Leader follower

leader-follower/etc/LeaderFollower.ucls

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+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.1.8" icons="true" automaticImage="PNG" always-add-relationships="false" generalizations="true" 
+  realizations="true" associations="true" dependencies="false" nesting-relationships="true">  
+  <class id="1" language="java" name="com.iluwatar.leaderfollower.App" project="leader-follower" 
+    file="/leader-follower/src/main/java/com/iluwatar/leaderfollower/App.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="364" y="21"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="2" language="java" name="com.iluwatar.leaderfollower.WorkStation" project="leader-follower" 
+    file="/leader-follower/src/main/java/com/iluwatar/leaderfollower/WorkStation.java" binary="false" 
+    corner="BOTTOM_RIGHT">    
+    <position height="223" width="185" x="40" y="-11"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="3" language="java" name="com.iluwatar.leaderfollower.Work" project="leader-follower" 
+    file="/leader-follower/src/main/java/com/iluwatar/leaderfollower/Work.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="97" width="122" x="313" y="139"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="4" language="java" name="com.iluwatar.leaderfollower.Worker" project="leader-follower" 
+    file="/leader-follower/src/main/java/com/iluwatar/leaderfollower/Worker.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="749" y="97"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <association id="5">    
+    <end type="SOURCE" refId="2" navigable="false">      
+      <attribute id="6" name="workers"/>      
+      <multiplicity id="7" minimum="0" maximum="2147483647"/>    
+    </end>    
+    <end type="TARGET" refId="4" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <association id="8">    
+    <end type="SOURCE" refId="4" navigable="false">      
+      <attribute id="9" name="queue"/>      
+      <multiplicity id="10" minimum="0" maximum="2147483647"/>    
+    </end>    
+    <end type="TARGET" refId="3" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <association id="11">    
+    <end type="SOURCE" refId="4" navigable="false">      
+      <attribute id="12" name="workstation"/>      
+      <multiplicity id="13" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="2" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <association id="14">    
+    <end type="SOURCE" refId="2" navigable="false">      
+      <attribute id="15" name="leader"/>      
+      <multiplicity id="16" minimum="0" maximum="1"/>    
+    </end>    
+    <end type="TARGET" refId="4" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <association id="17">    
+    <end type="SOURCE" refId="4" navigable="false">      
+      <attribute id="18" name="workers"/>      
+      <multiplicity id="19" minimum="0" maximum="2147483647"/>    
+    </end>    
+    <end type="TARGET" refId="4" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+    sort-features="false" accessors="true" visibility="true">    
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
+    <operations public="true" package="true" protected="true" private="true" static="true"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>

leader-follower/index.md

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+---
+layout: pattern
+title: Leader Follower
+folder: leader-follower
+permalink: /patterns/leader-follower/
+categories: Concurrency
+tags: Java
+---
+
+**Intent:** Leader Follower is a concurrency pattern where multiple threads can efficiently demultiplex
+ events and dispatch to event handlers.
+![alt text](./etc/LeaderFollower.png "Leader Follower")
+
+**Applicability:** Use the Leader Follower pattern when
+
+* multiple threads can receive events , process reponses and demutiplex connection using a shared HandleSet. 

leader-follower/pom.xml

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+<?xml version="1.0"?>
+<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>1.7.0</version>
+  </parent>
+  <artifactId>leader-follower</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

leader-follower/src/main/java/com/iluwatar/leaderfollower/App.java

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+package com.iluwatar.leaderfollower;
+
+import java.io.IOException;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * 
+ * Leader Follower is a concurrency pattern where multiple threads can efficiently demultiplex
+ * events and dispatch to event handlers.
+ * <p>
+ * In this example we use ThreadPool which basically acts as the ThreadPool. One of the Workers
+ * becomes Leader and listens on the {@link HandleSet} for work. HandleSet basically acts as the
+ * source of input events for the Workers, who are spawned and controlled by the {@link WorkStation}
+ * . When Work arrives which implements the {@link Handle} interface then the leader takes the work
+ * and calls the {@link ConcreteEventHandler}. However it also selects one of the waiting Workers as
+ * leader, who can then process the next work and so on.
+ *
+ */
+public class App {
+
+  public static void main(String[] args)
+      throws IOException, ClassNotFoundException, InterruptedException {
+    ExecutorService exec = Executors.newFixedThreadPool(4);
+    WorkStation station = new WorkStation(exec);
+    station.startWork();
+    exec.awaitTermination(5, TimeUnit.SECONDS);
+  }
+}

leader-follower/src/main/java/com/iluwatar/leaderfollower/ConcreteEventHandler.java

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+package com.iluwatar.leaderfollower;
+
+/**
+ * The ConcreteEventHandler. This is used by the {@link Worker} to process the newly arrived work.
+ * @author amit
+ *
+ */
+public class ConcreteEventHandler implements EventHandler{
+
+  @Override
+  public void handleEvent(Handle handle) {
+    System.out.println("Doing the work");
+    System.out.println("Travelled the distance " + handle.getPayLoad());
+  }
+
+}

leader-follower/src/main/java/com/iluwatar/leaderfollower/EventHandler.java

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+package com.iluwatar.leaderfollower;
+/**
+ * The EventHandler interface. All concrete EventHanlder implementations can process a unit of work
+ * represented by Handle
+ * @author amit
+ *
+ */
+public interface EventHandler {
+  public void handleEvent(Handle work);
+}

leader-follower/src/main/java/com/iluwatar/leaderfollower/Handle.java

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+package com.iluwatar.leaderfollower;
+
+/**
+ * Represents a Unit of Work which workers can process. In this example only the {@link Work}
+ * class implements this
+ * @author amit
+ *
+ */
+public interface Handle {
+  public int getPayLoad();
+}

