Possibility of duplicate Mongo ObjectId's being generated in two different collections?

Short Answer

Just to add a direct response to your initial question: YES, if you use BSON Object ID generation, then for most drivers the IDs are almost certainly going to be unique across collections. See below for what "almost certainly" means.

Long Answer

The BSON Object ID's generated by Mongo DB drivers are highly likely to be unique across collections. This is mainly because of the last 3 bytes of the ID, which for most drivers is generated via a static incrementing counter. That counter is collection-independent; it's global. The Java driver, for example, uses a randomly initialized, static AtomicInteger.

So why, in the Mongo docs, do they say that the IDs are "highly likely" to be unique, instead of outright saying that they WILL be unique? Three possibilities can occur where you won't get a unique ID (please let me know if there are more):

Before this discussion, recall that the BSON Object ID consists of:

[4 bytes seconds since epoch, 3 bytes machine hash, 2 bytes process ID, 3 bytes counter]

Here are the three possibilities, so you judge for yourself how likely it is to get a dupe:

1) Counter overflow: there are 3 bytes in the counter. If you happen to insert over 16,777,216 (2^24) documents in a single second, on the same machine, in the same process, then you may overflow the incrementing counter bytes and end up with two Object IDs that share the same time, machine, process, and counter values.

2) Counter non-incrementing: some Mongo drivers use random numbers instead of incrementing numbers for the counter bytes. In these cases, there is a 1/16,777,216 chance of generating a non-unique ID, but only if those two IDs are generated in the same second (i.e. before the time section of the ID updates to the next second), on the same machine, in the same process.

3) Machine and process hash to the same values. The machine ID and process ID values may, in some highly unlikely scenario, map to the same values for two different machines. If this occurs, and at the same time the two counters on the two different machines, during the same second, generate the same value, then you'll end up with a duplicate ID.

These are the three scenarios to watch out for. Scenario 1 and 3 seem highly unlikely, and scenario 2 is totally avoidable if you're using the right driver. You'll have to check the source of the driver to know for sure.

ObjectIds are generated client-side in a manner similar to UUID but with some nicer properties for storage in a database such as roughly increasing order and encoding their creation time for free. The key thing for your use case is that they are designed to guarantee uniqueness to a high probability even if they are generated on different machines.

Now if you were referring to the _id field in general, we do not require uniqueness across collections so it is safe to reuse the old _id. As a concrete example, if you have two collections, colors and fruits, both could simultaneously have an object like {_id: 'orange'}.

In case you want to know more about how ObjectIds are created, here is the spec: http://www.mongodb.org/display/DOCS/Object+IDs#ObjectIDs-BSONObjectIDSpecification