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Asynchronous Tasks

A Task[T] represent a chunk of work that yields a value of type T on completion.

Tasks allow fine control over what threads run the task and with what level of concurrency. They also allow the separation of the definition of work (when tasks are created and chained together) from its execution (when tasks are run and any side-effects occur). In this way, they play a similar role to the IO wrappers of Haskell, Cats-Effect or Scalaz.

The Monix Task Library

We will use Monix 3.x Tasks.

We choose Monix over Cats Effect IO because Monix offers a better default Execution Model for Tasks and more options than Cats-Effect does, and is relatively similar in other aspects.

We choose Monix tasks over Scala's default Futures because of Monix's Execution Model has more control and much more performant defaults than Scala Futures.

Tasks

🔍 Study Code

  • In the classic version, method scanReport returned String. What is its return type now? What about method pathScan?

📝 Write Code

  • The top level main still returns Unit. It builds a program, a chain of Tasks, but they'll have no effect until they're run. Use runSyncUnsafe(1.minute) to run the tasks. The word Unsafe in its name indicates that it will cause side-effects. The idea with Tasks is to run them at only one at the top of your top program.

  • There is a compile error in pathScan. The map method no longer has the correct type, because we want to traverse a list of subdirectories, converting each into a Task yielding their scan, and then roll the traversal into a single overall Task. Replace map with traverse, which is "like map, but for mapping to effectful values"

  • One more will appear problem. Tasks can be run asynchronously, so they need an implicit Scheduler available to dispatch them to. Add implicit val s = Scheduler(ExecutionModel.BatchedExecution(32)) at the top of Scanner object. The BatchedExecution(32) means that a chain of up to 32 Tasks will be run batched together in one thread before any context switch.

▶️ Run Code

Run the tests to verify your task based implementation still gives the correct output.

🔍 Study Code

  • Examine the tests for the Task version. Notice that the introduction of Task, which makes this version purely functional hasn't made the tests any easier to write. Still lots of messy files and directories created and cleaned-up. Why is this and what could you do about it?