Week 09: Asynchronous Computations and Futures

Useful resources:

• Documentation on Scala Futures and Promises

• Scala’s for-comprehensions

• Futures and Async: Slides and Video

• Code with examples, package futures

Summary of the Lecture

A short summary of the files from the accompanying code is given below, outlining the characteristic aspects of working with Java Futures, as well as Scala Futures and Promises. It is recommended that the files are explored in the order they appear in the following list:

Java Futures

• SortAndRead — reading a user input after having performed a costly operation (array sorting). This examples demonstrates the need for parallelisation.

• SortAndRead2 — a better version of the previous program. Sorting is moved into a separate future. Introducing Java’s thread pools and the class Future.

• SortAndRead3 — even better implementation, in which the user input is also moved into a separate future.

An Exercise on Java Futures

• FutureSort — an old friend, parallel quick-sort, now re-implemented with Java futures. An interesting question is why some of the offered thread pools, e.g., Executors.newFixedThreadPool(4) make it deadlock?

Scala Futures

• FutureDataType — a first example of using Scala’s futures, its isCompleted method and the implicit execution context (ExecutionContext.Implicits.global).

• FuturesCallbacks — demonstration of processing results of the futures asynchronously via callbacks (using foreach method).

• FutureExceptions — treatment of exceptions in futures, via onComplete method and pattern matching on the result class: Success or Failure.

• AwaitExample — an example showing the use of Await.result method to wait for a future to complete or fail otherwise.

Composing Scala Futures

• Options — a quick practice on building combinators for the Option type. Serves as an introduction to similar future combinators introduced next.

• ComposingFutures — an example of composing two futures running asynchronously, via the explicit flatMap combinator (or using the for notation). Check: what changes in the execution if the futures are inlined under the for comprehension?

• BlacklistedFiles — a large example involving composing futures in the implementation of getAllBlacklisted. Multiple ways to query the result are possible.

• ScalaFuturesTests — shows how to test futures.

Scala Promises

• ProducerConsumer — implementation of a producer-consumer with a promise, which serves as a single-assignment mailbox.

• ProducerConsumer2 — demonstration of isCompleted method of promises.

• ProducerConsumer3 — failing the promise and recovering after an exception in the future using the recover method.