Review Guidelines

Effective code review is a skill like any other professional skill you develop with experience. Effective code review requires trust. No one is perfect. Everyone makes mistakes. Trust builds over time.

This document will enumerate behaviors commonly observed and associated with competent reviews of changes purposed to the Swift code base. No one is expected to “follow these steps”. Guidelines are not rules, not all behaviors will be relevant in all situations.

Code review is collaboration, not judgement.

—Alistair Coles

Checkout the Change

You will need to have a copy of the change in an environment where you can freely edit and experiment with the code in order to provide a non-superficial review. Superficial reviews are not terribly helpful. Always try to be helpful. ;)

Check out the change so that you may begin.

Commonly, git review -d <change-id>

Run it

Imagine that you submit a patch to Swift, and a reviewer starts to take a look at it. Your commit message on the patch claims that it fixes a bug or adds a feature, but as soon as the reviewer downloads it locally and tries to test it, a severe and obvious error shows up. Something like a syntax error or a missing dependency.

“Did you even run this?” is the review comment all contributors dread.

Reviewers in particular need to be fearful merging changes that just don’t work - or at least fail in frequently common enough scenarios to be considered “horribly broken”. A comment in our review that says roughly “I ran this on my machine and observed description of behavior change is supposed to achieve” is the most powerful defense we have against the terrible scorn from our fellow Swift developers and operators when we accidentally merge bad code.

If you’re doing a fair amount of reviews - you will participate in merging a change that will break my clusters - it’s cool - I’ll do it to you at some point too (sorry about that). But when either of us go look at the reviews to understand the process gap that allowed this to happen - it better not be just because we were too lazy to check it out and run it before it got merged.

Or be warned, you may receive, the dreaded…

“Did you even run this?”

I’m sorry, I know it’s rough. ;)

Consider edge cases very seriously

Saying “that should rarely happen” is the same as saying “that will happen”

—Douglas Crockford

Scale is an amazingly abusive partner. If you contribute changes to Swift your code is running - in production - at scale - and your bugs cannot hide. I wish on all of us that our bugs may be exceptionally rare - meaning they only happen in extremely unlikely edge cases. For example, bad things that happen only 1 out of every 10K times an op is performed will be discovered in minutes. Bad things that happen only 1 out of every one billion times something happens will be observed - by multiple deployments - over the course of a release. Bad things that happen 1/100 times some op is performed are considered “horribly broken”. Tests must exhaustively exercise possible scenarios. Every system call and network connection will raise an error and timeout - where will that Exception be caught?

Run the tests

Yes, I know Gerrit does this already. You can do it too. You might not need to re-run all the tests on your machine - it depends on the change. But, if you’re not sure which will be most useful - running all of them best - unit - functional - probe. If you can’t reliably get all tests passing in your development environment you will not be able to do effective reviews. Whatever tests/suites you are able to exercise/validate on your machine against your config you should mention in your review comments so that other reviewers might choose to do other testing locally when they have the change checked out.


I went ahead and ran probe/ on my machine with both sync_method = rsync and sync_method = ssync - that works for me - but I didn’t try it with object_post_as_copy = false

Maintainable Code is Obvious

Style is an important component to review. The goal is maintainability.

However, keep in mind that generally style, readability and maintainability are orthogonal to the suitability of a change for merge. A critical bug fix may be a well written pythonic masterpiece of style - or it may be a hack-y ugly mess that will absolutely need to be cleaned up at some point - but it absolutely should merge because: CRITICAL. BUG. FIX.

You should comment inline to praise code that is “obvious”. You should comment inline to highlight code that you found to be “obfuscated”.

Unfortunately “readability” is often subjective. We should remember that it’s probably just our own personal preference. Rather than a comment that says “You should use a list comprehension here” - rewrite the code as a list comprehension, run the specific tests that hit the relevant section to validate your code is correct, then leave a comment that says:

I find this more readable:

diff with working tested code

If the author (or another reviewer) agrees - it’s possible the change will get updated to include that improvement before it is merged; or it may happen in a follow-up change.

