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CHAPTER 6 ACTORS AND CONCURRENCY
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Actors, Atomic Updates, and Transactions
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First, let s talk about ACID. Relational databases like PostgreSQL provide ACID transactions. ACID stands for atomic, consistent, isolated, and durable. Atomic means that if the transaction succeeds, all the rows affected by the transaction are changed at the same instant in the view of other transactions. Consistent means that all of the rows will be updated if the transaction succeeds. Isolated means that until the transaction is committed, no other transactions in the system see any updated rows. Durable means that the transaction is written to media, disk, before the commit is done. In this section, I m going to build a transaction system that is ACI but not D. There will be no durable writes to disk. Actors provide a great mechanism for multithreaded processing without explicit synchronization. Using synchronization, we can implement atomic updates by synchronizing the target of our updates and then performing updates on that target. Because Actors cannot be synchronized, we have to figure out another mechanism for performing atomic updates.9 Because it s possible to pass functions as messages to Actors, we can define messages that contain functions that perform atomic updates.
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Performing an Atomic Update
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Let s build a simple example for an inventory Actor (see Listing 6-4).
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Listing 6-4. Atomic Updates import scala.actors.Actor import Actor._ case case case case object GetInfo class Info(i: Map[String, Int]) class SetInfo(n: String, v: Int) class Update(n: String, f: Option[Int] => Int)
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object XAct1 extends Actor { private var info: Map[String, Int] = Map()
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9. An atomic update is an update that guarantees that the state of the thing being updated will not change between the time the values for calculating the update are retrieved from the thing and the time the update is applied.
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CHAPTER 6 ACTORS AND CONCURRENCY
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def act = loop { react { case GetInfo => reply(Info(info)) case SetInfo(n, v) => info += n -> v case Update(n, f) => info += n -> f(info.get(n)) } } this.start }
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First we do some importing:
import scala.actors.Actor import Actor._
Next, we define our messages:
case case case case object GetInfo class Info(i: Map[String, Int]) class SetInfo(n: String, v: Int) class Update(n: String, f: Option[Int] => Int)
The Update message is most interesting. It takes the name of the item to update and a function that performs the update. Next, let s define our Actor:
object XAct1 extends Actor { private var info: Map[String, Int] = Map() def act = loop { react { case GetInfo => reply(Info(info)) case SetInfo(n, v) => info += n -> v case Update(n, f) => info += n -> f(info.get(n)) } } this.start }
Note how the Update message is processed. Update is processed just like any other Actor message. That means while the Actor is processing Update, it cannot process any other message. But Update contains code in the form of a function. Update s handler applies the function. The function is processed in the scope of the Actor, but it does not have access
CHAPTER 6 ACTORS AND CONCURRENCY
to the Actor s state except for the value of the named item. Thus, the message is processed atomically and is thread-safe because the function is applied on the thread that s doing the processing the Update message. Let s see how we would call XAct1:
scala> XAct1 ! SetInfo("Apple", 4) scala> XAct1 ! SetInfo("Orange", 5) scala> XAct1 ! GetInfo
res2: Any = Info(Map(Apple -> 4, Orange -> 5))
scala> XAct1 ! Update("Apple", v => (v getOrElse 0) + 2)10 scala> XAct1 ! GetInfo
res4: Any = Info(Map(Apple -> 6, Orange -> 5))
Atomic updates are useful, but they do not tell the whole story. There may be times when we need transactions. We need to be able to have exclusive access to one or more Actor s state for an operation that spans both of the Actors.
Performing Transactions
It turns out that we can use the same techniques of immutable data structures and stateful message processing to build Actors that support transactions (see Listing 6-5).
Listing 6-5. Transactional Actors import scala.actors.Actor import Actor._ import scala.actors.TIMEOUT case case case case object GetInfo class Info(i: Map[String, Int]) class SetInfo(n: String, v: Int) class Update(n: String, f: Option[Int] => Int)
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