Use a consistent framework to avoid jumping into architecture too early.

Step 1 — Clarify requirements (5–10 minutes)

• Functional: What must users be able to do?
• Non-functional: What latency, availability, consistency, durability, security, and scale are required?
• Scope: What is explicitly out of scope?
• Actors: Who uses the system, and what permissions do they have?

Step 2 — Estimate scale (5 minutes)

• Daily active users and peak concurrent users.
• Requests per second, read/write ratio, and peak-to-average ratio.
• Storage growth, retention, bandwidth, and cache requirements.

Step 3 — Define APIs and data models (5–10 minutes)

• Identify primary entities and relationships.
• Define core read/write operations.
• Specify pagination, idempotency, authorization, and error behavior.

Step 4 — Draw the high-level architecture (10 minutes)

• Clients, DNS, CDN, load balancer, application services, caches, databases, queues, object storage, and external dependencies.
• Explain why each component exists.

Step 5 — Deep dive into critical paths (15–20 minutes)

• Select the hardest or most important flows.
• Trace requests through the system.
• Discuss bottlenecks, consistency, concurrency, retries, and failure handling.

Step 6 — Cover reliability and trade-offs (5–10 minutes)

• Identify single points of failure.
• Explain replication, recovery, observability, security, and capacity limits.
• State what you would improve first as traffic grows.

Interview rule: explain the simplest design that satisfies the requirements, then evolve it when a specific constraint justifies added complexity.