However, remember that style is non-material - it is useful to provide (via diff) suggestions to improve maintainability as part of your review - but if the suggestion is functionally equivalent - it is by definition optional.

Commit Messages

Read the commit message thoroughly before you begin the review.

Commit messages must answer the “why” and the “what for” - more so than the “how” or “what it does”. Commonly this will take the form of a short description:

  • What is broken - without this change

  • What is impossible to do with Swift - without this change

  • What is slower/worse/harder - without this change

If you’re not able to discern why a change is being made or how it would be used - you may have to ask for more details before you can successfully review it.

Commit messages need to have a high consistent quality. While many things under source control can be fixed and improved in a follow-up change - commit messages are forever. Luckily it’s easy to fix minor mistakes using the in-line edit feature in Gerrit! If you can avoid ever having to ask someone to change a commit message you will find yourself an amazingly happier and more productive reviewer.

Also commit messages should follow the OpenStack Commit Message guidelines, including references to relevant impact tags or bug numbers. You should hand out links to the OpenStack Commit Message guidelines liberally via comments when fixing commit messages during review.

Here you go: GitCommitMessages

New Tests

New tests should be added for all code changes. Historically you should expect good changes to have a diff line count ratio of at least 2:1 tests to code. Even if a change has to “fix” a lot of existing tests, if a change does not include any new tests it probably should not merge.

If a change includes a good ratio of test changes and adds new tests - you should say so in your review comments.

If it does not - you should write some!

… and offer them to the patch author as a diff indicating to them that “something” like these tests I’m providing as an example will need to be included in this change before it is suitable to merge. Bonus points if you include suggestions for the author as to how they might improve or expand upon the tests stubs you provide.

Be very careful about asking an author to add a test for a “small change” before attempting to do so yourself. It’s quite possible there is a lack of existing test infrastructure needed to develop a concise and clear test - the author of a small change may not be the best person to introduce a large amount of new test infrastructure. Also, most of the time remember it’s harder to write the test than the change - if the author is unable to develop a test for their change on their own you may prevent a useful change from being merged. At a minimum you should suggest a specific unit test that you think they should be able to copy and modify to exercise the behavior in their change. If you’re not sure if such a test exists - replace their change with an Exception and run tests until you find one that blows up.


Most changes should include documentation. New functions and code should have Docstrings. Tests should obviate new or changed behaviors with descriptive and meaningful phrases. New features should include changes to the documentation tree. New config options should be documented in example configs. The commit message should document the change for the change log.

Always point out typos or grammar mistakes when you see them in review, but also consider that if you were able to recognize the intent of the statement - documentation with typos may be easier to iterate and improve on than nothing.

If a change does not have adequate documentation it may not be suitable to merge. If a change includes incorrect or misleading documentation or is contrary to existing documentation is probably is not suitable to merge.

Every change could have better documentation.

Like with tests, a patch isn’t done until it has docs. Any patch that adds a new feature, changes behavior, updates configs, or in any other way is different than previous behavior requires docs. manpages, sample configs, docstrings, descriptive prose in the source tree, etc.

Reviewers Write Code

Reviews have been shown to provide many benefits - one of which is shared ownership. After providing a positive review you should understand how the change works. Doing this will probably require you to “play with” the change.

You might functionally test the change in various scenarios. You may need to write a new unit test to validate the change will degrade gracefully under failure. You might have to write a script to exercise the change under some superficial load. You might have to break the change and validate the new tests fail and provide useful errors. You might have to step through some critical section of the code in a debugger to understand when all the possible branches are exercised in tests.

When you’re done with your review an artifact of your effort will be observable in the piles of code and scripts and diffs you wrote while reviewing. You should make sure to capture those artifacts in a paste or gist and include them in your review comments so that others may reference them.


When I broke the change like this:


it blew up like this:

unit test failure

It’s not uncommon that a review takes more time than writing a change - hopefully the author also spent as much time as you did validating their change but that’s not really in your control. When you provide a positive review you should be sure you understand the change - even seemingly trivial changes will take time to consider the ramifications.

Leave Comments

Leave. Lots. Of. Comments.

