Parallel run
Running a new version of a system alongside the old one, with the same input, and comparing the results before switching over.
Also written parallel running, parallel testing, side-by-side run
A parallel run is a way of proving that a new system or version gives the same results as the old one. Both are run with the same input, often for several days or weeks, and their outputs are compared. Only when they agree, or every difference is explained, does the organisation switch to the new one.
It is used for large changes where a mistake would be costly: moving an application to a new platform, upgrading a compiler and recompiling every program, replacing a calculation engine, or rewriting part of a system.
The comparison is the hard part. Outputs might be files, database contents or reports, and some differences are expected, such as timestamps or the order of records. Teams write tools or use compare utilities to filter out the expected differences and find the real ones.
Parallel runs take time and effort, because both systems must be run and checked. That cost is accepted because the alternative, finding a difference in customers' balances after the switch, is far worse.
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