Event Loop Architecture & async/await Coroutines

1Concept

`asyncio` achieves concurrency on a single thread using cooperative multitasking managed by an Event Loop. Coroutines defined with `async def` yield control at `await` expressions, allowing other coroutines to execute while waiting for non-blocking I/O.

2Architecture Diagram

Event Loop Running on 1 Thread:
[ Coroutine A await socket ] ---> Suspended ---> [ Coroutine B executes ] ---> Socket ready ---> Resumes A

3Code Example

import asyncio

async def fetch_data(service: str, delay: float) -> str:
    print(f"Starting fetch from {service}...")
    await asyncio.sleep(delay) # Non-blocking cooperative sleep
    return f"{service} Payload"

async def main():
    result = await fetch_data("AuthService", 0.05)
    print(f"Received: {result}")

asyncio.run(main())

4Expected Output

Starting fetch from AuthService...
Received: AuthService Payload

5Key Takeaways

• ✓Never use blocking calls (like `time.sleep()` or `requests.get()`) inside coroutines; it blocks the entire event loop.
• ✓Coroutines do NOT run when called (`f()`); they must be awaited or scheduled as a Task.
• ✓`asyncio.run(main())` manages creating and tearing down the event loop.