What's the difference between findAndModify and update in MongoDB?

Question

I'm a little bit confused by the findAndModify method in MongoDB. What's the advantage of it over the update method? For me, it seems that it just returns the item first and then updates it. But why do I need to return the item first? I read the MongoDB: the definitive guide and it says that it is handy for manipulating queues and performing other operations that need get-and-set style atomicity. But I didn't understand how it achieves this. Can somebody explain this to me?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

One useful class of use cases is counters and similar cases. For example, take a look at this code (one of the MongoDB tests): find_and_modify4.js.

Thus, with findAndModify you increment the counter and get its incremented value in one step. Compare: if you (A) perform this operation in two steps and somebody else (B) does the same operation between your steps then A and B may get the same last counter value instead of two different (just one example of possible issues).

Answer 5

This is an old question but an important one and the other answers just led me to more questions until I realized: The two methods are quite similar and in many cases you could use either.

• Both findAndModify and update perform atomic changes within a single request, such as incrementing a counter; in fact the <query> and <update> parameters are largely identical
• With both, the atomic change takes place directly on a document matching the query when the server finds it, ie an internal write lock on that document for the fraction of a millisecond that the server confirms the query is valid and applies the update

There is no system-level write lock or semaphore which a user can acquire. Full stop. MongoDB deliberately doesn't make it easy to check out a document then change it then write it back while somehow preventing others from changing that document in the meantime. (While a developer might think they want that, it's often an anti-pattern in terms of scalability and concurrency ... as a simple example imagine a client acquires the write lock then is killed while holding it. If you really want a write lock, you can make one in the documents and use atomic changes to compare-and-set it, and then determine your own recovery process to deal with abandoned locks, etc. But go with caution if you go that way.)

From what I can tell there are two main ways the methods differ:

• If you want a copy of the document when your update was made: only findAndModify allows this, returning either the original (default) or new record after the update, as mentioned; with update you only get a WriteResult, not the document, and of course reading the document immediately before or after doesn't guard you against another process also changing the record in between your read and update
• If there are potentially multiple matching documents: findAndModify only changes one, and allows you customize the sort to indicate which one should be changed; update can change all with multi although it defaults to just one, but does not let you say which one

Thus it makes sense what HungryCoder says, that update is more efficient where you can live with its restrictions (eg you don't need to read the document; or of course if you are changing multiple records). But for many atomic updates you do want the document, and findAndModify is necessary there.

Answer 6

We used findAndModify() for Counter operations (inc or dec) and other single fields mutate cases. Migrating our application from Couchbase to MongoDB, I found this API to replace the code which does GetAndlock(), modify the content locally, replace() to save and Get() again to fetch the updated document back. With mongoDB, I just used this single API which returns the updated document.