By Product Engineering Team
The best compliment a rewrite can get is silence. No support tickets, no spike in cart abandonment, no "did something change?" tweets. That's the bar we hold ourselves to on any rewrite of a system that's actively making money for a client — and it's achievable, but only if you resist the urge to ship the whole thing at once.
Map production before you write a line of new code
Every checkout system accumulates edge cases that never made it into any spec: the currency-rounding rule added for one country, the retry logic for a payment provider that used to time out. We start every rewrite by mining production logs for every distinct path real traffic actually takes, not the happy path in the original design doc.
Build the new path next to the old one
Rather than replacing the old checkout outright, the new implementation runs alongside it behind a feature flag, routed by a small, reversible percentage of traffic. This means:
- A bug in the new path affects a bounded slice of orders, not all of them
- Comparing conversion and error rates side by side is trivial
- Rolling back is a flag flip, not a deploy
Widen the rollout only when the data says to
We don't advance the rollout percentage on a schedule — we advance it when the metrics that matter (checkout completion rate, payment error rate, p95 latency) hold steady at the current percentage for a full business cycle, including the highest-traffic day of the week. Slower than a big-bang launch, and it's why nobody noticed.