Suppose you have a complex object or set of objects in your database, which happens to be MongoDB, and you can’t simply perform a single db.coll.update() call to write to it. It could be an ordered list, because mongo only knows sets, or it could be multiple objects in multiple collections. Here’s how you can create a lock in MongoDB.

First create a new collection for locks, and add this document to it:

db.createCollection("locks");
db.locks.ensureIndex({ name: 1 }, { unique: 1 });
db.locks.insert({
    name: "mylock",
    locked: false,
    version: 1,
    uid: 0
});

To take control of the lock then, try to update the lock object and double check you have it.

var getLock = function() {
    this.myuid = Math.floor(Math.random()*10000000000);
    db.locks.update(
        { name: "mylock", locked: false },
        { $set: { locked: true, uid: myuid } },
        false, false);
    var haveLock = db.locks.count({ name: "mylock", uid: this.myuid });
    return haveLock;
}

Then, once your done modifying the object, release the lock and update the version.

var releaseLock = function() {
    db.locks.update(
        { name: "mylock", locked: true, uid: this.myuid},
        { $set: { locked: false }, $inc { version: 1 } },
        false, false);
}

This allows you to have multiple agents modifying and reading the same complex object and using a lock stored in MongoDB.