ObjectId

ObjectId is a special data type for MongoDB. Every document has an _id field from cradle to grave. It is the primary key for each document in a collection and has to be unique. If we omit this field in a create statement, it will be assigned automatically with an ObjectId.

Messing with the ObjectId is not advisable but we can use it (with caution!) for our purposes.

ObjectId is:

• 12-bytes
• Ordered; sorting by _id will sort by creation time for each document
• Storing the creation time that can be accessed by .getTimeStamp() in the shell

The structure of an ObjectId:

• a 4-byte value representing the seconds since the Unix epoch
• a 3-byte machine identifier
• a 2-byte process id
• a 3-byte counter, starting with a random value

By their structure, ObjectIds will be unique for all purposes; however since these are generated on the client side, one should check the underlying library's source code to verify that implementation is according to specification.