Understanding MongoDB ObjectId Data Type

What is ObjectId?

ObjectId is a 12-byte BSON type used as the default primary key in MongoDB documents.

It ensures each document has a unique identifier across distributed systems without requiring a centralized ID generator.

• Automatically generated if _id is not specified.
• Used as the primary key for document identification.
• Encodes creation time and machine/process information.

Structure of ObjectId

An ObjectId consists of 12 bytes divided into four parts that together guarantee uniqueness.

• 4 bytes: Timestamp (seconds since Unix epoch)
• 5 bytes: Random value unique to the machine and process
• 3 bytes: Incrementing counter, initialized randomly

Using ObjectId in MongoDB

When inserting a document without specifying _id, MongoDB generates an ObjectId automatically.

You can also create ObjectId manually in your application code for queries or document creation.

• Use ObjectId to query documents by their unique identifier.
• Extract the timestamp from ObjectId to find document creation time.
• Avoid manually setting _id unless you have a specific reason.