Concurrent Programming with Futures

Finagle uses futures [1] to encapsulate and compose concurrent operations such as network RPCs. Futures are directly analogous to threads — they provide independent and overlapping threads of control — and can be thought of as featherweight threads. They are cheap in construction, so the economies of traditional threads do not apply. It is no problem to maintain millions of outstanding concurrent operations when they are represented by futures.

Futures also decouple Finagle from the operating system and runtime thread schedulers. This is used in important ways; for example, Finagle uses thread biasing to reduce context switching costs.

Futures as containers

Common examples of operations that are represented by futures are:

• an RPC to a remote host
• a long computation in another thread
• reading from disk

Note that these operations are all fallible: remote hosts could crash, computations might throw an exception, disks could fail, etc. A Future[T], then, occupies exactly one of three states:

• Empty (pending)
• Succeeded (with a result of type T)
• Failed (with a Throwable)

While it is possible to directly query this state, this is rarely useful. Instead, a callback may be registered to receive the results once they are made available:

import com.twitter.util.Future

val f: Future[Int] = ???

f.onSuccess { res: Int =>
  println("The result is " + res)
}

which will be invoked only on success. Callbacks may also be registered to account for failures:

import com.twitter.util.Future

val f: Future[Int] = ???

f.onFailure { cause: Throwable =>
  println("f failed with " + cause)
}

Sequential composition

Registering callbacks is useful but presents a cumbersome API. The power of Futures lie in how they compose. Most operations can be broken up into smaller operations which in turn constitute the composite operation. Futures makes it easy to create such composite operations.

Consider the simple example of fetching a representative thumbnail from a website (ala Pinterest). This typically involves:

1. Fetching the homepage
2. Parsing that page to find the first image link
3. Fetching the image link

This is an example of sequential composition: in order to do the next step, we must have successfully completed the previous one. With Futures, this is called flatMap [3]. The result of flatMap is a Future representing the result of this composite operation. Given some helper methods — fetchUrl fetches the given URL, findImageUrls parses an HTML page to find image links — we can implement our Pinterest-style thumbnail extract like this:

import com.twitter.util.Future

def fetchUrl(url: String): Future[Array[Byte]] = ???
def findImageUrls(bytes: Array[Byte]): Seq[String] = ???

val url = "http://www.google.com"

val f: Future[Array[Byte]] = fetchUrl(url).flatMap { bytes =>
  val images = findImageUrls(bytes)
  if (images.isEmpty)
    Future.exception(new Exception("no image"))
  else
    fetchUrl(images.head)
}

f.onSuccess { image =>
  println("Found image of size " + image.size)
}

f represents the composite operation. It is the result of first retrieving the web page, and then the first image link. If either of the smaller operations fail (the first or second fetchUrl or if findImageUrls doesn’t successfully find any images), the composite operation also fails.

The astute reader may have noticed something peculiar: this is typically the job of the semicolon! That is not far from the truth: semicolons sequence two statements, and with traditional I/O operations, have the same effect as flatMap does above (the exception mechanism takes the role of a failed future). Futures are much more versatile, however, as we’ll see.

Concurrent composition

It is also possible to compose Futures concurrently. We can extend our above example to demonstrate: let’s fetch all the images. Concurrent composition is provided by Future.collect:

import com.twitter.util.Future

val collected: Future[Seq[Array[Byte]]] =
  fetchUrl(url).flatMap { bytes =>
    val fetches = findImageUrls(bytes).map { url => fetchUrl(url) }
    Future.collect(fetches)
  }

Here we have combined both concurrent and sequential composition: first we fetch the web page, then we collect the results of fetching all of the underlying images.

As with sequential composition, concurrent composition propagates failures: the future collected will fail if any of the underlying futures do [2].

It is also simple to write your own combinators that operate over Futures. This is quite useful, and gives rise to a great amount of modularity in distributed systems as common patterns can be cleanly abstracted.

Recovering from failure

Composed futures fail whenever any of their constituent futures fail. However it is often useful to recover from such failures. The rescue combinator on Future is the dual to flatMap: whereas flatMap operates over values, rescue operates over exceptions. They are otherwise identical. It is often desirable to handle only a subset of possible exceptions. To accommodate for this rescue accepts a PartialFunction, mapping a Throwable to a Future:

trait Future[A] {
  ..
  def rescue[B >: A](f: PartialFunction[Throwable, Future[B]]): Future[B]
  ..
}

The following retries a request infinitely should it fail with a TimeoutException:

import com.twitter.util.Future
import com.twitter.finagle.http
import com.twitter.finagle.TimeoutException

def fetchUrl(url: String): Future[http.Response] = ???

def fetchUrlWithRetry(url: String): Future[http.Response] =
  fetchUrl(url).rescue {
    case exc: TimeoutException => fetchUrlWithRetry(url)
  }

Other resources

• Effective Scala contains a section discussing futures
• As of Scala 2.10, the Scala standard library has its own futures implementation and API, described here. Note that this is largely similar to the API used in Finagle (com.twitter.util.Future), but there are still some naming differences.
• Akka’s documentation also has a section dedicated to futures.

Footnotes

[1]	Finagle uses its own Future implementation by a variety of reasons (fewer context switches, interruptibility, support for continuation-local variables, tail-call elimination), but mostly because it’s preceded SIP-14 by over a year.

[2]	Use Future.collectToTry to concurrently collect a sequence of futures while accumulating errors instead of failing fast.

[3]

The name flatMap may seem strange and unrelated to our present discussion, but its etymology is impeccable: it derives from a deeper relationship between the sort of sequential composition we do with futures, to a similar sort of composition we can perform over collections. See the this page for more details.