Project Retrospectives · Published March 11, 2026
Retrospective: Two Months of Parallel Running
Running old and new systems together cost extra, and prevented a potentially fatal cutover.
The client's legacy system had run for eight years and the business refused a one-shot switch. We proposed a parallel period. They agreed to one month and it stretched to two.
What parallel running revealed
An undocumented rule in the old system: for returns, freight is charged at half the original order. The new system charged in full, so totals diverged.
Some legacy customers had duplicate ids, and the new system's unique constraint rejected the import.
During monthly closing the old system processed a cross-year adjustment the new one had no logic for.
How to organise it
Key data from both systems is compared automatically every day, alerting past a threshold.
Staff enter data twice, which adds work but keeps the comparison meaningful.
Differences are resolved the same day by a named owner and become input for the next iteration.
Parallel running has a visible cost; a failed cutover does not. For systems carrying core business, the arithmetic is worth doing.
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