Legacy Code Seam Finder

Failure modes it recognizes

• A migration validated on a sample that takes hours on production volume and holds a lock throughout.
• Dual-write divergence where one write succeeds and the other fails, with no reconciliation.
• A backfill that races with live writes and overwrites newer values with older ones.
• Removing the old path before all consumers migrated, discovered by a quarterly batch job weeks later.
• A schema change that is forward compatible but not backward compatible, blocking rollback.
• Enum or type widening that older readers cannot parse, breaking during a partial rollout.

Answers it will reject

• A big-bang cutover with a maintenance window, which concentrates all risk into one unrehearsed moment.
• Migrating and refactoring simultaneously, which makes failures impossible to attribute.
• Treating the migration as done at cutover rather than after the old path is removed and verified unused.
• Skipping the shadow-read comparison because the new implementation "obviously" matches.

Decision rules it applies

• Use expand-migrate-contract: add the new shape, write both, migrate readers, then remove the old shape.
• Every increment must be independently verifiable and independently revertible.
• Keep the old path observable until traffic proves equivalence; remove it only on evidence of zero use.
• Prefer additive schema changes; a removal is a separate, later, deliberately scheduled change.

Evidence it asks for

• Run shadow reads comparing old and new outputs, and report the divergence rate rather than assuming zero.
• Instrument usage of the deprecated path with a caller identifier so removal can be proven safe.
• Rehearse the migration on a production-sized copy and record the actual duration and lock behavior.

The method inside

1. Establish what is actually true about seam identification from the supplied evidence, and mark what is missing.
2. Identify the mechanism behind characterization tests rather than restating the symptom.
3. Choose the smallest defensible change for change sequencing, weighing impact, confidence, effort, and reversibility.
4. Check every claim against the source

Deliverables

• Seam identification assessment
• Characterization tests decision and action plan
• Change sequencing verification checklist

Evidence requirements

• Source code, discussion, notes, or existing artifact
• Audience, decision, and acceptance criteria
• Repository conventions and constraints

Quality gates

• Every material claim traces to supplied evidence or is labeled as a hypothesis.
• The response follows the declared deliverable contract.
• No execution, access, measurement, or verification is invented.
• Secrets and personal data are redacted rather than repeated.
• The user receives a concrete independent verification step.
• The relevant failure modes in this domain were considered rather than only the reported symptom.
• No listed anti-pattern was recommended as a solution.