Finished Explanation for Leader/Followers #2239

leader-followers/README.md

@@ -3,28 +3,103 @@ title: Leader/Followers
 category: Concurrency
 language: en
 tag:
- - Performance
+- 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.
+The Leader/Followers design pattern is a pattern used to coordinate a selection of 'workers'. It allows tasks to execute concurrently
+with the Leader delegating tasks to the Follower threads for execution. It is a very common design pattern used in multithreaded
+situations such as servers, and works to help prevent ambiguity around delegation of tasks.
 
+## Explanation
+Real-world Example
+> The best real world example of Leader/Followers is a web server. Web servers have to be able to handle a multitude of incoming
+> connections all at once. In a web server, the Leader/Followers pattern works by using the Leader to listen to incoming requests
+> and accept connections. Once a connection is made to a client the Leader can then find a Follower thread to delegate the task
+> to for execution and return to the client. This means that the Leader does not have to wait to finish execution before it can
+> accept another incoming connection, and can focus on delegating tasks. This pattern is created to aid in concurrency of applicaitons,
+> allowing for many connections to work simultaneously.
+
+In plain words
+> You can picture the Leader as a traffic controller that has to direct traffic from one lane into 25 lanes. As a car comes in,
+> the Leader sends it down a road that isn't full or busy. This car can then have its request filled, or reach its destination.
+> If the Leader had to drive each car down the lane itself, the line would pile up and progress would be slow. But as the Leader
+> has Followers that can also drive the cars, the Leader can focus on making the line move quickly and ensuring traffic doesn't
+> back up.
+
+Wikipedia says
+> A concurrency pattern are those types of design patterns that deal with the multi-threaded programming paradigm.
+
+## Programmatic Example
+This example shows Java code that sets up a Leader that listens for client requests on port 8080. Once a request is sent,
+the leader will accept it and delegate it to a new Follower to execute. This means that the Leader can keep delegating,
+and ensure requests are fulfilled timely. This is only pseudocode and the working code would require a more concrete implementation
+of Leader and Followers.
+```java
+public class LeaderFollowerWebServer {
+
+    public static void main(String[] args) throws IOException {
+        int port = 8080; // the port that clients can reach the leader on
+        int numFollowers = 5; // the amount of followers we can delegate tasks to
+
+        ServerSocket serverSocket = new ServerSocket(port); // pseudocode for creating a socket to the server
+        ExecutorService executorService = Executors.newFixedThreadPool(numFollowers); // pseudocode to start execution for Followers
+
+        System.out.println("Web server started. Listening on port " + port);
+
+        while (true) {
+            Socket clientSocket = serverSocket.accept();
+            // Accept a new connection and assign it to a follower thread for processing
+            executorService.execute(new Follower(clientSocket));
+        }
+    }
+}
+
+class Follower implements Runnable {
+    private final Socket clientSocket;
+
+    public Follower(Socket clientSocket) {
+        this.clientSocket = clientSocket;
+    }
+
+    @Override
+    public void run() {
+        try {
+            // handle the client request, e.g., read and write data
+            // this is where you would implement your request processing logic.
+            // we will just close the socket
+            clientSocket.close();
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+    }
+}
+```
 ## 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.
+* You want to establish a concurrent application
+* You want faster response times on heavy load
+* You want an easily scalable program
+* You want to load balance a program
+
+## Consequences
+Consequences involved with using the Leader/Followers pattern
+
+* Implementing this pattern will increase complexity of the code
+* If the leader is too slow at delegating processes, some Followers may not get to execute tasks leading to a waste of resources
+* There is overhead with organising and maintaining a thread pool
+* Debugging is more complex
 
 ## 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)
+* ACE Thread Pool Reactor framework
+* JAWS
+* Real-time CORBA
 
 ## Credits
 
-* [Douglas C. Schmidt and Carlos O’Ryan - Leader/Followers](http://www.kircher-schwanninger.de/michael/publications/lf.pdf)
+* Douglas C. Schmidt and Carlos O’Ryan - Leader/Followers