Use case 1: Serial execution of HTTP calls
Let's say you're building a user interface which depends on two REST services. One provide a list of news article items, while the other is an image hosting service which return image binaries from URL's. The first service returns URL's which are to be used as input for the second service. This makes second operation dependent on the first - they have to be executed serially:
Use case 2: Parallel execution, wait for all
This time the user interface is composed of data from two independent services. You want to display a blank screen with a progress bar only while loading. Only when all data is fetched, you start rendering. This is what I dub the form-merge parallel execution:
Use case 3: Parallel execution, take first
In this case you're firing multiple HTTP queries to identical services, but only care for the response from the first to reply. It might be that you're building a performance-critical application, or you have multiple 3rd-party providers delivering the same service but with varying degree of reliability.
Implementation
Below is listed short examples illustrating the three patterns in the different languages, with support functions in the first chapter. For Java I've chosen to use the RxJava framework simply because that's what I know best. If you know how to do the same with plain Java (8), please let me know or contribute with a pull request to the repo!Support functions
// Resolves the input value after 300ms
function getSlowPromise(returnValue) {
return new Promise((resolve) => {
setTimeout(() => {
resolve(returnValue);
}, 300);
});
}// Returns input string after 250ms
public static async Task<string> GetSlowStringTask(String toReturn)
{
await Task.Delay(250);
return toReturn;
}
// Returns input int after 250ms
public static async Task<int> GetSlowIntTask(int toReturn)
{
await Task.Delay(250);
return toReturn;
}suspend fun getSlowStringJob(s: String) : String {
delay(250)
return s
}
suspend fun getSlowIntJob(i: Int) : Int {
delay(250)
return i
}
// Returns the deferred job which completes first
suspend fun <T> awaitFirst(jobs : Array<Deferred<T>>) : Deferred<T> {
var doneJob : Deferred<T>? = null
while (doneJob == null) {
for (job in jobs) {
if (job.isCompleted) {
doneJob = job
break;
}
yield()
}
}
return doneJob
}Pipeline / Serial execution
// createPromise is a method returning a promise which resolves to the input value
getSlowPromise('job1')
.then((result) => {
return getSlowPromise(result + '-job2');
}).then((result) => {
return getSlowPromise(result + '-job3');
}).then((result) => {
// Result from third job
});Observable<String> obs1 = Observable.just("Hello world");
obs1.flatMap(s -> {
Observable<Integer> obs2 = Observable.just(s.length());
return obs2;
}).flatMap(i -> {
Observable<String> obs3 = Observable.just("Length: " + i);
return obs3;
}).subscribe(s ->{
System.out.println(s);
// Will output "Length: 11"
});
var str = await GetSlowStringTask("Hello world");
var len = await GetSlowIntTask(str.Length);
var res = await GetSlowStringTask("Len: " + len);
Console.Out.WriteLine(res);launch(CommonPool) {
val res1 = getSlowStringJob("Hello")
val res2 = getSlowIntJob(res1.length)
val res3 = getSlowStringJob("First string length: " + res2)
System.out.println(res3)
}Parallel fork-join
// Assuming promise1, 2 and 3 are created
// before this code executing
Promise.all([
promise1,
promise2,
promise3
]).then((results) => {
// 'results' is array of results from each promise
})// obs1, 2 and 3 are Observable<String>'s
// Assuming each observable only emits 1 value
obs1.mergeWith(obs2)
.mergeWith(obs3)
.buffer(3)
.subscribe(resultArray -> {
// resultArray is String[3]
});
var tasks = new Task<int>[3];
tasks[0] = GetSlowIntTask(1);
tasks[1] = GetSlowIntTask(2);
tasks[2] = GetSlowIntTask(3);
Task.WaitAll(tasks);
for (int i = 0; i < 3; i++)
{
Console.Out.WriteLine("Res " + i + ": " + tasks[i].Result);
}val job1 = async(CommonPool) { getSlowIntJob(1) }
val job2 = async(CommonPool) { getSlowIntJob(2) }
val job3 = async(CommonPool) { getSlowIntJob(3) }
launch(CommonPool) {
val sum = job1.await() + job2.await() + job3.await()
System.out.println("Sum: " + sum)
}Parallel take first
Promise.race([
promise1,
promise2,
promise3
]).then((result) => {
// result is first to return
})// obs1, 2 and 3 are Observable<String>'s
// Assuming each observable only emits 1 value
obs1.mergeWith(obs2)
.mergeWith(obs3)
.first()
.subscribe(result -> {
// result is String
});
var tasks = new Task<int>[3];
tasks[0] = GetSlowIntTask(1);
tasks[1] = GetSlowIntTask(2);
tasks[2] = GetSlowIntTask(3);
int firstResult = Task.WaitAny(tasks);
Console.Out.WriteLine("Res " + firstResult);val job1 = async(CommonPool) { getSlowIntJob(1) }
val job2 = async(CommonPool) { getSlowIntJob(2) }
val job3 = async(CommonPool) { getSlowIntJob(3) }
launch(CommonPool) {
val jobs = arrayOf(job1, job2, job3)
val first = awaitFirst(jobs)
System.out.println("Finished first: " + first.getCompleted())
}Thanks to Espen Volden for contributing with Javascript examples
Article updated 18 Feb 2017 with Kotlin 1.1 Coroutines examples
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