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Rollback Is Something You Decide Is Possible Before You Deploy

Blue-green and canary are only worth their cost if reverting actually works, and the schema-sequencing nodes exist precisely to keep it working, so this node is the hinge between them. It presupposes the release shapes and leads directly into expand-then-contract. Its Apply question is a determinate judgement over a concrete release list - which of these steps makes the previous build unable to start - which no other node in this tier asks.