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Graph/event-loop-blocking-and-union-misrouting

Pattern

Async Blocking and Misrouting

event-loop-blocking-and-union-misrouting

Event-loop blocking and request handling mismatches occur when async endpoints perform blocking work (e.g., S3 downloads) or when FastAPI model validation selects the wrong overlapping Union branch, causing logs/admin views to hang or show incomplete/incorrect data.

Node Details

Public graph metadata

Updated6/28/2026

Connections

30 linked nodes

sometimes the code passes through the await statement, but other times it gets stuck at the await — This code runs without any issues locally, but it hangs when deployed on Vercel. Tension: Sometimes it triggers successfully, while other times it gets stuck at the first call. Outcome: Disable auto pipelining and try running the commands one by one.

requests with the forbidden fields sailed through to the write path. Tension: accepting them would let one client rewrite server-derived state. Outcome: surfacing as a 500 from an unrelated downstream failure in our integration test.

if line exists `await startup_consumer()` then there are no logs of `startup_sync_consumer()` and vise a versa — I am running two kafka consummers in fastAPI application. Tension: Looks like each consumer blocks event loop. Outcome: How to fix it?

<sys>:0: RuntimeWarning: coroutine 'Connection._cancel' was never awaited — Pytest in FastAPI + Postgres. Outcome: To fix this one method is to dispose the engine after every test route.

facing error that `event loop is closed` — I have been working on yet another API using FastAPI and trying to write test cases for the APIs. Tension: using `asyncpg` driver/library to connect to the main db and for testcases using `aiosqlite` driver/library. Outcome: re-run the tests and report back on if it mitigates the issue.

after some time I get a `503 Service Unavailable` error — When I make a POST request to this endpoint, it starts processing. Tension: especially when using ngrok. Outcome: What is the best way to handle long-running POST requests in FastAPI?

What I try to achieve is to have FastAPI listening to one port and a admin panel into another one, that displays in realti — FastAPI listening to one port and a admin panel into another one. Outcome: I moved the log into string generation upon `Log` class and `getLog` Actually receives the log as a string.

TypeError in FastAPI when using APIRoute with a Router — trying to implement an `APIRoute` class to handle logic before and after the request (logging). Outcome: Removing `async` before the function definition solved it.

some events are asynchronous and not come from a frontend request.

the two requests are executed "in parallel." — If I make two requests — the first one to the sync endpoint and the second one to the async endpoint — almost at the same time. Tension: I expected the event loop to be blocked. Outcome: the two requests are executed "in parallel.".

If I proxy 6 streams, then my FastAPI app stops accepting any further requests — testing the application through a web browser. Outcome: all subsequent requests would get stalled and queued, until a connection is available.

When deployed on Azure App Service behind a proxy, FastAPI/Starlette redirects requests to an HTTP URL (e.g., HTTP Location header in a 307) instead of preserving HTTPS. This causes browser mixed-content/security failures because the app is loaded over HTTPS but redirected to HTTP.

Waiting for background tasks to complete. — Whenever I gracefully shut down my FastAPI app by saving a file (with the `--reload` option). Tension: "Shutting down sessions" is never logged because it is waiting for all background tasks to be completed, which will never happen. Outcome: How am I supposed to close my sessions so that FastAPI can properly shut down?

Stripe payment flow between a FastAPI backend (creating PaymentIntents) and a Vue frontend (confirming using the PaymentIntent ID) is causing issues during the payment process on the frontend.

the reason for the various threads appearing in the logs — when you pass a synchronous generator to `EventSourceResponse`. Tension: you seem to be returning from the `/` endpoint. Outcome: `EventSourceResponse` will execute the `def` function in a separate thread from an external threadpool and then `await` it.

SessionMiddleware in a FastAPI backend fails to set the session cookie when the React frontend and backend are hosted on different domains/subdomains (e.g., XXX-ui.fly.dev vs XXX-api.fly.dev). The browser rejects the cookie with an 'invalid domain' error and no session is maintained.

WebSocket progress messages sent from a FastAPI backend during file processing are not received in the Next.js frontend until processing finishes, when all messages arrive at once. Backend logs show progress updates are generated successfully.

FastAPI cannot choose the correct model for a Union request body when one variant has an enum field

When deployed behind a Kubernetes/API-gateway URL prefix (e.g., https://api-gateway.example.com/myapp), FastAPI tried to load OpenAPI from a path missing the prefix (e.g., /api/v1/openapi.json), causing a 404 and failing to load API definitions.

Middleware registered on the main FastAPI app is also triggered for requests routed to a mounted sub-application, including subapi docs/openapi endpoints. This prevents isolating middleware behavior per sub-application.

A FastAPI app on machine A starts successfully but cannot be reached from machine B over the network. Connections fail despite the server running without errors.

Adding `py_eureka_client` to a FastAPI app during module import causes `RuntimeError: Cannot run the event loop while another loop is running` when running with `uvicorn --reload`.

Defining FastAPI routes in a loop causes each endpoint to behave like the last model

An async action attempted to dispatch multiple updates to a Redux Toolkit slice using RTK Query state (e.g., loading/error/data), but the dispatch/flow did not properly work. The goal was to store RTK Query results into the studentSlice state.

Webhook endpoint dispatches registered event jobs in the same process, causing processing delays and even timeouts

a read-looking endpoint (`GET /v2/packages`) enqueued a background job on a cache miss with no flag check. Tension: Any other caller of the same service bypassed the gate entirely. Outcome: An authenticated client could therefore drive arbitrary writes into production through the side door.

After calling the async register thunk, the user needs to be redirected only when registration succeeds; otherwise errors should be shown without navigating.

The middleware fetches an Azure token using async logic, but RTK-Query proceeds without waiting, so requests are sent before the token is available. As a result, the token is not present at the correct time for data fetching.

createListenerMiddleware does not fire when the async thunk finishes and extraReducers run

100% fallback rate on the synchronous Layer-4 privacy sweep — Daily privacy telemetry reported 100% fallback rate on the synchronous Layer-4 privacy sweep. Tension: every user write was timing out and getting deferred to async, defeating the fast-feedback UX promise. Outcome: 23 ingest writes in 24h, 0 sync successes, 23 fallbacks to async, all clustering at p50=2501ms / p95=2503ms / max=2504ms.

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