This is confusing. The MongdoDB doc on acknowledged write concern leads with: "With a receipt acknowledged write concern, the mongod confirms the receipt of the write operation." This implies the acknowledgement is just of receipt of the write; it does NOT imply that the client waits for success to continue. However, in the caption to an illustrative diagram, the docs say: "The client waits for acknowledgment of success or exception." That is much clearer – and somewhat contradictory, i.e. the acknowledgement is of success/failure, not simply of "receipt" of the write.