epoch - Can a 4 byte timestamp value in MongoDb ObjectId overflow?

Question

If at some time, the epoch is ffffffff, than the objectId created at this moment is something like :

ffffffff15580625bcb65364

Then, what could be the ObjectId created after 1 second?

question from:https://stackoverflow.com/questions/65842129/can-a-4-byte-timestamp-value-in-mongodb-objectid-overflow

Answer 1

Then, what could be the ObjectId created after [the Unix epoch rolls over in 32 bits]?

This would depend on the specific implementation, its programming language and their handling of math calculations.

It is possible that some implementations and languages would error when they retrieve the number of seconds since the Unix epoch as a 64-bit integer (which is quite common today) and then try to use a value which exceeds 32 bits in size for ObjectId generation. If this happens the driver will cease to be able to generate ObjectIds, consequently it may be unable to insert documents without _id values being provided by the application using some other generation strategy.

In other implementations the timestamp itself may roll over to zero, at which point the ObjectId generation will succeed with a very small timestamp value.

Yet other implementations may truncate (from either most or least significant side) the timestamp to coerce it into the 32 available bits of an ObjectId.

The ObjectId value itself doesn't actually have to have an accurate timestamp - it is required to be unique within the collection and it is "generally increasing" but MongoDB-the-database wouldn't care if ObjectId values wrapped to around zero at some point.