What's the difference between findAndModify and update in MongoDB?
If you fetch an item and then update it, there may be an update by another thread between those two steps. If you update an item first and then fetch it, there may be another update in-between and you will get back a different item than what you updated.
Doing it "atomically" means you are guaranteed that you are getting back the exact same item you are updating - i.e. no other operation can happen in between.
findAndModify returns the document, update does not.
If I understood Dwight Merriman (one of the original authors of mongoDB) correctly, using update to modify a single document i.e.("multi":false} is also atomic. Currently, it should also be faster than doing the equivalent update using findAndModify.
From the MongoDB docs (emphasis added):
By default, both operations modify a single document. However, the update() method with its multi option can modify more than one document.
If multiple documents match the update criteria, for findAndModify(), you can specify a sort to provide some measure of control on which document to update. With the default behavior of the update() method, you cannot specify which single document to update when multiple documents match.
By default, findAndModify() method returns the pre-modified version of the document. To obtain the updated document, use the new option. The update() method returns a WriteResult object that contains the status of the operation. To return the updated document, use the find() method. However, other updates may have modified the document between your update and the document retrieval. Also, if the update modified only a single document but multiple documents matched, you will need to use additional logic to identify the updated document.
Before MongoDB 3.2 you cannot specify a write concern to findAndModify() to override the default write concern whereas you can specify a write concern to the update() method since MongoDB 2.6.
When modifying a single document, both findAndModify() and the update() method atomically update the document.