second, people respond to certain types of reviews better when they come from a machine. If I leave the review "this line is longer than our style guide suggests" I'm nitpicking and being a pain in the ass, if a computer leaves that review, it's just doing it's job.
  • Everybody gets code reviewed. Code review isn't something senior engineers do to junior engineers, it's something everyone participates in. Code review can be a great equalizer, senior engineers shouldn't have special privledges, and their code certainly isn't above the review of others.
  • Do pre-commit code review. Some teams do post-commit code review, where a change is reviewed after it's already pushed to master. This is a bad idea. Reviewing a commit after it's already been landed promotes a feeling of inevitability or fait accompli, reviewers tend to focus less on small details (even when they're important!) because they don't want to be seen as causing problems after a change is landed.
  • All patches get code reviewed. Code review applies to all changes for the same reasons as you run your tests for all changes. People are really bad at guessing the implications of "small patches" (there's a near 100% rate of me breaking the build on change that are "so small, I don't need to run the tests"). It also encourages you to have a system that makes code review easy, you're going to be using it a lot! Finally, having a strict "everything gets code reviewed" policy helps you avoid arguments about just how small is a small patch.

So how do you start? First, get yourself a system. Phabricator, Github's pull requests, and Gerrit are the three systems I've used, any of them will work fine. The major benefit of having a tool (over just mailing patches around) is that it'll keep track of the history of reviews, and will let you easily do commenting on a line-by-line basis.

You can either have patch authors land their changes once they're approved, or you can have the reviewer merge a change once it's approved. Either system works fine.

As a patch author

Patch authors only have a few responsibilities (besides writing the patch itself!).

First, they need to express what the patch does, and why, clearly.

Second, they need to keep their changes small. Studies have shown that beyond 200-400 lines of diff, patch review efficacy trails off [1]. You want to keep your patches small so they can be effectively reviewed.

It's also important to remember that code review is a collaborative feedback process if you disagree with a review note you should start a conversation about it, don't just ignore it, or implement it even though you disagree.

As a review

As a patch reviewer, you're going to be looking for a few things, I recommend reviewing for these attributes in this order:

  • Intent - What change is the patch author trying to make, is the bug they're fixing really a bug? Is the feature they're adding one we want?
  • Architecture - Are they making the change in the right place? Did they change the HTML when really the CSS was busted?
  • Implementation - Does the patch do what it says? Is it possibly introducing new bugs? Does it have documentation and tests? This is the nitty-gritty of code review.
  • Grammar - The little things. Does this variable need a better name? Should that be a keyword argument?

You're going to want to start at intent and work your way down. The reason for this is that if you start giving feedback on variable names, and other small details (which are the easiest to notice), you're going to be less likely to notice that the entire patch is in the wrong place! Or that you didn't want the patch in the first place!

Doing reviews on concepts and architecture is harder than reviewing individual lines of code, that's why it's important to force yourself to start there.

There are three different types of review elements:

  • TODOs: These are things which must be addressed before the patch can be landed; for example a bug in the code, or a regression.
  • Questions: These are things which must be addressed, but don't necessarily require any changes; for example, "Doesn't this class already exist in the stdlib?"
  • Suggestions for follow up: Sometimes you'll want to suggest a change, but it's big, or not strictly related to the current patch, and can be done separately. You should still mention these as a part of a review in case the author wants to adjust anything as a result.

It's important to note which type of feedback each comment you leave is (if it's not already obvious).

Conclusion

Code review is an important part of a healthy engineering culture and workflow. Hopefully, this post has given you an idea of either how to implement it for your team, or how to improve your existing workflow.

[1]http://www.ibm.com/developerworks/rational/library/11-proven-practices-for-peer-review/

Sorry, no comments here. I'd much rather you wrote your own blog post instead. Comments are never going to happen. Stop trying to make comments happen.