Migration and Cutover: Two Systems, One Truth at a Time

Why this matters

A team migrates a database over a weekend with no rehearsal: the copy takes twice as long as guessed, the cutover lands mid-inconsistency, and Monday starts with two systems disagreeing about money. The rollback fails because the old system kept accepting writes after the copy. Every migration disaster shares one shape: the truth lived in two places with no rule saying which one counted, at exactly the moment it mattered. Checkpoints and rehearsals exist so Monday starts with one truth.

Concepts

The arc has five stations. Copy moves data without stopping the old system (snapshots, replication, bulk load). Dual-run operates both systems with writes mirrored or shadowed, so behavior compares live. Verify proves equality: row counts, checksums, sampled reads against both sides until the differences are zero and understood. Cut switches traffic at a declared checkpoint with the old path kept warm. Retire decommissions the old system explicitly, with its data archived, never left running as a silent second truth.

The checkpoint is the contract. It names the moment (traffic percentage, timestamp, transaction ID) after which the new system is authoritative, and the way back that stays valid until confidence holds: reverse replication, retained old system, tested re-cut procedure. A cutover without a way back is a jump, not a migration; jumps belong in no professional plan.

Rehearsal is non-negotiable and fixture-first. The L56 lab rehearses with checkpoints on small data; production rehearsals follow on copies, never on live systems. Each rehearsal measures what the plan guessed: copy duration, lag under load, verification time. The plan's numbers come from rehearsal, not from vendor slides or hope.

Worked example

A fixture store migrates from file to database across the five stations. The learner copies, dual-writes, verifies to zero diff, cuts at a checkpoint with the file path warm, then retires the file after a soak period. The checkpoint document names the moment, the verification quotes equality, and the way back stays tested until retirement.

Common wrong move

Migrating and cutting in one motion with no dual-run. The first comparison of old and new happens after the old is gone, when differences are mysteries instead of fixable gaps. Separate the copy from the cut by verification, always.