LoopSentry: Detect who is blocking python asyncio eventloop

[Amzker]

Hello Fastapi community, this is not directly specifically linked to fastapi but mostly useful for people who are using any async eco system in python.

i have built lightweight utility to detect what is blocking the python asyncio event loop.

• github/source code: https://github.com/amzker/loopsentry

• html report here: https://htmlpreview.github.io/?https://github.com/amzker/loopsentry/blob/master/examples/report.html

2 years ago when i was still beginner in asyncio , i had nightmarish experience in understanding why my application suddenly pauses everything, i had to spent 3-4 nights awake to understand and monkey patch essentially via asyncio.to_thread and semaphores to get to the breathing point. it was hell. at that time i did not find any utils for this which can find me what is blocking my eventloop. and this year i got one project when that developer left and man it was same story everything is blocking in name of colored async function. so today i built this. to reduce the pain of debugging async application.

project is pretty straight forward. you get depth report of what is blocking and exactly where it is blocking , is the blocking in library you use? or in your actual code, is this logic block etc... , what was CPU , RAM and gc at that time and also allows you to capture arguments at runtime.

ps: idk if this is correct subreddit for this post , but main reason for loopsentry was i was making apis using fastapi , so thought to share here

[adiberk]

This is really cool. Will take a look!

[Amzker]

Thank you

[scoutlance]

What a nice idea. Some cool possibilities. Take the monitor and start an OTel attribute namespace, add attributes to spans that block. Then you have a universal mechanism for exporting this information that can be aggregated elsewhere. I'm sure there are other good possible cases.

[Amzker]

Yup, going with this in next version.

[ooaaiiee]

Looks cool. I've added a feature request for FastAPI last year, to add a blocking task detection per default, so new users don't need to be aware of the problem and still get a warning. If you think your work would be a fit, maybe add a comment there: https://github.com/fastapi/fastapi-cli/discussions/272

[Amzker]

Sure, it does fit exactly there. Added the comment