Scalability, Availability, Reliability, Latency vs Throughput vs Bandwidth, Client-Server Architecture, Databases, SQL vs NoSQL, Load Balancing, Load Balancing Algorithms, Caching, Cache Invalidation, CDN, DNS, API Design, REST APIs, Authentication vs Authorization, Session-based vs Token-based Authentication, OAuth/OAuth2/OpenID Connect, JWT, Rate Limiting, Single Point of Failure, High Availability vs Fault Tolerance, CAP Theorem, Consistency Models, Data Replication, Read Replicas, Sharding, Data Partitioning, Consistent Hashing, Denormalization, Indexing, Microservices Architecture, Monolithic Architecture, Serverless Architecture, Event-Driven Architecture, Message Queues, Pub/Sub, Synchronous vs Asynchronous Communication.