Hi according to this reference: https://www.mongodb.com/blog/post/how-to-select--for-update-inside-mongodb-transactions When I lock a document with a field with a new ObjectID, the whole document is locked! Idea : Operations: i have three fields : A , B , C I locked field `A` with new ObjectID in transaction `T1`. i locked field `B` with new ObjectID in transaction `T2`. Behaviors (high performance) : - In transaction `T2`: if field `A` is updated, `writeConflict` error occurs. - In transaction `T1`: if field `B` is updated, `writeConflict` error occurs. - Outside of transactions: If field `A` is updated, it waits for `T1`. - Outside of transactions: If field `B` is updated, it waits for `T2`. - T1 and T2 transaction can be commit at same time. (high performance) https://www.mongodb.com/community/forums/t/lock-on-sub-document-in-mongo/140245