Comparison
Dual writevsParallel run
Dual write
you wrote to both databases for the whole migration, and spent a week chasing the rows where only one write landed.
Writing the same data to the old and new stores during a migration so either can serve reads. It is easy to describe and hard to do correctly, because two writes without a shared transaction will eventually diverge, and the divergence is silent. Teams that do it successfully plan for reconciliation from the start — a comparison job, a repair path, and a decision about which store is authoritative at each phase.
Full entry →Parallel run
both systems processed every transaction for a month and you only looked at where they disagreed.
Running the old and new implementations side by side on real input, with only the old one's output used, and comparing results. It is the most convincing evidence available that a replacement is correct, because it tests against real data rather than the data you imagined. It costs double compute and a genuine comparison mechanism, and the discipline is deciding in advance what disagreement rate means 'switch'.
Full entry →