pekko/akka-docs/scala/futures.rst

.. _futures-scala:

Futures (Scala)
===============

.. sidebar:: Contents

   .. contents:: :local:

Introduction
------------

In Akka, a `Future <http://en.wikipedia.org/wiki/Futures_and_promises>`_ is a data structure used to retrieve the result of some concurrent operation. This operation is usually performed by an ``Actor`` or by the ``Dispatcher`` directly. This result can be accessed synchronously (blocking) or asynchronously (non-blocking).

Use with Actors
---------------

There are generally two ways of getting a reply from an ``Actor``: the first is by a sent message (``actor ! msg``), which only works if the original sender was an ``Actor``) and the second is through a ``Future``.

Using an ``Actor``\'s ``?`` method to send a message will return a Future. To wait for and retrieve the actual result the simplest method is:

.. code-block:: scala

  val future = actor ? msg
  val result: Any = future.get()

This will cause the current thread to block and wait for the ``Actor`` to 'complete' the ``Future`` with it's reply. Due to the dynamic nature of Akka's ``Actor``\s this result will be untyped and will default to ``Nothing``. The safest way to deal with this is to cast the result to an ``Any`` as is shown in the above example. You can also use the expected result type instead of ``Any``, but if an unexpected type were to be returned you will get a ``ClassCastException``. For more elegant ways to deal with this and to use the result without blocking, refer to `Functional Futures`_.

Use Directly
------------

A common use case within Akka is to have some computation performed concurrently without needing the extra utility of an ``Actor``. If you find yourself creating a pool of ``Actor``\s for the sole reason of performing a calculation in parallel, there is an easier (and faster) way:

.. code-block:: scala

  import akka.dispatch.Future

  val future = Future {
    "Hello" + "World"
  }
  val result = future.get()

In the above code the block passed to ``Future`` will be executed by the default ``Dispatcher``, with the return value of the block used to complete the ``Future`` (in this case, the result would be the string: "HelloWorld"). Unlike a ``Future`` that is returned from an ``Actor``, this ``Future`` is properly typed, and we also avoid the overhead of managing an ``Actor``.

Functional Futures
------------------

A recent addition to Akka's ``Future`` is several monadic methods that are very similar to the ones used by Scala's collections. These allow you to create 'pipelines' or 'streams' that the result will travel through.

Future is a Monad
^^^^^^^^^^^^^^^^^

The first method for working with ``Future`` functionally is ``map``. This method takes a ``Function`` which performs some operation on the result of the ``Future``, and returning a new result. The return value of the ``map`` method is another ``Future`` that will contain the new result:

.. code-block:: scala

  val f1 = Future {
    "Hello" + "World"
  }

  val f2 = f1 map { x =>
    x.length
  }

  val result = f2.get()

In this example we are joining two strings together within a Future. Instead of waiting for this to complete, we apply our function that calculates the length of the string using the ``map`` method. Now we have a second Future that will eventually contain an ``Int``. When our original ``Future`` completes, it will also apply our function and complete the second Future with it's result. When we finally ``get`` the result, it will contain the number 10. Our original Future still contains the string "HelloWorld" and is unaffected by the ``map``.

Something to note when using these methods: if the ``Future`` is still being processed when one of these methods are called, it will be the completing thread that actually does the work. If the ``Future`` is already complete though, it will be run in our current thread. For example:

.. code-block:: scala

  val f1 = Future {
    Thread.sleep(1000)
    "Hello" + "World"
  }

  val f2 = f1 map { x =>
    x.length
  }

  val result = f2.get()

The original ``Future`` will take at least 1 second to execute now, which means it is still being processed at the time we call ``map``. The function we provide gets stored within the ``Future`` and later executed automatically by the dispatcher when the result is ready.

If we do the opposite:

.. code-block:: scala

  val f1 = Future {
    "Hello" + "World"
  }

  Thread.sleep(1000)

  val f2 = f1 map { x =>
     x.length
  }

  val result = f2.get()

Our little string has been processed long before our 1 second sleep has finished. Because of this, the dispatcher has moved onto other messages that need processing and can no longer calculate the length of the string for us, instead it gets calculated in the current thread just as if we weren't using a ``Future``.

Normally this works quite well as it means there is very little overhead to running a quick function. If there is a possibility of the function taking a non-trivial amount of time to process it might be better to have this done concurrently, and for that we use ``flatMap``:

.. code-block:: scala

  val f1 = Future {
    "Hello" + "World"
  }

  val f2 = f1 flatMap {x =>
    Future(x.length)
  }

  val result = f2.get()

Now our second Future is executed concurrently as well. This technique can also be used to combine the results of several Futures into a single calculation, which will be better explained in the following sections.

For Comprehensions
^^^^^^^^^^^^^^^^^^

Since ``Future`` has a ``map`` and ``flatMap`` method it can be easily used in a 'for comprehension':

.. code-block:: scala

  val f = for {
    a <- Future(10 / 2) // 10 / 2 = 5
    b <- Future(a + 1)  //  5 + 1 = 6
    c <- Future(a - 1)  //  5 - 1 = 4
  } yield b * c         //  6 * 4 = 24

  val result = f.get()

Something to keep in mind when doing this is even though it looks like parts of the above example can run in parallel, each step of the for comprehension is run sequentially. This will happen on separate threads for each step but there isn't much benefit over running the calculations all within a single Future. The real benefit comes when the ``Future``\s are created first, and then combining them together.

