Serialization "isolation level" in MongoDB, or the lost-update problem

Question

I realize MongoDB is a NoSQL solution, but I was wondering if it had some sort of equivalent to serialization-level transaction isolation level.

If not, how would you solve the lost-update problem in MongoDB?

I want to keep the revision history of some data in Mongo and each revision has to point to the one before it. How can I make sure no more than one latest revision exists for my data, and on the other hand that no revision is lost due to concurrent updates?

** Edit **

Oops, RTFM, it is indeed possible: http://www.mongodb.org/display/DOCS/Atomic+Operations

Not sure if I should close the question since the knowledge might be relevant to other people..

Answer 1

Yes, this is possible, so long as you keep the history in a single document. MongoDB supports atomic updates within the scope of a document, but not across multiple documents in a collection.

So, you could embed the history in an array, using a schema something like this:

{
    _id: 12345,
    value: "Apple",
    history:
    [
        { revisionid: 2, value: "Orange" },
        { revisionid: 1, value: "Pear" }
    ]
}

For example, you could insert a new document:

db.things.insert( { _id: 123, value: "Apple" } )

Then update it in one atomic operation:

db.things.update( { _id: 123 },
    {
        $set: { value: "Orange" },
        $push : { history : { revisionid: 1, value: "Apple" } }
    }
)