Fix issue iluwatar#179: Leader Followers Pattern (iluwatar#1189)

* add leader followers pattern

* use var and streams instead in App::execute

* use logger instead of printing to system output stream

Author: npczwh

leader-followers/README.md

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+---
+layout: pattern
+title: Leader/Followers
+folder: leader-followers
+permalink: /patterns/leader-followers/
+categories: Concurrency
+tags:
+ - Performance
+---
+
+## Intent
+The Leader/Followers pattern provides a concurrency model where multiple 
+threads can efficiently de-multiplex events and dispatch event handlers 
+that process I/O handles shared by the threads.
+
+## Class diagram
+![Leader/Followers class diagram](./etc/leader-followers.png)
+
+## Applicability
+Use Leader-Followers pattern when
+
+* multiple threads take turns sharing a set of event sources in order to detect, de-multiplex, dispatch and process service requests that occur on the event sources.
+
+## Real world examples
+
+* [ACE Thread Pool Reactor framework](https://www.dre.vanderbilt.edu/~schmidt/PDF/HPL.pdf)
+* [JAWS](http://www.dre.vanderbilt.edu/~schmidt/PDF/PDCP.pdf)
+* [Real-time CORBA](http://www.dre.vanderbilt.edu/~schmidt/PDF/RTS.pdf)
+
+## Credits
+
+* [Douglas C. Schmidt and Carlos O’Ryan - Leader/Followers](http://www.kircher-schwanninger.de/michael/publications/lf.pdf)

leader-followers/etc/leader-followers.png

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+@startuml
+package com.iluwatar.leaderfollowers {
+  class App {
+    + App()
+    - addTasks(taskSet : TaskSet) {static}
+    - execute(workCenter : WorkCenter, taskSet : TaskSet) {static}
+    + main(args : String[]) {static}
+  }
+  class Task {
+    - finished : boolean
+    - time : int
+    + Task(time : int)
+    + getTime() : int
+    + isFinished() : boolean
+    + setFinished()
+  }
+  class TaskHandler {
+    + TaskHandler()
+    + handleTask(task : Task)
+  }
+  class TaskSet {
+    - queue : BlockingQueue<Task>
+    + TaskSet()
+    + addTask(task : Task)
+    + getSize() : int
+    + getTask() : Task
+  }
+  class WorkCenter {
+    - leader : Worker
+    - workers : List<Worker>
+    + WorkCenter()
+    + addWorker(worker : Worker)
+    + createWorkers(numberOfWorkers : int, taskSet : TaskSet, taskHandler : TaskHandler)
+    + getLeader() : Worker
+    + getWorkers() : List<Worker>
+    + promoteLeader()
+    + removeWorker(worker : Worker)
+  }
+  class Worker {
+    - id : long
+    - taskHandler : TaskHandler
+    - taskSet : TaskSet
+    - workCenter : WorkCenter
+    + Worker(id : long, workCenter : WorkCenter, taskSet : TaskSet, taskHandler : TaskHandler)
+    + equals(o : Object) : boolean
+    + hashCode() : int
+    + run()
+  }
+}
+Worker -->  "-taskSet" TaskSet
+Worker -->  "-taskHandler" TaskHandler
+TaskSet -->  "-queue" Task
+Worker -->  "-workCenter" WorkCenter
+@enduml

leader-followers/etc/leader-followers.urm.puml

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+<?xml version="1.0"?>
+<!--
+
+    The MIT License
+    Copyright © 2014-2019 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>java-design-patterns</artifactId>
+        <version>1.23.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>leader-followers</artifactId>
+    <dependencies>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-core</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+</project>