Composing Futures
^^^^^^^^^^^^^^^^^

The example for comprehension above is an example of composing ``Future``\s. A common use case for this is combining the replies of several ``Actor``\s into a single calculation without resorting to calling ``get`` or ``await`` to block for each result. First an example of using ``get``:

.. code-block:: scala

  val f1 = actor1 ? msg1
  val f2 = actor2 ? msg2

  val a: Int = f1.get()
  val b: Int = f2.get()

  val f3 = actor3 ? (a + b)

  val result: String = f3.get()

Here we wait for the results from the first 2 ``Actor``\s before sending that result to the third ``Actor``. We called ``get`` 3 times, which caused our little program to block 3 times before getting our final result. Now compare that to this example:

.. code-block:: scala

  val f1 = actor1 ? msg1
  val f2 = actor2 ? msg2

  val f3 = for {
    a: Int    <- f1
    b: Int    <- f2
    c: String <- actor3 ? (a + b)
  } yield c

  val result = f3.get()

Here we have 2 actors processing a single message each. Once the 2 results are available (note that we don't block to get these results!), they are being added together and sent to a third ``Actor``, which replies with a string, which we assign to 'result'.

This is fine when dealing with a known amount of Actors, but can grow unwieldy if we have more then a handful. The ``sequence`` and ``traverse`` helper methods can make it easier to handle more complex use cases. Both of these methods are ways of turning, for a subclass ``T`` of ``Traversable``, ``T[Future[A]]`` into a ``Future[T[A]]``. For example:

.. code-block:: scala

  // oddActor returns odd numbers sequentially from 1
  val listOfFutures: List[Future[Int]] = List.fill(100)(oddActor ? GetNext)

  // now we have a Future[List[Int]]
  val futureList = Future.sequence(listOfFutures)

  // Find the sum of the odd numbers
  val oddSum = futureList.map(_.sum).get()

To better explain what happened in the example, ``Future.sequence`` is taking the ``List[Future[Int]]`` and turning it into a ``Future[List[Int]]``. We can then use ``map`` to work with the ``List[Int]`` directly, and we find the sum of the ``List``.

The ``traverse`` method is similar to ``sequence``, but it takes a ``T[A]`` and a function ``T => Future[B]`` to return a ``Future[T[B]]``, where ``T`` is again a subclass of Traversable. For example, to use ``traverse`` to sum the first 100 odd numbers:

.. code-block:: scala

  val oddSum = Future.traverse((1 to 100).toList)(x => Future(x * 2 - 1)).map(_.sum).get()

This is the same result as this example:

.. code-block:: scala

  val oddSum = Future.sequence((1 to 100).toList.map(x => Future(x * 2 - 1))).map(_.sum).get()

But it may be faster to use ``traverse`` as it doesn't have to create an intermediate ``List[Future[Int]]``.


Then there's a method that's called ``fold`` that takes a start-value, a sequence of ``Future``:s and a function from the type of the start-value and the type of the futures and returns something with the same type as the start-value, and then applies the function to all elements in the sequence of futures, non-blockingly, the execution will run on the Thread of the last completing Future in the sequence.

.. code-block:: scala

  val futures = for(i <- 1 to 1000) yield Future(i * 2) // Create a sequence of Futures
  
  val futureSum = Futures.fold(0)(futures)(_ + _)

That's all it takes!


If the sequence passed to ``fold`` is empty, it will return the start-value, in the case above, that will be 0. In some cases you don't have a start-value and you're able to use the value of the first completing Future in the sequence as the start-value, you can use ``reduce``, it works like this:

.. code-block:: scala

  val futures = for(i <- 1 to 1000) yield Future(i * 2) // Create a sequence of Futures

  val futureSum = Futures.reduce(futures)(_ + _)

Same as with ``fold``, the execution will be done by the Thread that completes the last of the Futures, you can also parallize it by chunking your futures into sub-sequences and reduce them, and then reduce the reduced results again.



This is just a sample of what can be done, but to use more advanced techniques it is easier to take advantage of Scalaz, which Akka has support for in its akka-scalaz module.


Scalaz
^^^^^^

There is also an `Akka-Scalaz`_ project created by Derek Williams for a more
complete support of programming in a functional style.

.. _Akka-Scalaz: https://github.com/derekjw/akka-scalaz


Exceptions
----------

Since the result of a ``Future`` is created concurrently to the rest of the program, exceptions must be handled differently. It doesn't matter if an ``Actor`` or the dispatcher is completing the ``Future``, if an ``Exception`` is caught the ``Future`` will contain it instead of a valid result. If a ``Future`` does contain an ``Exception``, calling ``get`` will cause it to be thrown again so it can be handled properly.

It is also possible to handle an ``Exception`` by returning a different result. This is done with the ``recover`` method. For example:

.. code-block:: scala

  val future = actor ? msg1 recover {
    case e: ArithmeticException => 0
  }

In this example, if an ``ArithmeticException`` was thrown while the ``Actor`` processed the message, our ``Future`` would have a result of 0. The ``recover`` method works very similarly to the standard try/catch blocks, so multiple ``Exception``\s can be handled in this manner, and if an ``Exception`` is not handled this way it will be behave as if we hadn't used the ``recover`` method.