I think the section "A bit more is happening here. After we resolve the collection by the stream type, we either take the provided expected position from options (if it was provided) as the current one or read it from the stream document.

Once we have it, we pass it on to the update statement.

If the document with a specified stream name and position wasn’t found, that means the stream doesn’t exist or the stream position is different. Even though MongoDB will try to insert the document, it’ll fail as we have the unique index constraint on the stream name. If no document was upserted, then that means that there was an optimistic concurrency conflict, and we can throw an error." is duplicated