I was under impression, that update w/upsert is atomic (since it operates on a single, though maybe not-yet-existing document). Unfortunately, when under moderate load we see duplicate key errors on id for upsert operations. I've read the docs and got the impression that $atomic modifier is not a solution (docs describe it as a way to do _multiple document updates concurrently). Is there something broken in the server, documentation or my own head? =) If it's a documentation issue (I suspect that, but I feel sorry for myself updating all our code that uses upserts), please update the docs with a clear statement that "upsert still can fail on non-unique IDs, use XXX instead".