[state_less]

You can get most of the benefit on smaller scales by building feature branches and ensuring they pass unit tests, deployment and integration testing before they're allowed to be merged to mainline.

It still depends on well written tests, lest your confidence be dashed when a human starts pushing buttons and pulling levers.

Also, don't break up tightly coupled code/modules into separate repos for the sake of microservices. Hard working developers will have to do two or more builds, PRs, possibly update semvers, etc... Find the right seams. If two repos tend to always change in lockstep, think about merging.

[Cthulhu_]

This is what we did for a while at a project; there's options in most git hosts nowadays that force any PR to be up to date with master before merging on the one hand, and to have a green pipeline on the other. That works fine for smaller projects, but because it's not automated you end up with quite a bit of manual labor (rebase on master, push, wait for CI, discover someone else merged into master first, repeat).

[draw_down]

It isn't just that the tests need to be good and humans break things sometimes. At a certain scale, the following happens enough to be a problem:

- changeset A is submitted, an integration branch is cut from latest master, and CI begins

- changeset B is submitted, an integration branch is cut from latest master, and CI begins

- changeset A's integration branch passes CI build/test, so A is merged into master

- changeset B's integration branch passes CI build/test, so B is merged into master

- however, changeset A + B interact in such a way that causes build and/or tests to fail

- build is now broken

You're probably thinking "that sounds like it wouldn't happen very often. Both changes would need to be submitted within some window such that changeset B's integration branch does not include changeset A, and vice-versa". Which is correct, but that's where the scale comes in. With enough engineers this starts happening more, and the more engineers you have the more unacceptable it is to have the build broken for any amount of time. And the more engineers the more code you have so the longer any individual build starts taking which lengthens the window during which the two conflicting changes could be submitted.

You need to do it in a way that serializes the changes because that's the only way to prevent this, but that takes too long. So the paper is about how to solve this problem.

[jadbox]

This is why you rebase and test before each feature branch is to be merged to master. The only issue comes up when someone decides to merge while someone else has already rebased and is running their tests... but when they try to merge to master they will see their branch is out of date and that they need to rebase again. For small teams, it's easy enough to let everyone know that you're merging and not to merge anything else in the meantime. In a larger company, I've seen queue tools that give teams a 'ticket' for their turn to merge into master. It's a little clunky, and probably wouldn't scale to huge engineering teams... but sometimes low tech solutions work just as well.