How to Implement At-Least-Once Delivery with Acknowledgment in Python

import time
import uuid
from collections import deque


class MockMessageBroker:
    def __init__(self):
        self.queue = deque()
        self.acked = set()

    def publish(self, payload: str) -> str:
        msg_id = str(uuid.uuid4())
        self.queue.append((msg_id, payload))
        return msg_id

    def poll(self):
        if self.queue:
            return self.queue.popleft()
        return None

    def acknowledge(self, msg_id: str):
        self.acked.add(msg_id)


def process_messages(broker, max_attempts=3, retry_delay=0.1):
    delivered = []
    while True:
        msg = broker.poll()
        if msg is None:
            break
        msg_id, payload = msg
        attempt = 1
        while attempt <= max_attempts:
            try:
                # Simulate processing; raise on first attempt for demo
                if attempt == 1 and payload == "order:42":
                    raise RuntimeError("mock failure")
                delivered.append((msg_id, payload, attempt))
                broker.acknowledge(msg_id)
                print(f"ACK {msg_id[:8]} for payload '{payload}' on attempt {attempt}")
                break
            except RuntimeError:
                print(f"Retry {msg_id[:8]} attempt {attempt} failed")
                attempt += 1
                time.sleep(retry_delay)
        else:
            print(f"Gave up on {msg_id[:8]} after {max_attempts} attempts")
    return delivered


if __name__ == "__main__":
    broker = MockMessageBroker()
    broker.publish("order:1")
    broker.publish("order:42")
    broker.publish("order:3")

    result = process_messages(broker)
    print(f"Delivered {len(result)} messages, acked ids: {len(broker.acked)}")