13Building Blocks of System Design: Mapping Requirements to Components14DNS, TCP, and TLS: The Network Path Before Your Service
15Load Balancing: Health Checks, Routing Algorithms, and Failure Domains16Caching and CDNs: Placement, Invalidation, and Freshness17Databases: SQL vs. NoSQL, Replication, and Sharding18Object Storage and Key-Value Stores: Separating Metadata From Content19Message Queues and Pub/Sub: Decoupling, Backpressure, and Delivery Guarantees20Rate Limiting Algorithms and Distributed Counters21Observability: Metrics, Logs, and Traces Tied to SLIs