Meet a 10-second latency and 99% availability target across 3-5 dependent services

An application serves each request by calling three to five dependent services. How do you make the application meet specific service-level goals, for example end-to-end latency under 10 seconds and availability of at least 99%?

This is less a standard "build a system" design round than an open-ended optimization discussion, held in a 45-minute slot. Once the goals are confirmed, the problem is broken down and discussed step by step. Expect the interviewer to switch to a new topic or add a requirement before the current thread is finished; keeping the changing requirements straight is part of the exercise.

Constraints and Clarifications

• The application depends on three to five services, the latency target is under 10 seconds, and the availability target is 99% or higher. Ask about any other numbers, or state them explicitly as assumptions.

Clarifying Questions

• Is the 10-second target a maximum or a percentile such as p99, and is it measured at the client or at the service?
• Does 99% availability mean the fraction of successful requests or uptime over a calendar window, and over what period is it measured?
• Which dependency calls are independent, and which need another call's output?
• What latency distribution and availability does each dependency currently deliver, and does the team own any of them?
• Are the requests reads, writes or both, and are the writes safe to retry?
• Which dependencies are essential to a useful response, and which could be skipped or served stale?

Part 1 — Latency under 10 seconds

Explain how you turn the 10-second end-to-end target into a budget for each dependency call, and what you change in the application when a dependency is too slow to fit.

What This Part Should Cover

• The dependency graph and its critical path, with sequential and parallel calls distinguished
• Per-call timeouts derived from the overall deadline, and propagation of that deadline downstream
• Tail latency under fan-out, and techniques that cut it
• What to do when the inherent work cannot finish within 10 seconds

Part 2 — Availability of at least 99%

Explain how the application reaches 99% availability when every request depends on several services, any of which can fail.

What This Part Should Cover

• How dependency availabilities combine, and the error budget that remains
• Mitigations that keep a failed dependency from failing the request
• Protecting the application from a slow or failing dependency
• How both objectives are measured and alerted on