The most dangerous sentence in IT: “We'll switch to the new system over the weekend.” If it fails, the company stands still on Monday. The strangler pattern is the more boring — and incomparably safer — road.
How the old system gets strangled
An API layer goes in front of the old system and all requests flow through it. Then functions move to the new solution one by one: orders this month, warehouse the next, invoicing after that. The old system keeps running alongside until only an empty shell remains — which you switch off without drama.
What that means in numbers
Migration takes longer — typically 9–18 months instead of “one weekend”. But the risk of an outage is close to zero, the team learns the new system gradually, and every phase delivers value immediately rather than at the very end. Cash flow is spread out: you pay per module, not half a million up front.
The first step is always the same: a dependency audit — what actually touches the old system. Ours takes 48 hours and produces a map, a migration order and a fixed price for phase one.