Databases labs
Databases look like black boxes until you see how they actually work: how an index narrows a search, how concurrent transactions interfere, how connection pools queue requests, and how data is split and copied across servers.
4 free interactive labs — How Database Indexes Work (B-Tree), What Happens When 100 Reads Hit the Database, SQL Isolation Levels Lab, Sharding & Replication. Each one has an easy-to-hard set of examples, a written explanation and a FAQ, and runs entirely in your browser.
See a B-tree lookup beat a full table scan, and why some queries ignore your index.
Connection pools, CPU workers, caches and replicas, as 100 requests arrive at once.
Run two transactions side by side and watch dirty reads, phantoms and write skew appear.
Split data across servers, copy it for reads, and see what each choice costs.