Async in action

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Multithreading, Parallel and Async

Multithreading

A form of concurrency that uses multiple threads of execution.

Thread, BackgroundWorker, low-level, best to start with high-level abstractions like thread pool.

Parallel

Divide work among multiple threads that run concurrently.

Maximize the use of multiple processors of CPU. In most cases, work against built-in parallelism on server-side.

Async

Use futures or callbacks to avoid unnecessary threads.

Async with callback

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function doTaskAsync(msg, cb) {
	console.log(msg);
	setTimeout(cb, 0);
}

doTaskAsync("a", function() {
	console.log("b");
	doTaskAsnyc("c", function() {
		console.log("d");
	});
});
console.log("e");

// a -> e -> b -> c -> d

Drawbacks:

  1. Not sequential
  2. Inversion of control
    Callback executed too early/late, too few/many times, swallow errors, …

Async with Promise

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function doTaskAsync(msg) {
	console.log(msg);
	return new Promise( function(resolve, reject) {
		setTimeout(resolve, 0);		
	});
}

doTaskAsync("a")
.then( function () {
	console.log("b");
	return doTaskAsync("c");
})
.then( function () {
	console.log("d");
})
.catch( function () {...} );

console.log("e");

// a -> e -> b -> c -> d

Async in ES7 (C# 5.0)

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async function doTasks() {
	try {
		await doTaskAsync("a");
		console.log("b");
		await doTaskAsync("c");
		console.log("d");
	}
	catch (err) {
		...
	}
	console.log("e");
}
doTasks();

// a -> b -> c -> d -> e

Async in Scala

(see Promise and Future)

Async in Java

Java 5

interface java.util.concurrent.Future<T>::get() is a blocking method.

Java 8

interface java.util.concurrent.CompletionStage<T> is Promise

class java.util.concurrent.CompletableFuture<T> implements Future<T>, CompletionStage<T>

That’s all.

Async and Reactive

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Rx.Observable.fromEvent(..., "event a")
	.do(x => console.log("b"))
	.switchMap(x => Rx.Observable.fromEvent(..., "event c"))
	.do(x => { console.log("d"); console.log("e"); });