Twitter Futures

History

Futures represent asynchronous values. Over time, the java community has experimented with a few different styles of working with them, starting with transforming them to synchronous code, where you would block until the future was satisfied. We can see remnants of this in the old java.util.concurrent.Future interface, where the only way of using the result of a future is Future#get, which blocks until the future returns. We can do somewhat better by polling until the future is satisfied, and doing other things in the mean time, but this is manual. Instead of managing it manually, we could use Future#get exclusively, which would force us to use a thread per concurrent request.

We wanted to try using an evented system instead, so we chose to describe what we want to do after we get a response, effectively a closure. This is a common API for asynchronous results, so it’s a natural match for asynchronous values. Scala had a Futures implementation, scala.actors.Future, but it was tightly coupled to their Actors library, which needed some work. We ended up working on our own Futures implementation, which turned into Twitter Futures. Around the same time, Akka was working on an actors implementation, and produced their own Futures implementation, which turned into the scala.concurrent.Future. More recently, Java came out with their own Future which you can attach closures to, java.util.concurrent.CompletableFuture.

Scheduling

Each of the modern Futures implementations, Scala, Java, and Twitter, represents a different way of scheduling the closures that should run on satisfaction.

It’s worth noting that our scheduler is configurable (this is also true for Scala Futures, on a more granular basis), so this model isn’t set in stone. With that said, we often unthinkingly program against the semantics of the default scheduler, so it would be a significant undertaking to change the scheduler.

Scala
Someone else run this. Typically this means that we schedule the closure to be run in a thread pool somewhere. This has the advantage that there’s a fixed cost of satisfying a future per closure hanging off of it, which is just the cost of scheduling each closure. It can also allow parallelism if you’re doing CPU-intensive work, and is kinder to blocking work. The disadvantage is that it requires doing the work on another thread, and comes with all of the costs of passing the message to the other thread. The way work gets scheduled in Scala can be changed on a per-closure basis, but the default is to be backed by a thread pool.
Java
I’ll run this now. This can be good for caching behavior, and lets us get better stack traces, because we can see the causal link between satisfying a Future and running the closures. The disadvantage is that this is no longer stack-safe, because hanging subsequent closures off will deepen the stack. Note that at each point of asynchrony, the stack trace is broken.
Twitter
I’ll run this later. This still helps with caching, but can be stack-safe. The disadvantage is that the stack traces aren’t as useful (in practice we sometimes “run it now”).

To illustrate this, imagine that we’re doing three pieces of work, doWorkA, doWorkB, doWorkC.

We write it as:

Future.value(doWorkA()).map { a => doWorkB(a) }.map { b => doWorkC(b) }

So what does this turn into at run time? The answer is different depending upon which model we have for Future.

With Scala, it becomes:

val executor = new ForkJoinPool(2 /* parallelism */)
val a = doWorkA()
executor.submit(new Runnable() { def run(): Unit = {
  val b = doWorkB(a)
  executor.submit(new Runnable() { def run(): Unit = doWorkC(b) })
})

With Java, it becomes:

def workA(): C = {
  val a = doWorkA()
  workB(a)
}
def workB(a: A): C = {
  val b = doWorkB(a)
  workC(b)
}
def workC(b): C = {
  doWorkC(b)
}
workA()

With Twitter, it becomes:

val a = doWorkA()
val b = doWorkB(a)
doWorkC(b)

We’ve chosen this style for performance and for stack safety, and have been quite happy with it historically.

Interruption

The other main difference that the different future types have is how they deal with cancellation. It can be useful to cancel asynchronous work to free up the resources, even though the thread itself is not blocked, for example if you’re consuming resources on a remote host. Note that this breaks the model of what a Future is, since it represents an asynchronous value. Being able to cancel to prevent work points out that the asynchronous work must be happening somewhere, and asks the future to keep a reference to where that work is being done, so it can be cancelled. Scala simply doesn’t implement cancellation, so that if you want to support it, you need to handle it in another way. Java only supports cancellation by immediately satisfying the future with CancellationException. If you want to subsequently cancel the work, you need to ensure that that Future handles the CancellationException gracefully. Twitter futures allow users to set interrupt handlers, and can propagate interruptions up the flatMap chain, so that if a future is no longer needed, it can cancel the underlying work that will later produce that future. This means that futures do the right thing by default, and we don’t need to treat each Future#flatMap specially to propagate interrupts. Interruptions are also advisory for Twitter futures, so that we can choose to ignore them if we wish. One nice thing about this style is that it’s analogous to the Java interrupt mechanism, where we in effect set a field that an implementor can choose to respect or not.

Guts

There are two main implementations of Twitter future, which are ConstFuture and Promise. ConstFuture is an already satisfied future–effectively a thin wrapper around a Try. Promise is what most real futures in the wild are, something that starts unsatisfied and gets satisfied later. Promise uses a state machine, which can be in one of a few different states, Waiting, Interruptible, Transforming, Interrupted, Linked, and Done. New Promises start in Waiting, and all of them are unsatisfied except for Done. You can move from Waiting to Interruptible by calling Promise#setInterruptHandler. If a Promise is constructed with a Future#transform method (like Future#flatMap, or Future#rescue) it’s in Transforming mode. If a promise is interrupted when Interruptible, it fires the interrupt handler, and moves to the Interrupted state. If it’s interrupted when Transforming, it delegates the interrupt to the underlying Future, and moves to the Interrupted state. When a future is constructed with a Future#transform method and the underlying future has been satisfied, but the newly constructed one hasn’t yet, the Promise moves to the Linked state, where it delegates all work to the new constructed one. This allows infinite future recursion without space leaks, which is quite neat. Scala 2.12 futures have since used our trick to do it themselves, so they can also do recursion without space leaks. Promise#updateIfEmpty and its derivatives, like Promise#setValue, will move a Promise from any state to Done, if they are not already satisfied, at which point its closures will be submitted to the scheduler to be run by the calling thread that satisfied the Promise.

Detachable

When more than one execution dependency depends on the result of a future, it can be dangerous to interrupt it, since you might inadvertantly fail other folks’ work. However, if you never interrupt it, you can end up with memory leaks if the future is never satisfied and we continuously add work to it. Detachable Promises are an attempt to fix this, so that we can mark promises as “may not be needed later, though the underlying future they depend on will be useful”, and must be constructed explicitly by saying Promise.attached(underlying) like here.