leader-followers/pom.xml

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+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import java.util.Random;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Leader/Followers pattern is a concurrency pattern. This pattern behaves like a taxi stand where
+ * one of the threads acts as leader thread which listens for event from event sources,
+ * de-multiplexes, dispatches and handles the event. It promotes the follower to be the new leader.
+ * When processing completes the thread joins the followers queue, if there are no followers then it
+ * becomes the leader and cycle repeats again.
+ *
+ * <p>In this example, one of the workers becomes Leader and listens on the {@link TaskSet} for
+ * work. {@link TaskSet} basically acts as the source of input events for the {@link Worker}, who
+ * are spawned and controlled by the {@link WorkCenter} . When {@link Task} arrives then the leader
+ * takes the work and calls the {@link TaskHandler}. It also calls the {@link WorkCenter} to
+ * promotes one of the followers to be the new leader, who can then process the next work and so
+ * on.
+ *
+ * <p>The pros for this pattern are:
+ * It enhances CPU cache affinity and eliminates unbound allocation and data buffer sharing between
+ * threads by reading the request into buffer space allocated on the stack of the leader or by using
+ * the Thread-Specific Storage pattern [22] to allocate memory. It minimizes locking overhead by not
+ * exchanging data between threads, thereby reducing thread synchronization. In bound handle/thread
+ * associations, the leader thread dispatches the event based on the I/O handle. It can minimize
+ * priority inversion because no extra queuing is introduced in the server. It does not require a
+ * context switch to handle each event, reducing the event dispatching latency. Note that promoting
+ * a follower thread to fulfill the leader role requires a context switch. Programming simplicity:
+ * The Leader/Followers pattern simplifies the programming of concurrency models where multiple
+ * threads can receive requests, process responses, and de-multiplex connections using a shared
+ * handle set.
+ */
+public class App {
+
+  /**
+   * The main method for the leader followers pattern.
+   */
+  public static void main(String[] args) throws InterruptedException {
+    var taskSet = new TaskSet();
+    var taskHandler = new TaskHandler();
+    var workCenter = new WorkCenter();
+    workCenter.createWorkers(4, taskSet, taskHandler);
+    execute(workCenter, taskSet);
+  }
+
+  /**
+   * Start the work, dispatch tasks and stop the thread pool at last.
+   */
+  private static void execute(WorkCenter workCenter, TaskSet taskSet) throws InterruptedException {
+    var workers = workCenter.getWorkers();
+    var exec = Executors.newFixedThreadPool(workers.size());
+    workers.forEach(exec::submit);
+    Thread.sleep(1000);
+    addTasks(taskSet);
+    exec.awaitTermination(2, TimeUnit.SECONDS);
+    exec.shutdownNow();
+  }
+
+  /**
+   * Add tasks.
+   */
+  private static void addTasks(TaskSet taskSet) throws InterruptedException {
+    var rand = new Random();
+    for (var i = 0; i < 5; i++) {
+      var time = Math.abs(rand.nextInt(1000));
+      taskSet.addTask(new Task(time));
+    }
+  }
+}

leader-followers/src/main/java/com.iluwatar.leaderfollowers/App.java

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+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+/**
+ * A unit of work to be processed by the Workers.
+ */
+public class Task {
+
+  private final int time;
+
+  private boolean finished;
+
+  public Task(int time) {
+    this.time = time;
+  }
+
+  public int getTime() {
+    return time;
+  }
+
+  public void setFinished() {
+    this.finished = true;
+  }
+
+  public boolean isFinished() {
+    return this.finished;
+  }
+
+}

leader-followers/src/main/java/com.iluwatar.leaderfollowers/Task.java

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+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The TaskHandler is used by the {@link Worker} to process the newly arrived task.
+ */
+public class TaskHandler {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(TaskHandler.class);
+
+  /**
+   * This interface handles one task at a time.
+   */
+  public void handleTask(Task task) throws InterruptedException {
+    var time = task.getTime();
+    Thread.sleep(time);
+    LOGGER.info("It takes " + time + " milliseconds to finish the task");
+    task.setFinished();
+  }
+
+}

leader-followers/src/main/java/com.iluwatar.leaderfollowers/TaskHandler.java

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+/*
+ * The MIT License
+ * Copyright © 2014-2019 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+package com.iluwatar.leaderfollowers;
+
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.BlockingQueue;
+
+/**
+ * A TaskSet is a collection of the tasks, the leader receives task from here.
+ */
+public class TaskSet {
+
+  private BlockingQueue<Task> queue = new ArrayBlockingQueue<>(100);
+
+  public void addTask(Task task) throws InterruptedException {
+    queue.put(task);
+  }
+
+  public Task getTask() throws InterruptedException {
+    return queue.take();
+  }
+
+  public int getSize() {
+    return queue.size();
+  }
+}