Do FastAPI and aiohttp share the same global event loop?

The collaborative working mechanism of Python asynchronous library

Asynchronous programming in Python has become a tool for handling highly concurrency and I/O-intensive tasks. However, when using multiple asynchronous libraries at the same time, ensuring compatibility between them is critical. This article will explore the collaboration mechanisms of two commonly used libraries, FastAPI and aiohttp.

For example, using aiohttp to initiate a network request in the FastAPI asynchronous view function involves two different asynchronous libraries. Do they each have independent event loops? The answer is no.

In Python's asynchronous programming model, there is only one global event loop. Asynchronous libraries such as FastAPI, aiohttp are all registered in this global event loop.

When FastAPI starts the application, it initializes this global event loop. All asynchronous tasks, including the asynchronous view functions of FastAPI, run in this event loop. When aiohttp is used in the view function to initiate a request, aiohttp also uses the same global event loop to manage its asynchronous operations.

Therefore, FastAPI and aiohttp share the same global event loop, ensuring that asynchronous tasks are executed in an orderly and efficient manner, avoiding event loop conflicts and performance issues.

The following code example verifies this:

 import asyncio
from fastapi import FastAPI
import aiohttp

app = FastAPI()

@app.get("/")
async def root():
    loop = asyncio.get_running_loop()
    print(f"FastAPI event loop: {loop}")

    async with aiohttp.ClientSession() as session:
        async with session.get("https://example.com") as resp:
            print(f"aiohttp event loop: {asyncio.get_running_loop()}")
            return await resp.text()

Run this code and you will find that FastAPI and aiohttp use the same event loop instance, confirming that they share the global event loop. Understanding this mechanism is crucial for writing efficient asynchronous code because it ensures coordination and consistency of asynchronous operations between different libraries.

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