Locking document in MongoDB while allowing queries to get non-locked records - mongodb

Question

In our product we have a core collection which can be accessed from a distributed set of workers.
I want to be able to get a document from the collection without any of the workers accidentally picking up the same document.
The best way I've come up with so far for managing to prevent duplicate records being loaded is the following:
Having 2 separate collections with the following basic structure:
core: { _id: '{mongoGeneratedId}', locked: false, lockTimeout: 0}
lock: { _id: null, lockTimeout: 0}
(lockTimeout would have a TTL index)
A worker would run a query that looks something like this:
db.core.findOne({
$or: [
{locked: false},
{lockTimeout < $currentTime}
]
})
and would have a record returned to it.
To test if the record has been grabbed by another worker and locked it would then try to insert a record into lock with a lockTimeout of 5 mins in the future and an id of the same id as your id from the core table.
If this fails, then we know that another worker pipped us to the post and we want to try to run the query again. If it succeeds, then we update core to have locked as true and have the lockTimeout as the same as the lockTimeout from the lock collection.
Apart from the addition of some form of slightly more complicated ordering to reduce the chances of 2 workers picking up the same record I believe this should work.
However, it doesn't feel elegant and I feel like there should be a better way that doesn't require me to create a secondary collection just to keep track of locking.
Does such a thing exist? Kind regards!

Answer 1

Try using the findAndModify command. This command atomically updates a document and returns the document (default pre-, optionally post-update). You can use the atomic update to lock the document as you grab it:
> db.queue.insert({ "x" : 1, "locked" : false })
> db.queue.findAndModify({
"query" : { "locked" : false },
"update" : { "$set" : { "locked" : true } }
})
{ "_id" : ObjectId("53ea6f0ef9b63e0dd3ca1a1f"), "x" : 1, "locked" : false }
You can also remove the document atomically. Check out the link for all of the features that could help for your queue-like use case and to read more about the command's behavior.