PRs are a great way of sharing information, and can help us be aware of the changes that are occuring in our codebase. They are also an excellent way of getting peer review on the work that we do, without the cost of working in direct pairs.
Ultimately though, the primary reason we use PRs is to encourage quality in the commits that are made to our code repositories
Done well, the commits (and their attached messages) contained within tell a story to people examining the code at a later date. If we are not careful to ensure the quality of these commits, we silently lose this ability.
Poor quality code can be refactored. A terrible commit lasts forever.
A good quality PR will have the following characteristics:
- It will be a complete piece of work that adds value in some way.
- It will have a title that reflects the work within, and a summary that helps to understand the context of the change.
- There will be well written commit messages, with well crafted commits that tell the story of the development of this work.
- Ideally it will be small and easy to understand. Single commit PRs are usually easy to submit, review, and merge.
- The code contained within will meet the best practises set by the team wherever possible.
A PR does not end at submission though. A code change is not made until it is merged and used in production.
A good PR should be able to flow through a peer review system easily and quickly.
PRs are a Github workflow tool, so it's important to understand that the PR title, summary and eventual discussion are not as trackable as the the commit history. If we ever move away from Github, we'll likely lose this infomation.
That said however, they are a very useful aid in ensuring that PRs are handled quickly and effectively.
Ensure that your PR title is scannable. People will read through the list of PRs attached to a repo, and must be able to distinguish between them based on title. Include a story/issue reference if possible, so the reviewer can get any extra context. Include a reference to the subsystem affected, if this is a large codebase.
Use keywords in the title to help people understand your intention with the PR, eg [WIP] to indicate that it's still in progress, so should not be merged.
Unless there is a good reason not to rebase - typically because more than one person has been working on the branch - it is often a good idea to rebase your branch to tidy up before submitting the PR.
Use git rebase -i master # or other reference, eg HEAD~5
For example:
- Merge 'oops, fix typo/bug' into their parent commit. There is no reason to create and solve bugs within a PR, unless there is educational value in highlighting them.
- Reword your commit messages for clarity. Once a PR is submitted, any rewording of commits will involve a rebase, which can then mess up the conversation in the PR.
In an ideal world, your PR will be one small(ish) commit, doing one thing - in which case your life will be made easier, since the commit message and PR title/summary are equivalent.
If your change contains more work than can be sensibly covered in a single commit though, do not try to squash it down into one. Commit history should tell a story, and if that story is long then it may require multiple commits to walk the reviewer through it.
Commit messages are invaluable to someone reading the history of the code base, and are critical for understanding why a change was made.
Try to ensure that there is enough information in there for a person with no context or understanding of the code to make sense of the change.
Where external information references are available - such as Issue/Story IDs, PR numbers - ensure that they are included in the commit message.
Remember that your commit message must survive the ravages of time. Try to link to something that will be preserved equally well -- another commit for example, rather than linking to master.
** Each commit message should include the reason why this commit was made. Usually by adding a sentence completing the form 'So that we...' will give an amazing amount of context to the history that the code change itself cannot **
Try to only fix one issue or add one feature within the pull request. The larger it is, the more complex it is to review and the more likely it will be delayed. Remember that reviewing PRs is taking time from someone else's day.
If you must submit a large PR, try to at least make someone else aware of this fact, and arrange for their time to review and get the PR merged. It's not fair to the team to dump large pieces of work on their laps without warning.
If you can rebase up a large PR into multiple smaller PRs, then do so.
It's a reviewers responsibility to ensure:
- Commit history is excellent
- Good changes are propagated quickly
- Code review is performed
- They understand what is being changed, from the perspective of someone examining the code in the future.
The importance of ensuring a quality commit history cannot be stressed enough. It is the historical context of all of the work that we do, and is vital for understanding the reasons why changes were made in the past. What is obvious now, will not be obvious 2-3 years in the future.
Without a decent commit history, we may as well be storing all our code in files ending yyyy-mm-dd. The commit history of a code base is what allows people to understand why a change was made - the when, what, and where are automatically evident.
When looking at a commit message, ask yourself the question - from the perspective of someone looking at this change without any knowledge of the codebase - 'do I understand why this change was made?'
If any commit within the PR does not meet this standard, the PR should be rebased until it does. We cannot fix a commit history once it is in place, unlike our ability to refactor crappy code or fix bugs.
A useful tip is simply asking the submitter to add a sentence to the commit message completing the sentence 'So that we...'.
Pull Requests are the fundamental unit of how we progress change. If PRs are getting clogged up in the system, either unreviewed or unmanaged, they are preventing a piece of work from being completed.
As PRs clog up in the system, merges become more difficult, as other features and fixes are applied to the same codebase. This in turn slows them down further, and often completely blocks progress on a given codebase.
There is a balance between flow and ensuring the quality of our PRs. As a reviewer you should make a call as to whether a code quality issue is sufficient enough to block the PR whilst the code is improved. Possibly it is more prudent to simply flag that the code needs rework, and raise an issue.
Any quality issue that will obviously result in a bug should be fixed.
To make sure PRs flow through the system speedily, we must scale the PR review process. It is not sufficient (or fair!) to expect one or two people to review all PRs to our code. For starters, it creates a blocker every time those people are busy.
Hopefully with the above guidelines, we can all start sharing the responsibility of being a reviewer.
NB: With this in mind - if you are the first to comment on a PR, you are that PRs reviewer. If you feel that you can no longer be responsible for the subsequent merge or closure of the PR, then flag this up in the PR conversation, so someone else can take up the role.
There's no reason why multiple people cannot comment on a PR and review it, and this is to be encouraged.
It's sometimes tempting to fix a bug in a PR yourself, or to rework a section to meet coding standards, or just to make a feature better fit your needs.
If you do this, you are no longer the reviewer of the PR. You are a collaborator, and so should not merge the PR.
It is of course possible to find a new reviewer, but generally change will be speedier if you require the original submitter to fix the code themselves. Alternatively, if the original PR is 'good enough', raise the changes you'd like to see as separate stories/issues, and rework in your own PR.
We are all busy people, and in the case of many PRs against our codebase we are not able or time-permitted to test the new code.
We need to assume that the submitter has tested their code to the point of being happy with the work to be merged to master and subsequently released.
If you, as a reviewer, are suspicious that the work in the PR has not been tested, raise this with the submitter. Find out how they have tested it, and refuse the work if they have not. They may not have a mechanism to test it, in which case you may need to help.
If, as a submitter, you know that this change is not fully tested, highlight this in the PR text, and talk to the reviewer.
Amazing article on how to effectively use the commit history as an incredible documentation source:
Github's recommendations for PRs, which has some useful tips on notifying teams and collegues via the github @-syntax.
The GOV.UK git styleguide:
An example of an extremely good commit message, for a change that would be highly confusing otherwise:
GDS advice on PRs: