Strangling a legacy system without a big-bang cutover
The replacement project that fails is the one with a date circled on a calendar eighteen months out. Everything depends on that weekend, nothing is provable until then, and the rollback plan is a sentence in a slide deck.
The alternative is to put a facade in front of the old system and move traffic through it a slice at a time. Each slice is small enough to reverse in minutes. The old system keeps running, which sounds wasteful and is in fact the cheapest insurance available.
Four systems became one without a single weekend cutover. The technique is old, unglamorous, and it works.
What makes this work in practice is discipline about the seam. The facade must be the only route in — the moment a client calls the legacy system directly, you have two systems of record and no way to move.
Arbor Lending replaced four systems this way over fourteen months. There was never a cutover weekend, and the only outage in the programme came from a supplier, not the migration.
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