Akka and ReactiveMongo

Question

I am trying to find the best approach to sharing the same pool of connection between actors withing the cluster workers. I have the following structure:

Master Actor -> Worker Actors(can be up to 100 or more) -> MongoDB

Between workers and MongoDB I want to put reactivemongo, however I am not sure how exactly to provide connection pool sharing between all actors.

According to reactivemongo documentation:

A MongoDriver instance manages an actor system; a connection manages a pool of connections. In general, MongoDriver or create a MongoConnection are never instantiated more than once. You can provide a list of one ore more servers; the driver will guess if it's a standalone server or a replica set configuration. Even with one replica node, the driver will probe for other nodes and add them automatically.

Should I just create it in Master actor and then bundle with each message? So, this would be in Master actor:

val driver = new MongoDriver
val connection = driver.connection(List("localhost"))

And then I pass connection to actors in a message. Or should I query a connection in each Work Actor and pass just driver in a message?

Any help is very appreciated. Thanks.

Answer 1

I would create the driver and connection in the master actor. I would then set up the the worker actors to take an instance of MongoConnection as a constructor argument so that each worker has a reference to the connection (which is really a proxy to a pool of connections). Then, in something like preStart, have the master actor create the workers (which I am assuming are routed) and supply the connection as an arg. A very simplified example could look like this:

class MongoMaster extends Actor{
  val driver = new MongoDriver
  val connection = driver.connection(List("localhost"))

  override def preStart = {
    context.actorOf(Props(classOf[MongoWorker], connection).withRouter(FromConfig()))
  } 

  def receive = {
    //do whatever you need here
    ...
  }
}

class MongoWorker(conn:MongoConnection) extends Actor{
  def receive = {
    ...
  }
}

This code is not exact, but at least it shows the high level concepts I described.

Answer 2

The answer by cmbaxter works as long as you don't need to instantiate the worker actors remotely. MongoConnection is not serializable.

I found this article https://github.com/eigengo/akka-patterns/wiki/Configuration very helpful. The basic idea is to implement a trait called Configured, which is populated by the main application. The actors can then use that trait to gain access to local, non-serializable objects such as MongoConnection.