Writes in MongoDB are persisted before the server returns acknowledgement. For a typical replica set deployment, this additionally means replicated and made durable on disk for a majority of nodes.

MongoDB has tunable durability guarantees; clients can opt-out of this behavior.

The layers are deep, but this is the function that handles waiting before acknowledgement:

https://github.com/mongodb/mongo/blob/20f42d9dc89999d119f35a...