Active Object Pattern

1. Introduction

The Active Object Pattern is a design pattern that decouples method execution from method invocation for objects that each reside in their own thread of control. This pattern is particularly useful in concurrent programming, where the execution of threads can be managed more effectively.

2. Key Concepts

• Decoupling: Separates the execution of a method from the invocation.
• Concurrency: Allows multiple operations to occur simultaneously.
• Asynchronous Messaging: Uses messages to communicate between threads.
• Thread Safety: Ensures that shared data is accessed safely by multiple threads.

3. Structure

The Active Object Pattern typically consists of the following key components:

• Active Object: The object that executes a method asynchronously.
• Method Request: Encapsulates a method invocation and its parameters.
• Scheduler: Manages the queue of method requests and dispatches them to the Active Object.
• Servant: The actual implementation of the Active Object's methods.

4. Implementation

Here is a basic implementation of the Active Object Pattern in Python:

import threading
import queue
import time

class MethodRequest:
    def __init__(self, receiver, method, *args):
        self.receiver = receiver
        self.method = method
        self.args = args

    def execute(self):
        return self.method(self.receiver, *self.args)

class ActiveObject:
    def __init__(self):
        self.request_queue = queue.Queue()
        self.thread = threading.Thread(target=self.run)
        self.thread.start()

    def run(self):
        while True:
            request = self.request_queue.get()
            if request is None:
                break
            request.execute()

    def enqueue(self, method, *args):
        request = MethodRequest(self, method, *args)
        self.request_queue.put(request)

    def shutdown(self):
        self.enqueue(None)
        self.thread.join()

    def example_method(self, value):
        print(f"Processing {value} in thread {threading.current_thread().name}")
        time.sleep(1)

# Example usage
active_obj = ActiveObject()
for i in range(5):
    active_obj.enqueue(active_obj.example_method, i)

active_obj.shutdown()
            

5. Best Practices

When implementing the Active Object Pattern, consider the following:

• Ensure thread safety by controlling access to shared resources.
• Use a robust messaging system to handle method requests.
• Always clean up resources by shutting down threads properly.
• Consider the overhead of context switching and thread management.

6. FAQ