Alex Minnaar

## Introduction to the Multithreading Problem and the Akka Actor Solution

27 Dec 2014

Nowadays, computers have multiple execution cores meaning that they can execute multiple tasks at the same time rather than sequentially. Obviously this makes things much faster but it also presents some new problems. The term multithreading refers to the process in which multiple threads execute code in the same program simultaneously. The inherent problem with multithreading lies in the fact that although each thread acts independently, their memory is shared. Therefore, it is possible for threads to change shared memory values without other threads knowing which can create problems. Let’s use a bank account as an example. Consider the following code that implements a bank account with deposit and withdraw methods.

class BankAccount{

private var balance = 0

def deposit(amount: Int): Unit =
if (amount > 0) balance = balance + amount

def withdraw(amount: Int): Int =
if (0 < amount && amount <= balance){
balance = balance - amount
balance
} else throw new Error("insufficient funds")

}


Here is a scenario where multithreading can cause problems. Let’s say that balance=40 dollars and thread A would like to withdraw 30 dollars. This satisfies both conditions in the if statement so thread A enters the code block where the 30 dollars is subtracted from the balance. However, before thread A changes the balance, a second thread, thread B, wants to withdraw 20 dollars. Since thread A has not yet changed the balance, thread B also satisfies the if statement and enters the code block where the balance can be changed. So thread A subtracts \$30 from the balance and then thread B subtracts 20 dollars from the balance leaving us with a balance of -10 dollars. Clearly, this is a problem!

Hopefully it is clear that the problem comes from the fact that the shared balance variable can be changed by any thread at any time so no thread can really be sure what value it holds. One solution is for a thread to be able to reserve the memory values that it will be using so that no other thread can change them. This is called locking.

## Using Locks (Synchronous)

As stated previously, locking tries to solve the multithreading problem by protecting the shared memory value (in this case balance). Scala does this with synchronization. Consider the same code as above but now each method definition is wrapped in this.synchronized.

class BankAccount{

private var balance = 0

def deposit(amount: Int): Unit = this.synchronized {
if (amount > 0) balance = balance + amount
}

def withdraw(amount: Int): Int = this.synchronized {
if (0 < amount && amount <= balance){
balance = balance - amount
balance
} else throw new Error("insufficient funds")
}

}


Now deposit and withdraw each exit as one atomic unit meaning that only one thread can access them at a time (all others are blocked and must wait until the blocking thread is finished). Now that all methods that can change balance are synchronized, conflicts such as the ones described above can no longer occur. Unfortunately, the problem still is not completely solved because synchronization produces a few new problems. For example, consider another BankAccount method called transfer which withdraws money from one account and deposits it into another with the following synchronized code.

def transfer(from: BankAccount, to: BankAccount, amount: Int): Unit = {
from.synchronized {
to.synchronized {
from.withdraw(amount)
to.deposit(amount)
}
}
}


The withdrawal and deposit steps must be synchronized so that no thread can access the balance between withdrawal and deposit (at this point the amount transfered would not be anywhere). The problem occurs when thread A wants to transfer money from account A to account B at the same time that thread B wants to transfer money from account B to account A. When this happens thread A would lock account A and thread B would lock account B and each thread would wait for the other to release the lock which would take forever! This is not good and it is called a dead-lock which is a common problem with synchronization. There are ways of dealing with dead-locks but they are complicated and can make your code difficult to read. In addition, stopping and starting threads when they become blocked turns out to be very bad for CPU utilization which will make your code run slower. It would be much better if we could deal with this multithreading problem in such a way that we do not have to use any kind of blocking. This is what the Akka actor model does.

## Using Actors (Asynchronous)

Akka actors are fully encapsulated entities. Changes to their internal state can only be done through passing known messages. Message passing between actors is one-way and completely asynchronous (i.e. unblocking) so when an actor sends a message it does not have to wait for a reply, it can continue performing other tasks. If multiple messages are sent to a single actor, it will process them sequentially in a queue (so internally an actor is single-threaded). If a received message changes an actor’s internal state, the change is reflected immediately after the message has been processed. Therefore, processing one message is the atomic unit of execution (it can never be interrupted).

