Project Retrospectives · Published April 4, 2026
Retrospective: Scaling Up the Night Before the Sale
The campaign page was load-tested, but login and coupons were what fell over.
It was the client's first major online sale, with ten times normal traffic expected. We load-tested a week ahead. The first ten minutes went fine; at minute fifteen logins began failing.
Bottlenecks nobody predicted
Load tests covered product browsing, not login. When the sale started, every user signed in at once and the auth service became the limit.
The coupon endpoint had no rate limit, so request volume exceeded database write capacity.
Cache warm-up was incomplete; hot data went back to the database at the exact moment of peak pressure.
Our readiness checklist
Load tests cover the full chain: login, browse, order and payment, mixed in realistic proportions.
Hot data is warmed before the event with staggered expiry times.
Engineers are on site during the event, checking core metrics every fifteen minutes and degrading by plan if needed.
Peak readiness is not about adding servers; it is knowing which link breaks first. Whatever the load test misses is where the risk lives.
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