NoSQL database

NoSQL covers the databases that deliberately abandon the relational model: document stores holding JSON-like records, key-value stores, wide-column stores, graph databases. Each trades some guarantee for some other advantage.

The advantages are real where the workload matches. A document store fits records whose shape varies genuinely between instances; a key-value store fits caching and sessions; a graph database fits data whose value is in the connections, such as recommendations or access hierarchies.

The trade is that consistency and relationships become the application's responsibility. Chosen for a good reason, that is a sensible exchange; chosen because a relational schema felt like effort, it usually means writing by hand — and eventually getting wrong — what the database would have enforced.