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Large rewrites fail in predictable ways. The strangler pattern is older and less glamorous, and it is the approach that works.
The big-bang rewrite has a consistent record: in our experience it has not once gone to plan. The reasons repeat. The legacy system does more than anyone documented. The business changes during the two years the rewrite takes. And the cut-over weekend arrives with a hundred unknowns and a single opportunity to be right.
The strangler pattern
The alternative is well established, which is why it works. Place a thin layer in front of the legacy system. Move one capability behind it into a new service. Run both. When the new path has handled a full cycle—a month-end, a season, a renewal period—retire the old one. Repeat.
Each slice is small enough to reason about, delivers value on its own, and releases capacity or cost that funds the next. The legacy system shrinks until, eventually, the last slice moves and it is gone.
Where it goes wrong
In two places. Teams select the first slice by how easy it is rather than by risk and value, and spend a year modernizing something no one was waiting for. And they skip the period of running both paths in parallel because it feels wasteful—when it is the entire safety mechanism.
How we sequence
We map the system into capabilities, score each on business value, the risk of leaving it in place, and the difficulty of moving it, and put the highest-value, lowest-risk slice first. Intake before payments; reporting before adjudication. The roadmap is the sequence, and the sequence is the strategy.
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