Dual writes are seductive because they seem to let a team move gradually from one system to another. They are dangerous because correctness becomes temporal the moment two destinations can disagree.
Dual writes are migration tools, not correctness guarantees
The pattern exists to create overlap during change. It should not be mistaken for a stable long-term correctness mechanism unless the team is willing to own comparison and reconciliation forever.
Comparison has to be first-class
A safe migration design includes a lane for comparing outputs, measuring divergence, and deciding who has authority during the overlap period.
Diagram
Migration control plane
Rollback must be credible
If rollback is only discussed in principle, the migration is not ready. Teams need to know how to stop, who becomes the source of truth, and what happens to writes already accepted by the new system.
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