What are the trade-offs between document databases and relational databases?

Document databases (like MongoDB) and relational databases (like PostgreSQL) differ primarily in data organization, query flexibility, and scalability. Document databases store data as JSON-like documents with dynamic schemas, making them ideal for unstructured or evolving data. Relational databases use tables with fixed schemas and enforce relationships through foreign keys, which suits structured data with strict consistency needs. The choice depends on the project’s requirements for schema flexibility, transactional guarantees, and scalability.

The first trade-off is schema flexibility versus data integrity. Document databases allow developers to store data without a predefined schema, enabling rapid iteration. For example, a product catalog with varying attributes (e.g., different fields for books vs. electronics) can be stored without altering a global schema. However, this flexibility can lead to inconsistent data formats if not managed carefully. Relational databases enforce rigid schemas, ensuring data consistency through constraints like data types and foreign keys. This is critical for applications like financial systems where data accuracy is non-negotiable, but schema changes require migrations that can slow development.

Another key difference is query capability and transactional support. Relational databases excel at complex queries involving joins across multiple tables, which is useful for reporting or applications with interconnected data (e.g., an e-commerce platform linking orders, users, and inventory). They also support ACID transactions, ensuring reliable updates across tables. Document databases often lack native join support, requiring denormalization or application-side logic to combine data, which increases complexity. While some document databases offer transaction support for single documents, multi-document transactions are less robust. For example, a social media app might prioritize fast writes for user posts (using a document database) over transactional consistency between posts and comments.

Finally, scalability patterns differ. Document databases are designed for horizontal scaling, distributing data across clusters to handle high read/write workloads (e.g., a real-time analytics platform). Relational databases traditionally scale vertically (upgrading hardware) or use limited horizontal scaling with sharding, which is complex to manage. However, relational databases often provide stronger consistency guarantees in distributed environments. A hybrid approach, like using a relational database for transactional data and a document database for user-generated content, can balance these trade-offs based on specific use cases.