It's not possible to lock a mongodb document. What if I need to?

Question

I know that I can\'t lock a single mongodb document, in fact there is no way to lock a collection either.

However, I\'ve got this scenario, where I think I need some

Answer 1

If the order of the elements in the array is not important for you then the $push operator should be safe enough to prevent threads from overwriting each others changes.

Answer 2

Instead of writing the question in another question, I try to answer this one: I wonder if this WiredTiger Storage will handle the problem I pointed out here: Limit inserts in mongodb

Answer 3

Hey the only way of which I think now is to add an status parameter and use the operation findAndModify(), which enables you to atomically modify a document. It's a bit slower, but should do the trick.

So let's say you add an status attribut and when you retrieve the document change the status from "IDLE" to "PROCESSING". Then you update the document and save it back to the collection updating the status to "IDLE" again.

Code example:

var doc = db.runCommand({
              "findAndModify" : "COLLECTION_NAME",
              "query" : {"_id": "ID_DOCUMENT", "status" : "IDLE"},
              "update" : {"$set" : {"status" : "RUNNING"} }
}).value

Change the COLLECTION_NAME and ID_DOCUMENT to a proper value. By default findAndModify() returns the old value, which means the status value will be still IDLE on the client side. So when you are done with updating just save/update everything again.

The only think you need be be aware is that you can only modify one document at a time.

Hope it helps.

Answer 4

Update: With MongoDB 3.2.2 using WiredTiger Storage implementation as default engine, MongoDB use default locking at document level.It was introduced in version 3.0 but made default in version 3.2.2. Therefore MongoDB now has document level locking.

Answer 5

Sounds like you want to use MongoDB's atomic operators: http://www.mongodb.org/display/DOCS/Atomic+Operations

Answer 6

As of 4.0, MongoDB supports Transactions for replica sets. Support for sharded clusters will come in MongoDB 4.2. Using transactions, commit up DB updates will be aborted if a conflicting write occurs, solving your issue.

Transactions are much more costly in terms of performance so don't use Transactions as an excuse for poor NoSQL schema design!