5.4. Leader/Followers

The Leader/Followers pattern is a concurrency design pattern that improves performance and simplifies synchronization in multi-threaded systems. This pattern is particularly useful for event-driven systems that process multiple requests concurrently, such as network servers or real-time applications. The pattern organizes threads into a leader and one or more followers, where the leader processes incoming requests and assigns them to followers for processing.

The Leader/Followers pattern is typically used when:

1. You want to improve performance and reduce synchronization overhead in multi-threaded systems.
2. You need to manage concurrency in event-driven systems that process multiple requests concurrently.
3. You want to simplify the design of a multi-threaded system by organizing threads into a leader and followers.

To implement the Leader/Followers pattern, follow these steps:

1. Identify the tasks that need to be processed concurrently in your system.
2. Create a thread pool with one thread designated as the leader and the rest as followers.
3. Implement a mechanism for the leader thread to listen for incoming requests or events and assign them to follower threads for processing.
4. Implement a mechanism for follower threads to process the assigned tasks and return the results to the leader thread.
5. Implement a mechanism for promoting a follower thread to the leader role when the current leader is busy processing a request.

Here’s a simple example of the Leader/Followers pattern in Java:

import java.util.concurrent.*;

class LeaderFollowers {
    private final ExecutorService executor;
    private final BlockingQueue<Runnable> taskQueue;

    public LeaderFollowers(int numberOfThreads) {
        executor = Executors.newFixedThreadPool(numberOfThreads);
        taskQueue = new LinkedBlockingQueue<>();
    }

    public void start() {
        for (int i = 0; i < numberOfThreads; i++) {
            executor.submit(this::run);
        }
    }

    public void run() {
        while (!Thread.currentThread().isInterrupted()) {
            try {
                Runnable task = taskQueue.take();
                task.run();
            } catch (InterruptedException e) {
                break;
            }
        }
    }

    public void submitTask(Runnable task) {
        taskQueue.add(task);
    }
}

// Client code
public class Client {
    public static void main(String[] args) {
        LeaderFollowers lf = new LeaderFollowers(4);
        lf.start();

        // Submit tasks to be processed by the leader and followers
        lf.submitTask(() -> System.out.println("Task 1"));
        lf.submitTask(() -> System.out.println("Task 2"));
        lf.submitTask(() -> System.out.println("Task 3"));
    }
}

In this example, the LeaderFollowers class represents the leader and followers in the system. The leader listens for incoming tasks and assigns them to follower threads for processing. Follower threads process the assigned tasks and return the results to the leader thread.

Advantages of the Leader/Followers pattern:

1. Improved performance: The pattern reduces synchronization overhead by organizing threads into a leader and followers, resulting in better overall performance.
2. Simplified synchronization: The pattern simplifies synchronization by reducing contention for shared resources among threads.
3. Scalability: The pattern supports the use of thread pools and can be extended to handle a larger number of tasks, improving the overall scalability of the system.

Disadvantages of the Leader/Followers pattern:

1. Increased complexity: The pattern introduces additional components such as thread pools, leader/follower roles, and task assignment mechanisms, which may make the codebase more complex and harder to manage.
2. Potential bottlenecks: The pattern relies on a single leader thread to manage incoming requests, which could become a performance bottleneck if the leader becomes overwhelmed with requests or spends too much time processing a request.

When using the Leader/Followers pattern, carefully consider its advantages and disadvantages. Use the pattern when you want to improve performance and simplify synchronization in multi-threaded systems, particularly event-driven systems that process multiple requests concurrently. Be aware of the potential limitations introduced by the pattern, such as increased complexity and potential bottlenecks, and ensure that it is applied judiciously to maintain a clean and efficient codebase. Provide clear documentation or guidance for developers so they can understand how the pattern is used and how to extend or modify it as needed.

Note: For complete list of design patterns click here