Given that: Sessions are cluster-wide and exist beyond the scope of a connection Each write can be uniquely identified by a session and statement ID Drivers can rely on SDAM and server selection to re-select the primary Drivers can safely retry single-document writes by resending the original command to the primary: If the write already executed, return the result we missed If the write never executed, do it now and return its result note: Single-document writes excludes updateMany and deleteMany. Doctrine ODM’s UnitOfWork generates many single-document writes, so it is a great candidate for retrying writes (or transactions).