Is Mongodb ObjectID unique between documents?

Question

Lets consider an example of blog site where I want to save questions and replies in two different documents. I know this can also be done with Embedding documents but lets assume we want two different documents.

And I have Like option on both question and reply. If user likes a question that particular question's objectID is stored inside a Like document and similarly for likes in reply.

Now I wonder whether the objectID's that I store in Like documents could create a conflict. Like if the question's objectid is same as any reply's objectid. IS it possible to have such conflicts??

In Mongodb ObjectID documentation the following is provided:

ObjectId is a 12-byte BSON type, constructed using:

a 4-byte value representing the seconds since the Unix epoch,

a 3-bytemachine identifier,

a 2-byte process id, and

a 3-byte counter, starting with a random value.

Answer 1

It is unique for all practical purposes. The non- uniqueness mentioned is a rather theoretical one.

The ObjectId is constructed out of

• a time stamp (seconds since epoch)
• a machine identifier
• the process id
• and a 3 byte counter starting with a random value each second

So, at median, unless you write in excess of 3080 documents per second per client, you should be safe. It is to note that _id has an unique constraint: Even if you'd try to write the same ObjectId twice, there'd be an exception.

Side note: even hash algorithms like MD5 or (to a lesser extent) SHA256 have a slight chance of collision. But with a unique constraint, you are safe.

Edit: Since an exception is thrown in the very, Very, VERY rare case that two identical ObjectIds are generated and tried to be written concurrently, you simply handle them by generating a new ObjectId and try to save the document again.

Answer 2

From the Documentation, it is most likely unique, not unique.