Schema Evolution Planner

Plan safe, staged database schema changes across application code, backfills, and cutovers.

Scope: Expand-contract and compatibility planning for live systems. NOT for greenfield schema design (database-architect) or DBA operations.

Canonical Vocabulary

Term	Definition
expand	Additive schema change compatible with existing code
backfill	Data migration that populates new structures from old data
dual-write	Temporarily writing old and new representations
dual-read	Temporarily reading from both old and new representations
cutover	The point where traffic or logic switches to the new path
contract	Removal of deprecated schema after compatibility window closes
compatibility window	Period where old and new code must both work

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