Design Monitoring and Reliability for 99.99% Availability

Explain how you would review a system architecture for monitoring and reliability, with a target availability of 99.99%. Begin by stating the user journey, architecture assumptions, and availability definition needed for your answer; no architecture diagram or measurement window is supplied here. ### Part 1 — Design monitoring and golden signals Which measurements would you collect at the user-facing boundary and within the system? Explain how they help reveal unhealthy behavior and direct investigation. #### What This Part Should Cover - Latency, traffic, errors, and saturation connected to the actual workload. - User-visible symptoms, dependency measurements, and gaps in observation. - Actionable alerts and enough context for investigation. ### Part 2 — Support the availability target Define how 99.99% would be measured, then describe the reliability mechanisms and operational practices needed to pursue it. #### What This Part Should Cover - A service-level indicator, observation window, and error budget. - Dependency and failure-domain analysis, recovery, and capacity. - Verification that failover and operational procedures work under realistic conditions. ### What a Strong Answer Covers - Monitoring and reliability choices tied to a clearly stated architecture. - A correct interpretation of 99.99%, including the difference between request-based and time-based measurement. - Evidence needed to assess the target rather than a guarantee based on a component checklist. ### Follow-up Questions - Why can all servers appear healthy while the user-facing availability objective is being missed? - How would a shared database or a correlated infrastructure failure change the value of adding more application replicas?