Electricity bills: the due-date rush

The story

The last days before the due date are the busiest of the month for a state electricity utility's app. Customers check a balance, then pay it, and an acknowledged payment must never be lost. Every request ends in one Postgres, the source of truth for accounts.

The target

• Load: 2,800 requests a second for 8 seconds, nine in ten of them balance lookups and one in ten payments.
• Latency: p99 within 100 ms.
• Budget: $1,650 a month at this load. Half the budget or less earns full cost marks; one and a half times it earns none.
• Faults: two 6-second drills, each with its fault landing 2 seconds in. In one the billing service runs twice as slow for 3 seconds; in the other one service instance crashes. A drill passes when under 1% of requests fail and p99 stays within 200 ms.

What is scored

Throughput 30%, latency 20%, cost 15% and resilience 35%, out of 1,000,000. Resilience weighs most this week: a fast, cheap design that falls over on one lost instance cannot win. Every entrant runs the same arrival stream; its seed is published when the week closes so anyone can replay a score.

How to enter

Choose Open in Sandbox. The canvas starts from the palette and starter design this challenge is built on. Build, test it here, then enter. You get five scored entries.