Engineering · April 15, 2026 · 11 min read
The strangler pattern in practice: replatforming without the launch weekend
How we re-platformed a 14-year-old claims system with zero downtime — traffic splitting, parity checks, and the rollback plan we never needed.
The Vantage claims platform was 14 years old, load-bearing, and terrifying to touch. The previous vendor quoted three years for a rewrite. We shipped the replacement in eleven months with zero minutes of downtime — by never having a launch day at all.
Strangle, don't rewrite
The strangler pattern replaces a system component by component while the old one keeps running. Each phase shifts a slice of traffic behind a flag, proves parity, and becomes the new normal. There is no big-bang cutover to bet the company on.
- Phase by risk × revenue: highest-risk, highest-value flows move first, while everyone is paying attention
- Nightly parity checks: both systems process the same inputs; drift pages a human
- Reversible always: every phase has a rollback tested in staging, not written in hope
- The old system earns trust for the new one: read-only mode came only after 60 clean days
What made it work
Not the architecture — the instrumentation. We spent the first month making both systems observable enough to compare honestly. Once parity was a dashboard instead of an argument, every stakeholder conversation got shorter.
The old vendor's three-year estimate wasn't wrong for a rewrite. It was wrong because a rewrite was the wrong plan.