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In my opinion, this approach can only work for very simple systems and for a very limited time. Because, if the System B's output is exactly the same as System A's output it must mean the B is exactly the same as A and that is impossible for any non-trivial system. So, as the system grows in complexity, the old tests become deprecated. Of course, it is ...


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Comparing the old system to the new system given identical inputs and expecting identical outputs is a legitimate test. There are problems, however: a) When the legacy system is retired, the Source of Truth is no longer available and comparisons are impossible. You'll have to be very comfortable that the systems work similarly enough to satisfy business ...


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Yes this is a useful strategy in converting from one system to another as a one time event (it might span a couple of iterations but the idea is very short term). You could effectively view this as BDD with 'half (the tests) already done'. You have the tests against data that works. You create a new back-end version and see if it does the same. As I ...


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