Pick a task

Each one is a concrete situation with numbers. There is no single right answer — your design is judged against the stated requirements, and anything extra counts against you.

• warm-up
  Three servers and a load balancer

  One app, three servers, nothing clever. Four components are the whole answer.

• async
  Photo recognition behind a slow vendor

  A vendor that takes three to seven minutes, offers no callback, and fails two per cent of the time.

• consistency
  Two hundred seats, five thousand buyers

  Two hundred seats, five thousand buyers in the same two seconds, and a payment provider that takes its time.

• reliability
  Webhooks that will not stop arriving

  Two seconds to answer, five retries, and the same event arriving twice.

The service is new and the pool is small on purpose — more get added as they are written. A task earns its place only once its failure modes can be named in a phrase, which takes a while. Check back for new ones.