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Graph/async-event-loop-blocking

Pattern

Async Event-Loop Blocking

async-event-loop-blocking

S3 downloads and scheduler loops run synchronously or without yielding, so the event loop can’t process FastAPI/CLI requests promptly; background tasks appear stalled and state updates lag or miss deadlines.

Node Details

Public graph metadata

Updated7/1/2026

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.

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?

background task blocks eventloop — reload = True or workers > 1. Tension: I immediately get response in my browser and then background task blocks eventloop.

the background job never runs — my `BackgroundTasks` only execute when declared inside an async dependency. Tension: No exception is raised, the route returns normally. Outcome: If I move the `bg.add_task()` call into the route itself, it works as expected.

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.

I am using FastAPI to make a `GET` request, which runs a `.jar` file that returns some text data. Tension: The Java program takes about `30` seconds to complete and I have attempted to run it asynchronously, but have been unable to.

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?

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

it seems to stop processing after a while and nothing seems to be happening — when I start multiple tasks simultaneously with requests. Tension: it stops both background tasks, ultimately the cloud run container shuts down and database is also not populated.

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.

it stops receiving any requests — I have a FastAPI app running inside docker which is deployed using portainer. Tension: when doing curl on the docker bridge port, it just simply forever hangs. Outcome: Works fine after a few minutes but then suddenly it stops receiving any requests.

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?

Pytest async tests for a FastAPI app fail with RuntimeError: “Task got Future attached to a different loop” when interacting with the database via SQLAlchemy.

How to serve a machine-learning model via FastAPI using dependencies when the model must be held in a stateful class instance that is updated on a schedule, rather than recreated per request.

The idea is to update this model once hourly without blocking the API's ability to respond. Tension: update this model once hourly without blocking the API's ability to respond to CPU-bound tasks. Outcome: this is all good.

Task exception was never retrieved — I used asyncio.create_task and `loop.run_in_excutor` in order to properly handle the subprocess code. Tension: I am getting the following error :-. Outcome: AttributeError("'function' object has no attribute 'submit'").

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.

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.

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

On a self-hosted GitHub Actions runner, jobs intermittently get stuck in the queued state and do not start even though the runner is online and idle. This happens randomly on various workflow steps/jobs.

the workflow hangs on the "Post Run actions/checkout@v3" step until it eventually times out — deploying an Angular app to Azure Static Web Apps using GitHub Actions. Tension: the build process completes without errors, but the workflow hangs. Outcome: Azure couldn't do; hence it got hang & the process ran indefinitely.

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

I see no way to initiate this event directly from the dashboard. — when using a hosted endpoint with ngrok or any other tunneling tool. Tension: I have to wait until the end of the month for the event to trigger. Outcome: my manager won't appreciate me waiting for that long.

I could start a process (using Python multiprocessing) - and kill it to cancel the download — potentially big files from S3. Tension: that would probably leave large chunks already in memory but not saved to the file "lost".

Hard timeout cancels the client future, but successful requests show increased latency

Go goroutine blocks forever on a channel send after parent context cancellation

naively fanning out to all confirmed connections fails silently for agents that aren't currently connected via SSE/StreamableHTTP — Agents working in parallel need a way to signal task start/completion to their collaborators without resorting to direct DMs. Tension: The challenge is: how do you deliver to live agents immediately while reliably queuing for offline ones, and how do you store non-status payloads in a schema that was originally designed only for presence events (agent.online/offline)?

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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