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The origin story of merge queues

mergify.com

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Re: The origin story of merge queues

#3

Post by zdw »

This seems to skip the idea of stacked commits plus automatic rebasing, which have been around in Gerrit and other tools for quite a while.

If you read between the lines, the underlying problem in most of the discussion is GitHub's dominance of the code hosting space coupled with it's less than ideal CI integration - which while getting better is stuck with baggage from all their past missteps and general API frailty.

Re: The origin story of merge queues

#6

Post by jd__ »

zdw wrote: post #3

This seems to skip the idea of stacked commits plus automatic rebasing, which have been around in Gerrit and other tools for quite a while. If you read between the lines, the underlying problem in most of the discussion is GitHub's dominance of the code hosting space coupled with it's less than ideal CI integration - which while getting better is stuck with baggage from all their past missteps and general API frailty…

That's a good point. To clarify, Gerrit itself didn't actually do merge queuing or CI gating. Its model was stacked commits: every change was rebased on top of the current tip of main before landing. That ensured a linear history but didn't solve the "Is the whole pipeline still green when we merge this?" problem.

That's why the OpenStack community built Zuul on top of Gerrit: it added a real gating system that could speculatively test multiple commits in a queue and only merge them if CI passed together. In other words, Zuul was Gerrit's version of a merge queue.

Re: The origin story of merge queues

#7

Post by sfink »

This covers part of the problem, the part where your tests are enough to indicate whether the changes are good enough to keep. In that scenario, relying only on fast-forward merges is good enough.

One trickier problem is when you don't know until later that a past change was bad: perhaps slow-running performance tests show a regression, or flaky tests turn out to have been showing a real problem, or you just want the additional velocity of pipelined landings that don't wait for all the tests to finish. Or perhaps you don't want to test every change, but then when things break you need to go back and figure out which change(s) caused the issue. (My experience is at Mozilla where all these things are true, and often.) Then you have to deal with backouts: do you keep an always-green chain of commits by backing up and re-landing good commits to splice out the bad, and only fast-forwarding when everything is green? Or do you keep the backouts in the tree, which is more "accurate" in a way but unfortunate for bisecting and code archaeology?

Re: The origin story of merge queues

#8

Post by Roguelazer »

I think this is ignoring a lot of prior art. Our deploys at Yelp in roughly 2010 worked this way -- you flagged a branch as ready to land, a system (`pushmaster` aka `pushhamster`) verified that it passed tests and then did an octopus merge of a bunch of branches, verified that that passed tests, deployed it, and then landed the whole thing to master after it was happy on staging. And this wasn't novel at Yelp; we inherited the practice from PayPal, so my guess is that most companies that care at all about release engineering have been doing it this way for decades and it was just a big regression when people stopped having professional release management teams and started just cowboy pushing to `master` / `main` on github some time in the mid 2010's.

Re: The origin story of merge queues

#9

Post by jes5199 »

I don't know what it's like now, but GitHub's internal merge queue circa 2017 was a nightmare. Every PR required you to set aside a full day of babysitting to get it getting merged/deployed - there were too many nondeterministic steps.

You'd join the queue, and then you'd have to wait for like 12 other people in front of you who would each spend up to a couple hours trying to get their merge branch to go green so it could go out. You couldn't really look away because it could be your turn out of nowhere - and you had to react to it like being on call, because the whole deployment process was frozen until your turn ended. Often that meant just clicking "retry" on parts of the CI process, but it was complicated, there were dependencies between sections of tests.

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