leader-follower/src/main/java/com/iluwatar/leaderfollower/HandleSet.java

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+package com.iluwatar.leaderfollower;
+
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.BlockingQueue;
+
+/**
+ * The {@link HandleSet} class. All work arrive here, workers receive work from here.
+ * @author amit
+ *
+ */
+public class HandleSet {
+
+  private BlockingQueue<Work> queue = new ArrayBlockingQueue<>(100);
+
+  public void fireEvent(Work input) throws InterruptedException {
+    queue.put(input);
+  }
+
+  public Work getPayLoad() throws InterruptedException {
+    return queue.take();
+  }
+
+
+}

leader-follower/src/main/java/com/iluwatar/leaderfollower/Work.java

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+package com.iluwatar.leaderfollower;
+
+/**
+ * A unit of work to be processed by the Workers. Implements Handle. 
+ * @author amit
+ *
+ */
+public class Work implements Handle {
+  public final int distance;
+
+  public Work(int distance) {
+    this.distance = distance;
+  }
+
+  @Override
+  public int getPayLoad() {
+    return distance;
+  }
+
+
+}

leader-follower/src/main/java/com/iluwatar/leaderfollower/WorkStation.java

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+package com.iluwatar.leaderfollower;
+
+import java.util.List;
+import java.util.Random;
+import java.util.concurrent.ArrayBlockingQueue;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+public class WorkStation {
+  private Worker leader;
+  private List<Worker> workers = new CopyOnWriteArrayList<>();
+  private ExecutorService executorService = Executors.newFixedThreadPool(4);
+
+  public WorkStation(ExecutorService executorService) {
+    this.executorService = executorService;
+  }
+
+  public void startWork() throws InterruptedException {
+
+    HandleSet handleSet = new HandleSet();
+    ConcreteEventHandler concreteEventHandler = new ConcreteEventHandler();
+    Worker worker = new Worker(handleSet, workers, 1, this, concreteEventHandler);
+    Worker worker2 = new Worker(handleSet, workers, 2, this ,concreteEventHandler);
+    Worker worker3 = new Worker(handleSet, workers, 3, this,concreteEventHandler);
+    Worker worker4 = new Worker(handleSet, workers, 4, this,concreteEventHandler);
+    workers.add(worker);
+    workers.add(worker2);
+    workers.add(worker3);
+    workers.add(worker4);
+    this.leader = workers.get(0);
+    executorService.submit(worker);
+    executorService.submit(worker2);
+    executorService.submit(worker3);
+    executorService.submit(worker4);
+    Random rand = new Random(1000);
+    handleSet.fireEvent(new Work(Math.abs(rand.nextInt())));
+    handleSet.fireEvent(new Work(Math.abs(rand.nextInt())));
+    handleSet.fireEvent(new Work(Math.abs(rand.nextInt())));
+    handleSet.fireEvent(new Work(Math.abs(rand.nextInt())));
+    handleSet.fireEvent(new Work(Math.abs(rand.nextInt())));
+    // queue.
+    Thread.sleep(1000);
+  }
+
+  public Worker getLeader() {
+    return this.leader;
+  }
+
+  public void setLeader(Worker leader) {
+    this.leader = leader;
+  }
+
+  public void addWorker(Worker worker) {
+    if (this.workers.size() <= 0) {
+      this.leader = worker;
+      workers.add(worker);
+    }
+  }
+
+  public List<Worker> getWorkers() {
+    return workers;
+  }
+}

leader-follower/src/main/java/com/iluwatar/leaderfollower/Worker.java

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+package com.iluwatar.leaderfollower;
+
+import java.util.List;
+
+public  class Worker implements Runnable {
+
+  private Object leader = new Object();
+  private HandleSet handleSet;
+  private List<Worker> workers;
+  private long id;
+  private WorkStation workstation;
+  private ConcreteEventHandler concreteEventHandler;
+
+  public Worker(HandleSet queue, List<Worker> workers, long id,
+      WorkStation workstation, ConcreteEventHandler concreteEventHandler) {
+    super();
+    this.handleSet = queue;
+    this.workers = workers;
+    this.id = id;
+    this.workstation = workstation;
+    this.concreteEventHandler = concreteEventHandler;
+  }
+
+  public void becomeLeader() {
+    synchronized (leader) {
+      leader.notifyAll();
+    }
+  }
+
+
+
+  @Override
+  public void run() {
+    while (!Thread.interrupted()) {
+      try {
+        if (workstation.getLeader() != null && !workstation.getLeader().equals(this)) {
+          // System.out.println("ID " +id + " is follower");
+          synchronized (leader) {
+            leader.wait();
+          }
+
+        }
+      //  
+        workers.remove(this);
+        System.out.println("Leader: " +id);
+        Work work = handleSet.getPayLoad();
+        if (workers.size() > 0) {
+          workstation.getWorkers().get(0).becomeLeader();
+          workstation.setLeader(workstation.getWorkers().get(0));
+        }
+        else {
+          workstation.setLeader(null);
+        }
+        concreteEventHandler.handleEvent(work);
+        Thread.sleep(100);
+        System.out.println("The Worker with the ID " + id + " completed the task");
+        workstation.addWorker(this);
+      } catch (InterruptedException e) {
+        System.out.println("Thread intreuppted");
+      }
+    }
+  }
+
+  @Override
+  public int hashCode() {
+    final int prime = 31;
+    int result = 1;
+    result = prime * result + (int) (id ^ (id >>> 32));
+    return result;
+  }
+
+  @Override
+  public boolean equals(Object obj) {
+    if (this == obj)
+      return true;
+    if (obj == null)
+      return false;
+    if (getClass() != obj.getClass())
+      return false;
+    Worker other = (Worker) obj;
+    if (id != other.id)
+      return false;
+    return true;
+  }
+
+}