How the internet works (DNS, HTTP, TCP/IP)

Client-Server architecture

REST vs GraphQL vs gRPC

Latency vs Throughput

CAP Theorem

ACID vs BASE

Horizontal vs Vertical Scaling

Load Balancing (Round Robin, Least Connections, Consistent Hashing)

Caching (Redis, Memcached, CDN, cache eviction policies)

Database Sharding + Partitioning

Replication (master-slave, multi-master)

Rate Limiting

SQL vs NoSQL tradeoffs

Indexing strategies

Query optimization

Connection pooling

PostgreSQL internals

MongoDB, Cassandra, DynamoDB tradeoffs

Message Queues (Kafka, RabbitMQ, BullMQ)

Event-driven architecture

Microservices vs Monolith

Docker + Kubernetes basics

CI/CD pipelines

API Gateways

Design Twitter/X feed

Design WhatsApp

Design YouTube

Design Uber

Design Rate Limiter

Design Distributed Cache

Design Notification System

Design Search Autocomplete

Design Web Crawler

Design Pastebin

Design Instagram