Catch event-loop blocking in FastAPI and see which requests were in flight.
When something blocks your event loop (via time.sleep(), blocking I/O, or CPU work), LoopGuard detects it and narrows it down to the requests that were in flight when the loop stalled. The sentinel measures loop lag, so it cannot name the single guilty handler — it reports every request that was active during the stall.
Install
pip install fastapi-loopguard
Quick Start
from fastapi import FastAPI
from fastapi_loopguard import LoopGuardMiddleware
app = FastAPI()
app.add_middleware(LoopGuardMiddleware)
Enforcement Modes
| Mode | Behavior | Use Case |
|---|---|---|
"warn" |
Console warnings + headers | Default |
"strict" |
HTTP 503 + error page | Development / CI |
"log" |
Silent logging | Production |
from fastapi_loopguard import LoopGuardConfig
# Development: diagnostic headers on every response
config = LoopGuardConfig(dev_mode=True)
# Development / CI: fail loudly with an educational 503
config = LoopGuardConfig(enforcement_mode="strict")
# Production: silent logging
config = LoopGuardConfig(enforcement_mode="log")
app.add_middleware(LoopGuardMiddleware, config=config)
What You Get
Strict Mode
Returns an educational 503 page that explains what went wrong and how to fix it:
Warn Mode
Adds diagnostic headers to every response for debugging:
Log Mode
Writes structured logs listing the requests that were in flight:
1 maintainer
Release files for fastapi-loopguard 0.5.0
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Total release size: 404.2 kB
Release files / fastapi_loopguard-0.5.0.tar.gz
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