Proper way to access shared resource in Scala actors

Question

In Java, one would synchronize methods or blocks that access a shared resource that is needed in a multi-threaded environment.

I'm wondering how the "Scala Actors" way of doing this would work.

Suppose I have a connection pool of java.sql.Connection objects that I wish to provide thread-safe access to. I implement it as an actor that receives messages and sends the sender a connection back.

It seems there's three ways to do this:

1. Use a Future
2. Use !?
3. Have the class needing the Connection also be an actor

Code:

sealed abstract class ConnectionPoolMessage
case class NewConnection extends ConnectionPoolMessage
case class CloseConnection(c:Connection) extends ConnectionPoolMessage

class ConnectionPool extends Actor {
  def act() {
    while (true) {
      receive() {
        case NewConnection => sender ! getConnectionFromPool
        case CloseConnection(conn) => returnConnection(conn)
      }
    }
  }
}

// Here, my "do stuff" method is all in one place, and I block waiting
// on the Future from the pool; however this could take forever and cause trouble
class UsingFuture {
  val pool = new ConnectionPool
  def doSomething() {
    val connectionFuture = pool !! NewConnection
    val connection = connectionFuture() // concerned that we can't timeout here
    // do stuff with my Connection instance
    pool ! CloseConnection(connection)  
  }
}


// here, I wait and just use a timeout
// Seems clean to me, I guess.
class UsingBangQuestion {
  val pool = new ConnectionPool
  def doSomething() {
    pool !?(TIMEOUT,NewConnection) match {
      case Some(conn) => {
        // do something with connection
        pool ! CloseConnection(conn)
      }
      case None => throw new RuntimeException("timed out")
    }
  }
}

// here, I don't worry about timeouts, cause I only use the
// the connection when I receive a message back with it.  
// The problem is that I now have to split my logic up
// with two methods
class AsAnActor extends Actor {
  val pool = new ConnectionPool
  def startSomething() {
    start
    pool ! NewConnection
  }
  def act() {
    receive() {
      case conn:Connection => finishSomething(conn)
    }
  }
  def finishSomething(conn:Connection) {
    // do stuff with my Connection
    pool ! CloseConnection(conn)
  }
}

The Future version seems cleanest, except for the fact that I could block forever.

Any thoughts, or is my whole conception of this wrong?

Answer 1

It may be bad style but one way is to mix imperative and functional styles by having your actor (which requires connections) to have the connection pool plugged in directly and use synchronization to get a Connection. To be honest, I don't really see what is wrong with this approach; I much prefer it to the !!or !? one, which just screams deadlock (or even livelock)!

I guess one other way would be to send a message to your pool which expressed the work which needed to be done with the connection and a possible target for the result:

class DbWork(f: Connection => Unit)
class DbWorkWithResult[T](f:Connection => T, target: OutputChannel[Any])

And then you might use it thus:

pool ! new DbWork( { (conn: Connection) => //do something 
                 })

Or:

pool ! new DbWorkWithResult[Int]( (conn: Connection) => //return int
                 }, self)

Answer 2

The Actors way of doing it is not sharing resources. Have all access fed to a single Actor whose job is to deal with access to the shared resource.

This way, the resource itself isn't shared among threads. The Actor is.