Update README.md

Author: iluwatar

priority-queue/README.md

@@ -10,19 +10,203 @@ tags:
 ---
 
 ## Intent
-Prioritize requests sent to services so that requests with a higher priority are received and processed more quickly than those of a lower priority. This pattern is useful in applications that offer different service level guarantees to individual clients.
+
+Prioritize requests sent to services so that requests with a higher priority are received and 
+processed more quickly than those of a lower priority. This pattern is useful in applications that 
+offer different service level guarantees to individual clients.
 
 ## Explanation
-Applications may delegate specific tasks to other services; for example, to perform background processing or to integrate with other applications or services. In the cloud, a message queue is typically used to delegate tasks to background processing. In many cases the order in which requests are received by a service is not important. However, in some cases it may be necessary to prioritize specific requests. These requests should be processed earlier than others of a lower priority that may have been sent previously by the application.
+
+Applications may delegate specific tasks to other services; for example, to perform background 
+processing or to integrate with other applications or services. In the cloud, a message queue is 
+typically used to delegate tasks to background processing. In many cases the order in which requests 
+are received by a service is not important. However, in some cases it may be necessary to prioritize 
+specific requests. These requests should be processed earlier than others of a lower priority that 
+may have been sent previously by the application.
+
+Real world example
+
+> Imagine a video processing service with free and premium customers. The requests coming from the
+> paying premium customers should be prioritized over the others. 
+
+In plain words
+
+> Priority Queue enables processing of high priority messages first, regardless of queue size or
+> message age. 
+
+Wikipedia says
+
+> In computer science, a priority queue is an abstract data type similar to regular queue or stack 
+> data structure in which each element additionally has a "priority" associated with it. In a 
+> priority queue, an element with high priority is served before an element with low priority. 
+
+**Programmatic Example**
+
+Looking at the video processing example from above, let's first see the `Message` structure.
+
+```java
+public class Message implements Comparable<Message> {
+
+  private final String message;
+  private final int priority; // define message priority in queue
+
+  public Message(String message, int priority) {
+    this.message = message;
+    this.priority = priority;
+  }
+
+  @Override
+  public int compareTo(Message o) {
+    return priority - o.priority;
+  }
+  ...
+}
+```
+
+Here's `PriorityMessageQueue` that handles storing the messages and serving them in priority
+order.
+
+```java
+public class PriorityMessageQueue<T extends Comparable> {
+
+  ...
+
+  public T remove() {
+    if (isEmpty()) {
+      return null;
+    }
+
+    final var root = queue[0];
+    queue[0] = queue[size - 1];
+    size--;
+    maxHeapifyDown();
+    return root;
+  }
+
+  public void add(T t) {
+    ensureCapacity();
+    queue[size] = t;
+    size++;
+    maxHeapifyUp();
+  }
+
+  ...
+}
+```
+
+`QueueManager` has a `PriorityMessageQueue` and makes it easy to `publishMessage` and
+`receiveMessage`.
+
+```java
+public class QueueManager {
+
+  private final PriorityMessageQueue<Message> messagePriorityMessageQueue;
+
+  public QueueManager(int initialCapacity) {
+    messagePriorityMessageQueue = new PriorityMessageQueue<>(new Message[initialCapacity]);
+  }
+
+  public void publishMessage(Message message) {
+    messagePriorityMessageQueue.add(message);
+  }
+
+  public Message receiveMessage() {
+    if (messagePriorityMessageQueue.isEmpty()) {
+      return null;
+    }
+    return messagePriorityMessageQueue.remove();
+  }
+}
+```
+
+`Worker` constantly polls `QueueManager` for highest priority message and processes it.
+
+```java
+public class Worker {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(Worker.class);
+
+  private final QueueManager queueManager;
+
+  public Worker(QueueManager queueManager) {
+    this.queueManager = queueManager;
+  }
+
+  public void run() throws Exception {
+    while (true) {
+      var message = queueManager.receiveMessage();
+      if (message == null) {
+        LOGGER.info("No Message ... waiting");
+        Thread.sleep(200);
+      } else {
+        processMessage(message);
+      }
+    }
+  }
+
+  private void processMessage(Message message) {
+    LOGGER.info(message.toString());
+  }
+}
+```
+
+Here's the full example how we create an instance of `QueueManager` and process messages using
+`Worker`.
+
+```java
+    var queueManager = new QueueManager(100);
+
+    for (var i = 0; i < 100; i++) {
+      queueManager.publishMessage(new Message("Low Message Priority", 0));
+    }
+
+    for (var i = 0; i < 100; i++) {
+      queueManager.publishMessage(new Message("High Message Priority", 1));
+    }
+
+    var worker = new Worker(queueManager);
+    worker.run();
+```
+
+Program output:
+
+```
+Message{message='High Message Priority', priority=1}
+Message{message='High Message Priority', priority=1}
+Message{message='High Message Priority', priority=1}
+Message{message='High Message Priority', priority=1}
+Message{message='High Message Priority', priority=1}
+Message{message='High Message Priority', priority=1}
+Message{message='High Message Priority', priority=1}
+Message{message='High Message Priority', priority=1}
+Message{message='High Message Priority', priority=1}
+Message{message='High Message Priority', priority=1}
+Message{message='Low Message Priority', priority=0}
+Message{message='Low Message Priority', priority=0}
+Message{message='Low Message Priority', priority=0}
+Message{message='Low Message Priority', priority=0}
+Message{message='Low Message Priority', priority=0}
+Message{message='Low Message Priority', priority=0}
+Message{message='Low Message Priority', priority=0}
+Message{message='Low Message Priority', priority=0}
+Message{message='Low Message Priority', priority=0}
+Message{message='Low Message Priority', priority=0}
+No Message ... waiting
+No Message ... waiting
+No Message ... waiting
+```
+
 
 ## Class diagram
+
 ![alt text](./etc/priority-queue.urm.png "Priority Queue pattern class diagram")
 
 ## Applicability
-Use the Priority Queue pattern when
+
+Use the Priority Queue pattern when:
 
 * The system must handle multiple tasks that might have different priorities.
-* Different users or tenants should be served with different priority..
+* Different users or tenants should be served with different priority.
 
 ## Credits
 

priority-queue/src/main/java/com/iluwatar/priority/queue/Application.java

@@ -37,20 +37,19 @@ public class Application {
    */
   public static void main(String[] args) throws Exception {
 
-    var queueManager = new QueueManager(100);
+    var queueManager = new QueueManager(10);
 
     // push some message to queue
     // Low Priority message
-    for (var i = 0; i < 100; i++) {
+    for (var i = 0; i < 10; i++) {
       queueManager.publishMessage(new Message("Low Message Priority", 0));
     }
 
     // High Priority message
-    for (var i = 0; i < 100; i++) {
+    for (var i = 0; i < 10; i++) {
       queueManager.publishMessage(new Message("High Message Priority", 1));
     }
 
-
     // run worker
     var worker = new Worker(queueManager);
     worker.run();

priority-queue/src/main/java/com/iluwatar/priority/queue/QueueManager.java

@@ -45,7 +45,7 @@ public void publishMessage(Message message) {
 
 
   /**
-   * recive message from queue.
+   * Receive message from queue.
    */
   public Message receiveMessage() {
     if (messagePriorityMessageQueue.isEmpty()) {