alt Hacker NewsThe canonical example here is a feature for a website that requires both backend and frontend work. The frontend depends on the backend, but the backend does not depend on the frontend. This means that the first commit is "independent" in the sense that it can land without the second, but the second is not, hence, a stack. The root of the stack can always be landed independently of what is on top of it, while the rest of the stack is dependent.
> If they’re not, why do you want to review them independently?
For this example, you may want review from both a backend engineer and a frontend engineer. That said, see this too though:
> that seems like something you can already do by just breaking a PR into commits and letting people look at one of those at a time.
If you do this in a PR, both get assigned to review the whole thing. Each person sees the code that they don't care about, because they're grouped together. Notifications go to all parties instead of the parties who care about each section. Both reviews can proceed independently in a stack, whereas they happen concurrently in a PR.
> If you approve change A and reject change B, then the feature is only half done.
It depends on what you mean by "the feature." Seen as one huge feature, then yes, it's true that it's not finished until both land. But seen as two separate but related features, it's fine to land the independent change before the dependent one: one feature is finished, but the other is not.
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> If you do this in a PR, both get assigned to review the whole thing. Each person sees the code that they don't care about, because they're grouped together.
There are two separate issues you’re bringing up:
- Both groups being “assigned” the PR: fixable with code owners files. It’s more elegant than assigning diffs to people: groups of people have ownership over segments of the codebase and are responsible for approving changes to it. Solves the problem way better IMO.
- Both groups “seeing” all the changes: I already said GitHub lets you view single commits during PR review. That is already a solved problem.
And I didn’t even bring up the fact that you can just open a second PR for the frontend change that has the backend commit as the parent. Yes, the second PR is a superset of the first, but we’ve already established that (1) the second change isn’t orthogonal to the first one and can’t be merged independently anyway, and (2) reviewers can select only the commits that are in the frontend range. Generally you just mark the second PR as draft until the first one merges (or do what Gitlab does and mark it as “depends on” the first, which prevents it from merging until the first one is done.) The first PR being merged will instantly make the second PR’s diff collapse to just the unique changes once you rebase/merge in the latest main, too.
All of this is to explain how we can already do pretty much all of this. But in reality, it’s silly to have people review change B if change A hasn’t landed yet. A reviewer from A may completely throw the whole thing out and tell you to start over, or everything could otherwise go back to the drawing board. Making reviewers look at change B before this is done, is a potential for a huge waste of time. But then you may think reviewers from change B may opt to make the whole plan go back to the drawing board too, so what makes A so special? And the answer is it’s both a bad approach: just make the whole thing in one PR, and discuss it holistically. Code owners files are for assigning ownership, and breaking things into separate commits is to help people look at a subset of the changes. (Or just, like, have them click on the folder in the source tree they care about. This is not a problem that needs a whole new code review paradigm.)
If the layers of a stack have a disjoint set of reviewers things are viewed in separation which might lead to issues if there is no one reviewing the full picture.