A popular web comic has stated that WTFs/Minute is the only valid measurement of code quality.

If something initially strikes you as questionable - you should jot down a note so you can loop back around to it.

However, because of the distributed nature of authors and reviewers it’s imperative that you try your best to answer your own questions as part of your review.

Do not say “Does this blow up if it gets called when xyz” - rather try and find a test that specifically covers that condition and mention it in the comment so others can find it more quickly. Or if you can find no such test, add one to demonstrate the failure, and include a diff in a comment. Hopefully you can say “I thought this would blow up, so I wrote this test, but it seems fine.”

But if your initial reaction is “I don’t understand this” or “How does this even work?” you should notate it and explain whatever you were able to figure out in order to help subsequent reviewers more quickly identify and grok the subtle or complex issues.

Because you will be leaving lots of comments - many of which are potentially not highlighting anything specific - it is VERY important to leave a good summary. Your summary should include details of how you reviewed the change. You may include what you liked most, or least.

If you are leaving a negative score ideally you should provide clear instructions on how the change could be modified such that it would be suitable for merge - again diffs work best.


Scoring is subjective. Try to realize you’re making a judgment call.

A positive score means you believe Swift would be undeniably better off with this code merged than it would be going one more second without this change running in production immediately. It is indeed high praise - you should be sure.

A negative score means that to the best of your abilities you have not been able to your satisfaction, to justify the value of a change against the cost of its deficiencies and risks. It is a surprisingly difficult chore to be confident about the value of unproven code or a not well understood use-case in an uncertain world, and unfortunately all too easy with a thorough review to uncover our defects, and be reminded of the risk of… regression.

Reviewers must try very hard first and foremost to keep master stable.

If you can demonstrate a change has an incorrect behavior it’s almost without exception that the change must be revised to fix the defect before merging rather than letting it in and having to also file a bug.

Every commit must be deployable to production.

Beyond that - almost any change might be merge-able depending on its merits! Here are some tips you might be able to use to find more changes that should merge!

  1. Fixing bugs is HUGELY valuable - the only thing which has a higher cost than the value of fixing a bug - is adding a new bug - if it’s broken and this change makes it fixed (without breaking anything else) you have a winner!

  2. Features are INCREDIBLY difficult to justify their value against the cost of increased complexity, lowered maintainability, risk of regression, or new defects. Try to focus on what is impossible without the feature - when you make the impossible possible, things are better. Make things better.

  3. Purely test/doc changes, complex refactoring, or mechanical cleanups are quite nuanced because there’s less concrete objective value. I’ve seen lots of these kind of changes get lost to the backlog. I’ve also seen some success where multiple authors have collaborated to “push-over” a change rather than provide a “review” ultimately resulting in a quorum of three or more “authors” who all agree there is a lot of value in the change - however subjective.

Because the bar is high - most reviews will end with a negative score.

However, for non-material grievances (nits) - you should feel confident in a positive review if the change is otherwise complete correct and undeniably makes Swift better (not perfect, better). If you see something worth fixing you should point it out in review comments, but when applying a score consider if it need be fixed before the change is suitable to merge vs. fixing it in a follow up change? Consider if the change makes Swift so undeniably better and it was deployed in production without making any additional changes would it still be correct and complete? Would releasing the change to production without any additional follow up make it more difficult to maintain and continue to improve Swift?

Endeavor to leave a positive or negative score on every change you review.

Use your best judgment.

A note on Swift Core Maintainers

Swift Core maintainers may provide positive reviews scores that look different from your reviews - a “+2” instead of a “+1”.

But it’s exactly the same as your “+1”.

It means the change has been thoroughly and positively reviewed. The only reason it’s different is to help identify changes which have received multiple competent and positive reviews. If you consistently provide competent reviews you run a VERY high risk of being approached to have your future positive review scores changed from a “+1” to “+2” in order to make it easier to identify changes which need to get merged.

Ideally a review from a core maintainer should provide a clear path forward for the patch author. If you don’t know how to proceed respond to the reviewers comments on the change and ask for help. We’d love to try and help.