system-designlisted

# System Design Reference ## Core Concepts ### Networking - **HTTP**: Request-response, stateless. Default for most APIs. - **WebSockets**: Bidirectional, persistent. Chat, gaming, collaborative editing. - **SSE**: Server to client only, over HTTP. Live feeds, notifications. Simpler than WebSockets when you do not need client to server. - **gRPC**: Binary (protobuf), bidirectional streaming. Service to service. - **L4 load balancer**: TCP-level, fast, supports persistent connections. - **L7 load balancer**: HTTP-level, routes by path/header, content-aware. ### API Design - **REST**: Resource-based, stateless, HTTP verbs. Default for public APIs. - **GraphQL**: Client specifies exact data shape. Good when clients have diverse data needs. - **gRPC**: Protobuf, streaming. Best for internal service to service. - Pagination: cursor-based > offset-based at scale (offset degrades with depth). ### Data Modeling - **Relational (Postgres)**: Structured data, strong consistency, complex queries, ACID. Default choice. - **NoSQL**: Flexible schema, horizontal scaling, high write throughput. Choose for specific access patterns, not by default. - Start normalized, denormalize hot paths only when needed. - NoSQL does not mean "no relationships". It means different access pattern optimization. ### Caching - Cache hit ~1ms (Redis) vs 20-50ms (DB). 20-50x speedup. - **When**: Read-heavy, data does not change often, can tolerate staleness. - **Invalidation**: Short TTLs, write-through, or a