System Design Fundamentals

A framework for any design question

1. Clarify requirements — functional + non-functional (scale, latency, consistency, availability).
2. Estimate scale — QPS, data size, read/write ratio.
3. API design — endpoints, request/response, pagination.
4. High-level components — client → gateway/LB → services → data stores / caches.
5. Data model & storage — SQL vs NoSQL, indexing, partitioning.
6. Scale it — horizontal scaling, load balancing, caching, async/queues, CDN.
7. Resilience — timeouts, retries, circuit breakers, graceful degradation → Scalability & Resilience.
8. Trade-offs — state them explicitly.

Building blocks

• Load balancer — L4 (transport) vs L7 (application); round-robin, least-connections.
• Caching — cut latency + load → Caching.
• Database — replication (read scaling), sharding/partitioning (write scaling), indexing.
• Message queue — decouple + smooth spikes → Messaging.
• CDN — static assets near users.
• API gateway — routing, auth, rate limiting, aggregation.

Key theory

• CAP — during a partition, choose Consistency or Availability → see DB Principles.
• Consistency models — strong vs eventual.
• Statelessness — externalize session → any instance serves any request → horizontal scale.
• Idempotency — safe retries (idempotency keys for writes).
• Pagination — offset vs cursor/keyset (cursor scales for deep pages).

Scaling axes

Vertical (bigger box) vs horizontal (more boxes). Prefer horizontal + stateless behind a load balancer.