This article is for the Scala programmer who has at least used or heard about Futures before. You can also find this over at the Rock the JVM blog or on YouTube in video form or embedded below:
In this article, I’m going to address the problem of "deterministic" Futures in Scala. You probably know by now that Futures are inherently non-deterministic, in the sense that if you create a Future
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val myFuture = Future {
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// you have no future, you are DOOMED!
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42
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// JK.
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}
You know the value inside will be evaluated on "some" thread, at "some" point in time, without your control.
The Scenario
Here, I will speak to the following scenario which comes up often in practice. Imagine you’re designing a function like this:
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def gimmeMyPreciousValue(yourArg: Int): Future[String]
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with the assumption that you’re issuing a request to some multi-threaded service which, is getting called all the time. Let’s also assume that the service looks like this:
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object MyService {
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def produceThePreciousValue(theArg: Int): String = "The meaning of your life is " + (theArg / 42)
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def submitTask[A](actualArg: A)(function: A => Unit): Boolean = {
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// send the function to be evaluated on some thread, at the discretion of the scheduling logic
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true
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}
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}
The service has two API methods:
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A "production" function that is completely deterministic.
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A submission function that has a pretty terrible API because the function argument will be evaluated on one of the service’s threads, and you can’t get the returned value back from another thread’s call stack.
Let’s assume this important service is also impossible to change, for various reasons (API breaks, etc). In other words, the "production" logic is completely fixed and deterministic. However, what’s not deterministic is when the service will actually end up calling the production function. In other words, you can’t implement your function as:
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def gimmeMyPreciousValue(yourArg: Int): Future[String] = Future {
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MyService.produceThePreciousValue(yourArg)
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}
because spawning up the thread responsible for evaluating the production function is not up to you.
The Solution
Introducing Promises — a "controller" and "wrapper" over a Future. Here’s how it works. You create a Promise, get its Future and use it (consume it) with the assumption that it will be filled in later:
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// create an empty promise
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val myPromise = Promise[String]()
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// extract its future
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val myFuture = myPromise.future
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// do your thing with the future, assuming it will be filled with a value at some point
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val furtherProcessing = myFuture.map(_.toUpperCase())
Then pass that promise to someone else, perhaps an asynchronous service:
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val asyncCall(promise: Promise[String]): Unit = {
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promise.success("Your value here, your majesty")
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}
And at the moment, the promise contains a value. Its future will automatically be fulfilled with that value, which will unlock the consumer.
How to Use it
For our service scenario, here’s how we would implement our function:
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def gimmeMyPreciousValue(yourArg: Int): Future[String] = {
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// create promise now
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val thePromise = Promise[String]()
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// submit a task to be evaluated later, at the discretion of the service
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// note: if the service is not on the same JVM, you can pass a tuple with the arg and the promise so the service has access to both
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MyService.submit(yourArg) { x: Int =>
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val preciousValue = MyService.producePreciousValue(x)
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thePromise.success(preciousValue)
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}
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// return the future now, so it can be reused by whoever's consuming it
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thePromise.future
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}
We create a promise and then we return its future at the end for whoever wants to consume it. In the middle, we submit a function that will be evaluated at some point out of our control. At that moment, the service produces the value and fulfills the Promise, which will automatically fulfill the Future for the consumer.
This is how we can leverage the power of Promises to create "controllable" Futures, which we can fulfill at a moment of our choosing. The Promise class also has other methods, such as
failure, trySuccess/ tryFailure and more. I hope this was useful!
