Collection ID length in MongoDB

Question

i am new to mongodb and stack overflow.

I want to know why on mongodb collection ID is of 24 hex characters? what is importance of that?

Answer 1

Why is the default _id a 24 character hex string?

The default unique identifier generated as the primary key (_id) for a MongoDB document is an ObjectId. This is a 12 byte binary value which is often represented as a 24 character hex string, and one of the standard field types supported by the MongoDB BSON specification.

The 12 bytes of an ObjectId are constructed using:

• 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)

What is the importance of an ObjectId?

ObjectIds (or similar identifiers generated according to a GUID formula) allow unique identifiers to be independently generated in a distributed system.

The ability to independently generate a unique ID becomes very important as you scale up to multiple application servers (or perhaps multiple database nodes in a sharded cluster). You do not want to have a central coordination bottleneck like a sequence counter (eg. as you might have for an auto-incrementing primary key), and you will want to insert new documents without risk that a new identifier will turn out to be a duplicate.

An ObjectId is typically generated by your MongoDB client driver, but can also be generated on the MongoDB server if your client driver or application code or haven't already added an _id field.

Do I have to use the default ObjectId?

No. If you have a more suitable unique identifier to use, you can always provide your own value for _id. This can either be a single value or a composite value using multiple fields.

The main constraints on _id values are that they have to be unique for a collection and you cannot update or remove the _id for an existing document.

Answer 2

Now mongoDB current version is 4.2. ObjectId size is still 12 bytes but consist of 3 parts.

ObjectIds are small, likely unique, fast to generate, and ordered. ObjectId values are 12 bytes in length, consisting of:

• a 4-byte timestamp value, representing the ObjectId’s creation, measured in seconds since the Unix epoch

• a 5-byte random value

• a 3-byte incrementing counter, initialized to a random value

Create ObjectId and get timestamp from it

> x = ObjectId()
ObjectId("5fdedb7c25ab1352eef88f60")
> x.getTimestamp()
ISODate("2020-12-20T05:05:00Z")

Reference

Read MongoDB official doc