Core System Design Principles Every Developer Should Know Early

A software developer has outlined foundational system design concepts they consider essential beyond typical interview prep, drawing on experience building a real product called TokenPulse. The CAP theorem explains that distributed systems can only guarantee two of three properties—consistency, availability, and partition tolerance—forcing practical trade-offs that should guide every database and caching decision. On scaling, the advice is to start with vertical scaling, which is simpler and requires no architectural changes, and move to horizontal scaling only when genuinely necessary, as Stack Overflow famously ran at scale on just five servers for years. Horizontal scaling, when needed, requires stateless application servers where sessions are stored in tools like Redis and files in object storage. Caching is highlighted as the highest-leverage performance improvement available, with four key layers including browser cache, CDN, application-level cache, and database query cache.