* 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

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.

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.