What is a document ID in a document database?

A document ID is a unique identifier assigned to each document in a document database. It serves as the primary key, enabling developers to retrieve, update, or delete specific documents efficiently. Unlike relational databases, where primary keys are often integers or composite values, document databases typically use simple, flexible identifiers like strings, UUIDs, or database-generated values. For example, MongoDB uses an _id field that can be a user-provided value or an auto-generated ObjectId (a 12-byte hexadecimal value). This ID is essential for basic operations: querying by ID is the fastest way to access a document, as databases optimize for this lookup by default.

Document IDs play a critical role in scalability and data distribution. In distributed systems, document databases often partition data across nodes using the ID to determine where a document is stored. For instance, MongoDB uses the _id field in its sharding logic to distribute documents evenly. This ensures that read and write operations scale horizontally. Additionally, document IDs enforce uniqueness within a collection (a group of documents), preventing conflicts. If two documents have the same ID, the database will reject the second insertion or overwrite the existing document, depending on the operation. This uniqueness constraint is enforced by the database, eliminating the need for manual checks.

When working with document IDs, developers should consider trade-offs between auto-generated and custom IDs. Auto-generated IDs (like MongoDB’s ObjectId) guarantee uniqueness and simplify code, but they lack semantic meaning. Custom IDs (e.g., a username or product SKU) can make queries more intuitive but require careful validation to avoid duplicates. For example, using a user’s email as an ID might seem practical, but it risks conflicts if emails change or formatting rules evolve. Security is another consideration: predictable sequential IDs (like incrementing integers) can expose system internals, making auto-generated random IDs safer. Finally, most document databases index the ID field automatically, ensuring fast lookups—a feature developers should leverage when designing queries.