System Design

Phase 1: Clarify requirements (always do this first)

Ask before designing:

1. Scale: How many users/requests/day? Read-heavy or write-heavy?
2. Consistency: Strong (banking) or eventual (social feed)?
3. Availability target: 99.9% (8.7h/yr downtime) or 99.99% (52min/yr)?
4. Latency budget: p99 < 100ms? < 1s?
5. Geography: Single region or multi-region?

Capacity estimation shortcuts

1M users/day active → ~12 req/s avg, ~120 req/s peak (10x)
1KB per request → 1M req/day = ~1GB/day = ~365GB/year
Read:write ratio 10:1 (typical social) → optimize read path first
1 server handles ~1000 req/s (rule of thumb for I/O-bound services)

Component breakdown template

Client layer  → Web / Mobile / API consumers
CDN           → Static assets, edge caching
API Gateway   → Rate limiting, auth, routing, SSL termination
Services      → Domain-specific services (User, Order, Payment, Notification)
Cache         → Redis for hot data (sessions, rate limits, computed results)
Database      → Primary DB + Read replicas
Message queue → Async operations, event-driven decoupling
Storage       → Object storage for files (S3/GCS)
Monitoring    → Metrics, logs, traces, alerts

Database selection guide

Need	Choose
ACID transactions, relations	PostgreSQL
High-scale document store	MongoDB
Key-value, cache, pub/sub	Redis
Time-series data	TimescaleDB / InfluxDB
Graph relationships	Neo4j
Full-text search	Elasticsearch
Analytical/OLAP	ClickHouse / BigQuery

Caching strategies

Cache-aside (read):  App checks cache → miss → DB → write to cache
Write-through:        Write to cache AND DB simultaneously (consistent, slower writes)
Write-behind:         Write to cache → async flush to DB (fast writes, risk of loss)
Read-through:         Cache handles DB reads automatically

TTL guidelines:
- Sessions: 15-30 min
- User profile: 5 min
- Product catalog: 1 hour
- Config/settings: 24 hours

Message queue patterns

When to use queues:
✓ Async processing (email, PDF generation, notifications)
✓ Rate-limiting downstream services
✓ Decoupling services (order → payment → shipping)
✓ Fan-out (1 event → multiple consumers)

Queue selection:
- RabbitMQ: complex routing, request-reply, low latency
- Kafka: high throughput, event log/replay, stream processing
- SQS: managed, simple, AWS-native, at-least-once delivery
- Redis Streams: lightweight, same infra as cache

API design decisions

REST:    Standard CRUD, simple clients, team familiarity (default choice)
GraphQL: Multiple clients with different data needs, reduce over-fetching
gRPC:    Internal service-to-service, binary protocol, streaming needed
WebSocket: Real-time bidirectional (chat, live updates, collaborative tools)

Scaling patterns

Vertical (scale up):   More CPU/RAM — quick, limited ceiling
Horizontal (scale out): More instances — requires stateless services
Database read replicas: Offload read traffic (good for 80%+ read workloads)
Database sharding:      Shard by user_id, geography — last resort, complex
CQRS:                   Separate read/write models — when read/write patterns diverge heavily

Common design mistakes

Mistake	Better approach
Over-engineering for scale you don't have	Start monolith, extract services at clear pain points
Synchronous calls to all dependencies	Use async queues for non-critical paths
No caching strategy	Cache at API layer + DB query results
Storing sessions in DB	Use Redis; DB sessions don't scale horizontally
Single point of failure	Redundancy at every critical layer