Databases

Introduction Databases are at the heart of virtually every software system — from web applications to financial platforms to machine learning pipelines. But how do they actually work? What happens when you run a SELECT query? How are rows stored on disk? How do multiple transactions run concurrently without corrupting data? This post is a comprehensive guide to database theory, covering the full lifecycle of database design and operation: from conceptual modeling to physical storage, from SQL queries to concurrency control. Whether you are studying for an exam, preparing for a systems design interview, or simply curious about what happens beneath the ORM layer, this guide has you covered. ...

Basic Application Architecture Developer Perspective A typical application is composed of: Build & Deploy Layer — CI/CD pipeline Server Layer — handles incoming requests Storage Layer — persists application data (can be external) Logging Storage Server — logs all events Metric Storage Server — logs all metrics Alert Server — alerts when something goes down Stateless vs Stateful Services Stateless: the server holds no session data between requests. Any instance can serve any request. Enables effortless horizontal scaling. Stateful: the server holds session data (e.g. WebSocket connections, in-memory cart). Requires sticky sessions (session affinity at the load balancer) or externalizing state to Redis/a DB. Stateless Stateful Horizontal scaling Trivial — add instances freely Hard — requires sticky sessions or external state Fault tolerance High — any replica serves the request Low — losing a server loses its session data Examples REST APIs, static servers WebSocket servers, game servers, video call nodes User Perspective Users simply make requests to the server and receive responses. ...