In terms of our bank account example, let’s create a BankAccount actor that can receive Deposit and Withdraw messages. In Scala, we create an actor by extending the Actor trait and implementing its receive method. We must also define the messages that it can send and recieve in the actor’s companion object. The following is an actor-based BankAccount implementation.

object BankAccount {

case class Deposit(amount: BigInt) {
require(amount > 0)
}

case class Withdraw(amount: BigInt) {
require(amount > 0)
}

case object Done

case object Failed

}

//Actor that receives messages to perform actions of a bank account
class BankAccount extends Actor {

import BankAccount._

var balance = BigInt(0)

//Deposit messages add amount to balance state
case Deposit(amount) =>
balance += amount
sender ! Done

//Withdraw messages subtract amount from balance state
case Withdraw(amount) if amount <= balance =>
balance -= amount
sender ! Done

//Any other message would return a failure to the sender
case _ => sender ! Failed
}

}


In the BankAccount companion object four messages are defined Deposit, Withdraw, Done, and Failed. In the receive method in the BankAccount class defines how to change the balance variable when either the Deposit or the Withdraw method is received. Once this is finished, a Done message is passed back to the actor that sent it via the sender variable (an actor’s reference is always tied to the message it sends). If any other message is received, then a Failed message is sent back to the sending actor.

But we also want to use an actor to transfer money between accounts. You may remember that this had the potential to produce a dead-lock when done synchronously. We can avoid this using actors because now blocking is replaced with enqueuing messages. Let’s call this actor WireTransfer. This actor can receive a Transfer message which contains three fields - a reference to the sending BankAccount actor, a reference to the receiving BankAccount actor, and the amount to be transferred. When WireTransfer receives this message, it sends a Withdraw message (defined within the BankAccount companion object) to the sending actor, awaits a successful Done response, then sends a Deposit message to the recieving actor. The following code implements the WireTransfer actor.

object WireTransfer {

case class Transfer(from: ActorRef, to: ActorRef, amount: BigInt)

case object Done

case object Failed

}

//actor implementing the actions of a wire transfer between two bank account actors
class WireTransfer extends Actor {

import WireTransfer._

//If Transfer message is received, send withdraw message to 'from' and wait for reply
case Transfer(from, to, amount) =>
from ! BankAccount.Withdraw(amount)
context.become(awaitFrom(to, amount, sender))
}

//If Withdraw was successful, send deposit to other bank account actor, or else give them a failure message
case BankAccount.Done =>
to ! BankAccount.Deposit(amount)
context.become(awaitTo(customer))
case BankAccount.Failed =>
customer ! Failed
context.stop(self)
}

//If deposit was successful, send 'Done' to original actor that sent Transfer message
case BankAccount.Done =>
customer ! Done
context.stop(self)
}
}


When the actor first receives the Transfer message, it sends a Withdraw message to the BankAccount actor referenced in the from field of the message. Then the actor must wait until a Done message is received (it does this with the context.become() method) at which point it sends a Deposit message to the other BankAccount actor.

We can also test this transfer process by creating a new actor that creates two BankAccount actors and a WireTransfer actor and then sends a Transfer message to the WireTransfer actor which references both of the BankAccount actors. But before the Transfer message is sent, the actor must deposit some money in the first BankAccount actor so that there is some money available to transfer. Let’s call this actor TransferMain and here is its implementation.

class TransferMain extends Actor {

//First create two BankAccount actors
val accountA = context.actorOf(Props[BankAccount], "accountA")
val accountB = context.actorOf(Props[BankAccount], "accountB")

//send a deposit message to accountA
accountA ! BankAccount.Deposit(100)

//If a 'Done' message is received back, call a transfer function
case BankAccount.Done => transfer(70)
}

//transfer function creates a transacton actor and sends a 'Transfer' message to it between
//accountA and accountB for the specified amount.
def transfer(amount: BigInt): Unit = {

val transaction = context.actorOf(Props[WireTransfer], "transfer")

transaction ! WireTransfer.Transfer(accountA, accountB, amount)

case WireTransfer.Done =>
println("successs")
context.stop(self)
case WireTransfer.Failed =>
println("failed")
context.stop(self)
})

}
}


As you can see from the above code, 100 dollars is deposted in accountA via the Deposit message. Then, when the Done message is recieved, the transfer function is called with an argument of 70 dollars. Inside the transfer function, a WireTransfer actor is created and a Transfer message is sent to it with the appropriate arguments. It then waits for either a Done or Failed message in return.

Hopefully this blog post has shed some light on the multithreading problem and how the Akka actor model tries to solve it.