System Design Template (Use This Every Time)

This is a repeatable structure to keep you from rambling.

1) Requirements

Functional

• What does the system do?
• Who are the users?
• What are the core APIs?

Non-functional

• latency targets (p50/p95/p99)
• availability target (e.g., 99.9%)
• consistency requirements
• data retention and privacy constraints

2) Back-of-the-envelope numbers

Estimate:

• QPS (reads/writes)
• peak factor
• data size per object
• storage growth
• bandwidth

The point is not perfect math—it’s to justify design choices.

3) High-level architecture

Start simple:

• clients → API layer → core services → storage

Then add scaling elements:

• caching
• queues/streams
• read replicas
• sharding/partitioning

4) Data model

• key entities
• indexes needed
• partition key and why

5) Deep dives (pick 1–3)

• caching strategy and invalidation
• consistency model
• rate limiting
• search/indexing
• async processing

6) Failure modes and resilience

• timeouts, retries (with jitter), circuit breakers
• dependency degradation plan
• disaster recovery (RPO/RTO)

7) Observability and operations

• SLIs/SLOs
• dashboards + alerts
• runbooks and rollback strategy

8) Security and privacy

• authn/authz
• data encryption (in transit/at rest)
• least privilege

9) Tradeoffs recap

End with 3–5 explicit tradeoffs:

• what you optimized for
• what you sacrificed
• what you’d do next if given more time