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Add explanation to Async Method Invocation pattern (iluwatar#1680)
* iluwatar#590 make the example more interesting and add an explanation * iluwatar#590 fix checkstyle Co-authored-by: Subhrodip Mohanta <hello@subho.xyz>
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‎async-method-invocation/README.md

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---
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## Intent
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Asynchronous method invocation is pattern where the calling thread
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Asynchronous method invocation is a pattern where the calling thread
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is not blocked while waiting results of tasks. The pattern provides parallel
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processing of multiple independent tasks and retrieving the results via
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callbacks or waiting until everything is done.
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## Explanation
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Real world example
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> Launching space rockets is an exciting business. The mission command gives an order to launch and
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> after some undetermined time, the rocket either launches successfully or fails miserably.
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In plain words
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> Asynchronous method invocation starts task processing and returns immediately before the task is
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> ready. The results of the task processing are returned to the caller later.
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Wikipedia says
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> In multithreaded computer programming, asynchronous method invocation (AMI), also known as
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> asynchronous method calls or the asynchronous pattern is a design pattern in which the call site
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> is not blocked while waiting for the called code to finish. Instead, the calling thread is
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> notified when the reply arrives. Polling for a reply is an undesired option.
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**Programmatic Example**
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In this example, we are launching space rockets and deploying lunar rovers.
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The application demonstrates the async method invocation pattern. The key parts of the pattern are
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`AsyncResult` which is an intermediate container for an asynchronously evaluated value,
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`AsyncCallback` which can be provided to be executed on task completion and `AsyncExecutor` that
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manages the execution of the async tasks.
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```java
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public interface AsyncResult<T> {
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boolean isCompleted();
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T getValue() throws ExecutionException;
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void await() throws InterruptedException;
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}
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```
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```java
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public interface AsyncCallback<T> {
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void onComplete(T value, Optional<Exception> ex);
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}
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```
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```java
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public interface AsyncExecutor {
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<T> AsyncResult<T> startProcess(Callable<T> task);
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<T> AsyncResult<T> startProcess(Callable<T> task, AsyncCallback<T> callback);
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<T> T endProcess(AsyncResult<T> asyncResult) throws ExecutionException, InterruptedException;
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}
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```
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`ThreadAsyncExecutor` is an implementation of `AsyncExecutor`. Some of its key parts are highlighted
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next.
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```java
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public class ThreadAsyncExecutor implements AsyncExecutor {
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@Override
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public <T> AsyncResult<T> startProcess(Callable<T> task) {
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return startProcess(task, null);
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}
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@Override
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public <T> AsyncResult<T> startProcess(Callable<T> task, AsyncCallback<T> callback) {
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var result = new CompletableResult<>(callback);
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new Thread(
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() -> {
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try {
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result.setValue(task.call());
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} catch (Exception ex) {
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result.setException(ex);
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}
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},
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"executor-" + idx.incrementAndGet())
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.start();
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return result;
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}
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@Override
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public <T> T endProcess(AsyncResult<T> asyncResult)
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throws ExecutionException, InterruptedException {
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if (!asyncResult.isCompleted()) {
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asyncResult.await();
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}
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return asyncResult.getValue();
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}
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}
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```
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Then we are ready to launch some rockets to see how everything works together.
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```java
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public static void main(String[] args) throws Exception {
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// construct a new executor that will run async tasks
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var executor = new ThreadAsyncExecutor();
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// start few async tasks with varying processing times, two last with callback handlers
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final var asyncResult1 = executor.startProcess(lazyval(10, 500));
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final var asyncResult2 = executor.startProcess(lazyval("test", 300));
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final var asyncResult3 = executor.startProcess(lazyval(50L, 700));
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final var asyncResult4 = executor.startProcess(lazyval(20, 400), callback("Deploying lunar rover"));
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final var asyncResult5 =
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executor.startProcess(lazyval("callback", 600), callback("Deploying lunar rover"));
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// emulate processing in the current thread while async tasks are running in their own threads
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Thread.sleep(350); // Oh boy, we are working hard here
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log("Mission command is sipping coffee");
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// wait for completion of the tasks
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final var result1 = executor.endProcess(asyncResult1);
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final var result2 = executor.endProcess(asyncResult2);
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final var result3 = executor.endProcess(asyncResult3);
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asyncResult4.await();
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asyncResult5.await();
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// log the results of the tasks, callbacks log immediately when complete
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log("Space rocket <" + result1 + "> launch complete");
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log("Space rocket <" + result2 + "> launch complete");
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log("Space rocket <" + result3 + "> launch complete");
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}
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```
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Here's the program console output.
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```java
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21:47:08.227 [executor-2] INFO com.iluwatar.async.method.invocation.App - Space rocket <test> launched successfully
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21:47:08.269 [main] INFO com.iluwatar.async.method.invocation.App - Mission command is sipping coffee
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21:47:08.318 [executor-4] INFO com.iluwatar.async.method.invocation.App - Space rocket <20> launched successfully
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21:47:08.335 [executor-4] INFO com.iluwatar.async.method.invocation.App - Deploying lunar rover <20>
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21:47:08.414 [executor-1] INFO com.iluwatar.async.method.invocation.App - Space rocket <10> launched successfully
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21:47:08.519 [executor-5] INFO com.iluwatar.async.method.invocation.App - Space rocket <callback> launched successfully
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21:47:08.519 [executor-5] INFO com.iluwatar.async.method.invocation.App - Deploying lunar rover <callback>
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21:47:08.616 [executor-3] INFO com.iluwatar.async.method.invocation.App - Space rocket <50> launched successfully
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21:47:08.617 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <10> launch complete
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21:47:08.617 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <test> launch complete
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21:47:08.618 [main] INFO com.iluwatar.async.method.invocation.App - Space rocket <50> launch complete
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```
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# Class diagram
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![alt text](./etc/async-method-invocation.png "Async Method Invocation")
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## Applicability
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Use async method invocation pattern when
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Use the async method invocation pattern when
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* You have multiple independent tasks that can run in parallel
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* You need to improve the performance of a group of sequential tasks
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* You have limited amount of processing capacity or long running tasks and the
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caller should not wait the tasks to be ready
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* You have a limited amount of processing capacity or long-running tasks and the caller should not wait for the tasks to be ready
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## Real world examples
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‎async-method-invocation/src/main/java/com/iluwatar/async/method/invocation/App.java

