* Deploy everything * It explodes * Rollback everything * Spend two weeks finding problem in one system and then fix it * Deploy everything * It explodes * Rollback everything * Spend two weeks finding a new problem that was created while you were fixing the last problem * Repeat ad nauseum
Migrating iteratively gives you a foundation to build upon with each component
Does it handle queries, trigger CI actions, run jobs?
Of course, you need some way of producing test loads similar to those found in production. One way would be to take a snapshot of production, tap incoming requests for a few weeks, log everything, then replay it at "as fast as we can" speed for testing; another way would be to just mirror production live, running the same operations in test as run in production.
Alternatively, you could take the "chaos monkey" approach (https://www.folklore.org/Monkey_Lives.html), do away with all notions of realism, and just fuzz the heck out of your test system. I'd go with that, first, because it's easy, and tends to catch the more obvious bugs.
How do you handle duplicate requests to external services? Are you going to run credit cards twice? Send emails twice? If not, how do you know it's working with fidelity?
But you need to have pieces that are independent enough to run some here and some there, and ideally pieces that can fail without taking down the whole system.
1. Stateful systems (databases, message brokers) are hard to switch back-and-forth; you often want to migrate each one as few times as possible.
2. If something goes sideways -- especially performance-wise -- it can be hard to tell the reason if everything changed.
3. It takes a long time (months/years) to complete the migration. By doing it incrementally, you can reap the advantages of the new infra, and avoid maintaining two things.
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All that said, GitHub is doing something wrong.
Business by spreadsheet is super hard for this reason - if you try to charge the maximum you can before people get angry and leave then you're a tiny outage/issue/controversy/breach from tipping over the wrong side of that line.