29 Live Migration Stack – The Grimoire of Software Magic Words

30.1 Related Runes

0926 migration - Stored reality changing shape without being lost. Good migrations are staged, inspectable, and reversible long enough to discover what the data actually contains.
0943 rollback {DB} - Permission to retreat toward a previously trusted state. A system without rollback mistakes hope for safety.
0347 schema {Data} - The declared shape of stored meaning.
1461 idempotent - The word that makes retries survivable: same request, same lasting effect.
1567 schema drift - The stored world and the assumed world have silently stopped matching. This is how migrations keep hurting after everyone thinks they are done.

Migration Without Data Loss
Stack Grammar
Failure Modes
Canon Map | Step | Spell | Artifact | Advance when | | —- | —————– | —————————————– | ——————————— | | 1 | Inventory | Dependency and blast-radius map | Moving parts are known | | 2 | Prepare rollback | Backup, snapshot, flag, or dual-read path | Reversal is real | | 3 | Expand compatibly | Backward-compatible new structure | Old paths still work | | 4 | Backfill | New representation | Old path is not cut off | | 5 | Compare parity | Parity report | Old and new paths agree | | 6 | Cut over | Read or write switch | Parity holds and rollback is live | | 7 | Observe | Live metrics and divergence samples | Safety window remains stable | | 8 | Contract | Removed old paths | Safety window closes cleanly |

{
  "steps": [
    5,
    6,
    7
  ],
  "until": "Confidence is sufficient for cutover and contraction"
}

Rollback immediately using the prepared boundary and preserve divergence samples.

It forces reversibility to exist before forward motion becomes expensive.