ObjectId

ObjectId is a 12-byte identifier used by MongoDB as the default value for the _id field of documents. It is typically represented as a 24-character hexadecimal string and is designed to be unique within a deployment while also encoding creation time information.

The 12 bytes are arranged in three parts: a 4-byte Unix timestamp (in seconds since the epoch), a 5-byte value that is generally tied to the host and process to help distinguish different instances, and a 3-byte counter that increments with each new ObjectId generated in the same context. The timestamp makes ObjectIds roughly sortable by creation time, while the random value and counter ensure uniqueness across machines, processes, and moments of generation.

ObjectIds can be generated on the client side or by the server, allowing documents to be created without an immediate database round trip. The 3-byte counter in each process increments for every new ObjectId within the same second and wraps around after reaching its maximum. The 5-byte value varies between hosts or processes to further reduce the chance of collision.

In MongoDB, the _id field is indexed by default, making ObjectId an efficient and unique primary key for documents. Many language drivers expose utilities to convert between the ObjectId and its hexadecimal representation, and to extract the embedded timestamp (for example, toDate or getTimestamp methods). ObjectId is part of BSON, and while it provides useful ordering and uniqueness properties, the embedded timestamp means creation times can be inferred from the value.