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import lombok.extern.slf4j.Slf4j;
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/**
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* This application demonstrates the async method invocation pattern. Key parts of the pattern are
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* <code>AsyncResult</code> which is an intermediate container for an asynchronously evaluated
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* value, <code>AsyncCallback</code> which can be provided to be executed on task completion and
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* <code>AsyncExecutor</code> that manages the execution of the async tasks.
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* In this example, we are launching space rockets and deploying lunar rovers.
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*
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* <p>The application demonstrates the async method invocation pattern. The key parts of the
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* pattern are <code>AsyncResult</code> which is an intermediate container for an asynchronously
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* evaluated value, <code>AsyncCallback</code> which can be provided to be executed on task
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* completion and <code>AsyncExecutor</code> that manages the execution of the async tasks.
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*
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* <p>The main method shows example flow of async invocations. The main thread starts multiple
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* tasks with variable durations and then continues its own work. When the main thread has done it's
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final var asyncResult1 = executor.startProcess(lazyval(10, 500));
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final var asyncResult2 = executor.startProcess(lazyval("test", 300));
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final var asyncResult3 = executor.startProcess(lazyval(50L, 700));
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final var asyncResult4 = executor.startProcess(lazyval(20, 400), callback("Callback result 4"));
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final var asyncResult4 = executor.startProcess(lazyval(20, 400),
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callback("Deploying lunar rover"));
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final var asyncResult5 =
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executor.startProcess(lazyval("callback", 600), callback("Callback result 5"));
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executor.startProcess(lazyval("callback", 600), callback("Deploying lunar rover"));
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// emulate processing in the current thread while async tasks are running in their own threads
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Thread.sleep(350); // Oh boy I'm working hard here
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log("Some hard work done");
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Thread.sleep(350); // Oh boy, we are working hard here
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log("Mission command is sipping coffee");
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// wait for completion of the tasks
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final var result1 = executor.endProcess(asyncResult1);
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asyncResult5.await();
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// log the results of the tasks, callbacks log immediately when complete
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log("Result 1: " + result1);
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log("Result 2: " + result2);
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log("Result 3: " + result3);
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log("Space rocket <" + result1 + "> launch complete");
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log("Space rocket <" + result2 + "> launch complete");
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log("Space rocket <" + result3 + "> launch complete");
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}
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/**
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private static <T> Callable<T> lazyval(T value, long delayMillis) {
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return () -> {
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Thread.sleep(delayMillis);
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log("Task completed with: " + value);
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log("Space rocket <" + value + "> launched successfully");
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return value;
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};
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}
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if (ex.isPresent()) {
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log(name + " failed: " + ex.map(Exception::getMessage).orElse(""));
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} else {
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log(name + ": " + value);
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log(name + " <" + value + ">");
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}
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};
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}

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