Mutex with MongoDB

Question

I have multiple servers sharing a common mongodb. In the DB there is a list of jobs, the servers have to finish. As I want to divide the load over all servers and want to avoid multiple servers doing the same job, I want to "Lock" that job.

My Idea is:

• Set the element as taken if it is not yet taken:

  db.collection.update({done: false, taken: false},{$set: {taken: true, takenBy: myIp}});

• Check if the server got the mutex on this element: db.collection.findOne({taken: true, takenBy: myIp})

Would that be the best way to "synchronize" multiple worker servers over a mongodb (does the server do updates in a single transaction like mysql) or could the server do multiple of this first commands at once?

Answer 1

The key MongoDB feature in this area is that an update to a single document is atomic. From the docs:

In MongoDB, a write operation is atomic on the level of a single document, even if the operation modifies multiple embedded documents within a single document.

When a single write operation modifies multiple documents, the modification of each document is atomic, but the operation as a whole is not atomic and other operations may interleave. However, you can isolate a single write operation that affects multiple documents using the $isolated operator.

So for your update of:

db.collection.update({done: false, taken: false},{$set: {taken: true, takenBy: myIp}});

this means that it will atomically find a doc that matches the criteria and then update it. So yes, that will work well to assign a task to a given server.

See this other post for more details on implementing a shared work queue in MongoDB. A key point that's mentioned there is the use of findAndModify to both perform the update and return the updated doc.