MongoDB vs Redis: Documents vs Key-Value Cache

MongoDB and Redis are both NoSQL databases but solve fundamentally different problems. MongoDB is a general-purpose document database designed for rich querying, flexible schemas, and durable persistence. Redis is an in-memory data structure store designed for sub-millisecond latency, simple access patterns, and ephemeral or semi-persistent data. Understanding when to use each — and when to use both together — is a critical architectural skill.

MongoDB excels when you need: rich query capability (filter by any field, nested documents, arrays); flexible schema (different documents can have different fields); durable persistence (data survives restarts with configurable write concerns); large document sizes (up to 16 MB vs Redis's string size limits); and aggregation pipelines for complex server-side transformations. MongoDB is the right choice for your application's primary data store.