Unable to view logs on a function app using FastAPI synchronous routes

Question

python 3.9
azure-functions 1.11.2
fastapi 0.82.0
Function app runtime: 4

I wrote a very simple function with a single FastAPI route to test how logging works. The code I wrote is as follows:

import azure.functions as func  
from fastapi import FastAPI, APIRouter  
import logging  
  
FUNC_PREFIX = "/api"  
app = FastAPI(  
    docs_url=f"{FUNC_PREFIX}/docs",  
    openapi_url=f"{FUNC_PREFIX}/openapi.json",  
)  
  
azrouter = APIRouter()  
  
@azrouter.get(  
    "/",  
    description="Test log",  
    response_model=str,  
)  
def get_bonuses():  
    logging.info("Test log")  
    return "Test logging"  
  
  
def main(req: func.HttpRequest, context: func.Context) -> func.HttpResponse:  
    return func.AsgiMiddleware(app).handle(req, context)  
  
  
app.include_router(azrouter, prefix=FUNC_PREFIX)  

The API works fine, but unfortunately the log doesn't work properly. Locally nothing is printed on the console and the same thing happens on the function monitor.

If I change the FastAPI route from synchronous to asynchronous as follow everything works fine. I can see both the log locally and on the function monitor

@azrouter.get(  
    "/",  
    description="Test log",  
    response_model=str,  
)  
async def get_bonuses():  
    logging.info("Test log")  
    return "Test logging"  

Why does this behavior occur? Is it possible to have synchronous routes with working log? If so how?

Answer 1

@a.salogni Thank you for reaching out to Microsoft Q&A. I found issue #11 with fast-api samples related to logging on synchronous operations and from the related main issue #1158, our product team has identified the cause and the fix is being worked upon. I would recommend tracking the main issue for the update. Sorry for the inconvenience caused by this issue.

I hope this answers your question and feel free to add a comment if you have any other questions. Please ‘Upvote’ if it helped so that it can help others in the community.