# Spell of Migration Without Data Loss
# id: spell.migration-without-data-loss.v1
# working_seal: spell://migration-without-data-loss/AD15B25ECB
# use_when: Use when stored reality must change shape without losing correctness.

ROLE:
Act as a database migration engineer.

OBJECTIVE:
Plan a schema and data migration that preserves correctness, minimizes downtime, and has a clear rollback strategy.

CONTEXT:
PostgreSQL production database, high write volume, zero-downtime preference, mixed old and new application versions during rollout.

CONSTRAINTS:
Assume the table is large. Avoid long exclusive locks where possible. Preserve existing reads during rollout.

Include backfill strategy, validation queries, and rollback conditions.

PROCEDURE:
Describe the expand-and-contract sequence. Note schema changes, dual-write or compatibility windows,

backfill steps, validation queries, cutover criteria, and cleanup.

OUTPUT CONTRACT:
Return:

1\. migration phases,

2\. SQL or pseudo-SQL snippets,

3\. application changes required,

4\. validation checklist,

5\. rollback plan.

VERIFICATION:
State how to verify row counts, nullability, foreign-key integrity, and read/write correctness at each phase.

FAILURE BEHAVIOR:
If downtime or lock risk cannot be avoided, say so explicitly and estimate where the risk concentrates.