The problem

Friday, 4 p.m. A team ships a new checkout page behind an if. They want 10% of users to see it first, plus the 40 people of the support team.

At 4:20 the error rate on the new page climbs. The team wants it off now. A deploy to 200 servers takes 25 minutes, and every one of those minutes loses orders.

A feature flag service lets them change that if from an admin screen. You design that service, with at most 30 seconds between the click and the last server using the new rule.