Parallelism and Concurrency

We will now start exploring one of the main effects that people use Cats Effect for: parallelism and concurrency.

Exercise: Parallelism vs Concurrency

What's the difference between parallelism and concurrency?

Let's be clear on the difference between parallelism and concurrency:

  • Parallelism means multiple things running at the same time, even though we might not be able to see this. For example, SIMD instructions on your CPU do multiple operations at the same time, but you cannot observe the intermediate results and hence cannot observe the parallelism. From the programmer's point of view you call the instruction on the CPU and get back the result.

  • Concurrency means we can observe multiple things happening in overlapping time periods, even though they might not actually be running at the same time (i.e. they might not be parallel.) An example is Javascript. It has a single-threaded runtime but we can still observe race conditions between different callback functions.

Parallelism in Cats Effect

If we have two IO values that don't depend on each other, we should be able to run them in parallel. For example, the following two values could be run in parallel.

val a = IO.println("A")
val b = IO.println("B")

We know that mapN expresses a computation that depends on two independent IO values, but always runs them from left to right.

(a, b).mapN((_, _) => "Done").unsafeRunSync()
// A
// B

The reason for this is type class coherence, which is a fancy of way of saying "if you can implement a type class in different ways, all those different ways should give the same result." To understand this we need to know what a type class is, the two type classes in use here (monad and applicative), and their relationship (monad can implement applicative.) This is out of scope for us here. If you want to know more, Functional Programming Strategies has the details.

To get parallelism we can use parMapN. Try the following, and you should see b occasionally runs before a.

(a, b).parMapN((_, _) => "Done").unsafeRunSync()

Exercise: Parallelism or Concurrency?

Does parMapN display parallelism, concurrency, neither, or both?

It's both. It depends on the effects you run with parMapN, so it's not really a property of parMapN but of both parMapN and the effects that are being run.

If the effects being run only compute values, so we have no way of observing they are doing stuff, then we have just parallelism. However, if we can observe them being run, as we can when we use IO.println, then we have concurrency.

How does parMap work? It's either dark magic or an instance of the Parallel type class. Despite it's name, Parallel is not about running code in parallel. It's just about having a different implementation of mapN and friends with different semantics. For IO these different semantics are, coincidently, about parallelism, but you can call parMapN on Either and get something else.

Exercise: Parallel Either

What does parMapN do on Either? Create some code examples illustrating the difference between the usual mapN.

Note: you will need to import cats.syntax.all.* to make mapN etc. available.

parMapN accumulates errors. Here's an example. Start by defining two failed Eithers.

import cats.syntax.all.*

val failed1: Either[List[String], Int] = Left(List("Oh no! I failed!"))
val failed2: Either[List[String], Int] = Left(List("Oh no! I also failed!"))

Now let's see what happens when we use mapN.

(failed1, failed2).mapN((a, b) => a + b)
// res2: Either[List[String], Int] = Left(value = List("Oh no! I failed!"))

We get only the first failure.

Now we use parMapN.

(failed1, failed2).parMapN((a, b) => a + b)
// res3: Either[List[String], Int] = Left(
//   value = List("Oh no! I failed!", "Oh no! I also failed!")
// )

We get both failures.

Exercise: Parallel Helpers

There are a bunch of par methods. Here are the most useful ones, with simplified type signatures.

  • (F[A], ..., F[Z]).parTupled: F[(A, ..., Z)]
  • (F[A], ..., F[Z]).parMapN((A, ..., Z) => AA): AA
  • G[F[A]].parSequence: F[G[A]]
  • G[A].parTraverse(A => F[B]): F[G[B]]

Complete the challenge in code/src/main/scala/parallelism/01-parallelism.scala to increase your familiarity with parMapN and friends.

  1. This is simply IO(Blackhold.consumeCPU(999999999L))

  2. consume.timed will run the blackhole and return the time it took. You probably want to out the time.

  3. This is mapN versus parMapN.

  4. This is sequence versus parSequence.

  5. You can use traverse and parTraverse to achieve this.

Exercise: Job Manager

Complete code/src/main/scala/parallelism/02-job-manager.scala to use the tools in a somewhat realistic context.