How do document databases compare to key-value stores?

Document databases and key-value stores are both NoSQL databases optimized for scalability and flexibility, but they serve different purposes based on data structure and query requirements. Document databases store semi-structured data (like JSON or XML) where each document can have nested fields and varying schemas. Key-value stores, in contrast, treat data as opaque values associated with unique keys, prioritizing simple read/write operations over complex queries. The choice between them depends on how structured your data is and what kinds of queries you need to perform.

The primary difference lies in data organization and query capabilities. Document databases, such as MongoDB or Couchbase, allow querying based on the internal structure of documents. For example, you could search for all users with a “subscription.status” field set to “active” or filter products by price ranges. These databases support secondary indexes and aggregation pipelines for complex operations. Key-value stores like Redis or Amazon DynamoDB, however, limit queries to key lookups. If you need data from a value (e.g., extracting a user’s email from a stored JSON blob), you must retrieve and parse the entire value in your application, as the database doesn’t interpret its contents.

Use cases also differ significantly. Key-value stores excel in high-speed scenarios requiring simple access patterns, such as session management, caching, or feature flag storage. For instance, Redis is often used to cache web page fragments using keys like “homepage_v2.” Document databases are better suited for content management, user profiles, or catalogs where data varies in structure. An e-commerce platform might use MongoDB to store product details, where each item has unique attributes (e.g., “screen_size” for monitors vs. “capacity” for storage drives). While key-value stores offer lower latency for basic operations, document databases provide richer querying at the cost of slightly higher complexity. Choosing between them depends on whether your application prioritizes raw speed or flexible data modeling.