A two-phase commit Transaction Monad. See: http://en.wikipedia.org/wiki/Two-phase_commit_protocol

twophase.scala

// Run this with scala <filename>

/**
 * A Two-phase commit Monad
 */
trait Transaction[+T] {
  def map[U](fn: T => U): Transaction[U] = flatMap { t => Constant(fn(t)) }
  def flatMap[U](fn: T => Transaction[U]): Transaction[U] =
    FlatMapped(this, fn)

  // In a real system, this should be wrapped in a Future
  def query(id: Long): Either[Transaction[T], Prepared[T]]

  final def run(inid: Long): T = {
    @annotation.tailrec
    def go(id: Long, eith: Either[Transaction[T], Prepared[T]]): T = eith match {
      case Left(trans) => go(id + 1L, trans.query(id + 1L))
      case Right(prepped) => prepped.commit.get
    }
    go(inid, query(inid))
  }
}

/** Represents the state when commit is possible */
trait Prepared[+T] {
  def init: T
  // In a real system, these should be wrapped in Futures
  def commit: Committed[T]
  def rollback: Transaction[T]
}

/** A Type wrapper just used to mark success. */
case class Committed[+T](get: T)

// The most trivial instance
case class Constant[+T](get: T) extends Transaction[T] { self =>
  def query(l: Long) = Right(new Prepared[T] {
    def init = get
    def commit = Committed(get)
    def rollback = self
  })
}

/** Implementation of flatMap (non-trampolined, so super big transactions can fail) */
case class FlatMapped[R,T](init: Transaction[R], fn: R => Transaction[T]) extends Transaction[T] {
  def query(id: Long) = init.query(id) match {
    case Left(ninit) =>
      Left(FlatMapped(ninit, fn)) // didn't work
    case Right(iprep) =>
      val next = fn(iprep.init)
      next.query(id) match {
        case Left(nretry) => Left(FlatMapped(iprep.rollback, fn))
        case Right(rprep) => Right(new Prepared[T] {
          def init = rprep.init
          def commit = {
            // See how we need a Future (or some Monad) to sequence these
            iprep.commit
            rprep.commit
          }
          lazy val rollback = {
            // See how we need a Future (or some Monad) to sequence these
            rprep.rollback
            FlatMapped(iprep.rollback, fn)
          }
        })
      }
  }
}

import java.util.concurrent.atomic.{AtomicReference => Atom}

// Wrapper for an atomic-ref
class Atomic[T](init: T) {
  private val state = new Atom[Either[(Long, T, T), T]](Right(init))

  // The two things we can do: read and alter
  def read: Transaction[T] = new Map(state, identity[T])
  def map(fn: T => T): Transaction[T] = new Map(state, fn)
}

class Map[T](atom: Atom[Either[(Long, T, T), T]], fn: T => T) extends Transaction[T] {
  @annotation.tailrec
  private def unset(id: Long): Unit =
    atom.get match {
      case Right(_) => ()
      case left@Left((thisId, t, _)) =>
        if(thisId != id) ()
        else if(atom.compareAndSet(left, Right(t))) ()
        else unset(id)
    }

  private def rollbackId(id: Long): Unit =
    atom.get match {
      case Right(_) => ()
      case left@Left((thisId, _, rb)) =>
        if(thisId != id) ()
        else if(atom.compareAndSet(left, Right(rb))) ()
        else rollbackId(id)
    }

  private def prep(id: Long, myRes: T) =
    Right(new Prepared[T] {
      def init = myRes
      lazy val commit = {
        unset(id)
        Committed(myRes)
      }
      lazy val rollback = {
        rollbackId(id)
        new Map(atom, fn)
      }
    })

  private def tryApply(eith: Either[(Long, T, T), T], id: Long, t: T, rollbackv: T) = {
    val myRes = fn(t)
    if(atom.compareAndSet(eith, Left((id, myRes, rollbackv))))
      prep(id, myRes)
    else
      Left(new Map(atom, fn))
  }

  def query(id: Long) = {
    atom.get match {
      case r@Right(t) => tryApply(r, id, t, t) // start of request
      case left@Left((existingId, before, rb)) =>
        if(existingId == id) {
          // Part of the same request
          tryApply(left, id, before, rb)
        }
        else Left(new Map(atom, fn))
    }
  }
}

object Test {
def test(count: Int) = {
  val mem = new Atomic[Int](42)

  /** Thread that just does subtraction */
  val sub = new Thread {
    override def run {
      @annotation.tailrec
      def go(cnt: Int, m: Int): Int = {
        if(cnt > 0) {
          // Compose monadically
          val r = (for {
            x <- mem.read
            y <- mem.map { _ => x - 1}
          } yield y).run(cnt) // make sure the id does not collide with add
          go(cnt - 1, m max r)
        }
        else
          m
      }
      println("max seen by subber: " + go(count, 42))
    }
  }
  /** Thread that just does addition */
  val add = new Thread {
    override def run {
      @annotation.tailrec
      def go(cnt: Int, m: Int): Int = {
        if(cnt > 0) {
          // Compose monadically
          val r = (for {
            x <- mem.read
            y <- mem.map { _ => x + 1}
          } yield y).run(2*count + cnt) // make sure the id does not collide with sub
          go(cnt - 1, r min m)
        }
        else
          m
      }
      println("min seen by adder: " + go(count, 42))
    }
  }
  sub.start
  add.start
  sub.join
  add.join
  println("42 is always the answer (no race conditions) ==> " + mem.read.run(-count))
}
}

Test.test(1000000)