28 January 2021
Pull Requests are a mechanism popularized by github, used to help facilitate merging of work, particularly in the context of open-source projects. A contributor works on their contribution in a fork (clone) of the central repository. Once their contribution is finished they create a pull request to notify the owner of the central repository that their work is ready to be merged into the mainline. Tooling supports and encourages code review of the contribution before accepting the request. Pull requests have become widely used in software development, but critics are concerned by the addition of integration friction which can prevent continuous integration.
Pull requests essentially provide convenient tooling for a development workflow that existed in many open-source projects, particularly those using a distributed source-control system (such as git). This workflow begins with a contributor creating a new logical branch, either by starting a new branch in the central repository, cloning into a personal repository, or both. The contributor then works on that branch, typically in the style of a Feature Branch, pulling any updates from Mainline into their branch. When they are done they communicate with the maintainer of the central repository indicating that they are done, together with a reference to their commits. This reference could be the URL of a branch that needs to be integrated, or a set of patches in an email.
Once the maintainer gets the message, she can then examine the commits to decide if they are ready to go into mainline. If not, she can then suggest changes to the contributor, who then has opportunity to adjust their submission. Once all is ok, the maintainer can then merge, either with a regular merge/rebase or applying the patches from the final email.
Github's pull request mechanism makes this flow much easier. It keeps track of the clones through its fork mechanism, and automatically creates a message thread to discuss the pull request, together with behavior to handle the various steps in the review workflow. These conveniences were a major part of what made github successful and led to "pull request" becoming a fundamental part of the developer's lexicon.
So that's how pull requests work, but should we use them, and if so how? To answer that question, I like to step back from the mechanism and think about how it works in the context of a source code management workflow. To help me think about that, I wrote down a series of patterns for managing source code branching. I find understanding these (specifically the Base and Integration patterns) clarifies the role of pull requests.
In terms of these patterns, pull requests are a mechanism designed to implement a combination of Feature Branching and Pre-Integration Reviews. Thus to assess the usefulness of pull requests we first need to consider how applicable those patterns are to our situation. Like most patterns, they are sometimes valuable, and sometimes a pain in the neck - we have to examine them based on our specific context. Feature Branching is a good way of packaging together a logical contribution so that it can be assessed, accepted, or deferred as a single unit. This makes a lot of sense when contributors are not trusted to commit directly to mainline. But Feature Branching comes at a cost, which is that it usually limits the frequency of integration, leading to complicated merges and deterring refactoring. Pre-Integration Reviews provide a clear place to do code review at the cost of a significant increase in integration friction. 
That's a drastic summary of the situation (I need a lot more words to explain this further in the feature branching article), but it boils down to the fact that the value of these patterns, and thus the value of pull requests, rest mostly on the social structure of the team. Some teams work better with pull requests, some teams would find pull requests a severe drag on the effectiveness. I suspect that since pull requests are so popular, a lot of teams are using them by default when they would do better without them.
While pull requests are built for Feature Branches, teams can use them within a Continuous Integration environment. To do this they need to ensure that pull requests are small enough, and the team responsive enough, to follow the CI rule of thumb that everybody does Mainline Integration at least daily. (And I should remind everyone that Mainline Integration is more than just merging the current mainline into the feature branch).
The wide usage of pull requests has encouraged a wider use of code review, since pull requests provide a clear point for Pre-Integration Review, together with tooling that encourages it. Code review is a Good Thing, but we must remember that a pull request isn't the only mechanism we can use for it. Many teams find great value in the continuous review afforded by Pair Programming. To avoid reducing integration frquency we can carry out post-integration code review in several ways. A formal process can record a review for each commit, or a tech lead can examine risky commits every couple of days. Perhaps the most powerful form of code review is one that's frequently ignored. A team that takes the attitude that the codebase is a fluid system, one that can be steadily refined with repeated iteration carries out Refinement Code Review every time a developer looks at existing code. I often hear people say that pull requests are necessary because without them you can't do code reviews - that's rubbish. Pre-integration code review is just one way to do code reviews, and for many teams it isn't the best choice.
AcknowledgementsChris Ford, Dan Mutton, Jeremy Huiskamp, Kief Morris, Pramod Sadalage, and Ryan Boucher commented on drafts of this post on our internal mailing list.
1: A colleague of mine recently calculated the time a client spent waiting for pull requests that had no comments (true of 91% of them). Total time waiting in 2020 for 7000 PRs was 130,000 hours. This figure included time elapsed over nights and weekends.