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Graph/async-blocked-by-sync-io

AntiPattern

Async Work Blocked By Sync IO

async-blocked-by-sync-io

Async FastAPI lifecycles and endpoints get broken when synchronous IO and non-awaited background tasks are used—blocking the event loop or running after request context teardown—so state commits, consumers, and shutdown persistence happen at the wrong time.

Node Details

Public graph metadata

Updated7/26/2026

Connections

30 linked nodes

MATCHESSolution

Load the async user data with a Suspense-compatible async pattern (e.g., using `leo-query`’s `query`/`subscribe`) so the component waits until `currentUser` is available before rendering.

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Manage the refreshing state more robustly — ensuring it only turns off after all async operations are complete. Tension: remove the arbitrary timeout. Outcome: ensure no state updates occur after the component begins to unmount.

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myEndpoint must return a Promise — Please note, that above method is synchronous just to check if that makes difference, but it is not. Tension: my template is keep displaying "Loading..".

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delete the async from `fake_execute` — your `AsyncContextMock` needs to handle both use cases in your endpoint `await conn.execute()` and `async with conn.execute()`. Outcome: just make it a regular function so it returns your context manager directly, no coroutine wrapper.

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spawn it via `asyncio.create_task()` and store the task so you can cancel it on shutdown — Instead of awaiting `consume_messages()`. Outcome: store the task so you can cancel it on shutdown.

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single worker running async loop. Outcome: So you have a single worker running async loop and you block that loop.

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If you convert all operations to perform asynchronously (using `aiosqlite`) , the issue will be mitigated. — When a route in FastAPI is listed as asynchronous (`async def`). Tension: that doesn't fully solve the inherent single writer problem of Sqlite. Outcome: Here's a fully operational update of the FastAPI server logic:.

MATCHESSolution

Solution 1 — within an `async def` endpoint.

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use httpx or aiohttp e.g: — async def test_api(request: Request):. Tension: requests(which is synchronous) inside async function. Outcome: async with client.stream("POST", "http://127.0.0.1:8000/test_api",json={} ) as response: async for chunk in response.aiter_bytes(chunk_size=1234): yield chunk.

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python interpreter just pauses one task and looks at other — In your scenerio where you use asynchronous. Tension: it is not completly usesless. Outcome: Think it like a round-robin.

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This issue on github offers a solution github. — since fastapi and fastHTML are focusing on diffrent logics. Tension: there are diffrent tools included. Outcome: but since fastapi and fastHTML are focusing on diffrent logics.

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You can use a global dependency: — FastAPI. Tension: there's no way to set auth tokens in the docs. Outcome: app.openapi = custom_openapi.

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Move the persistence/cleanup logic into code that runs at the end of the background task (or otherwise extend lifecycle explicitly for the background task), rather than relying on the request-scoped `asynccontextmanager` teardown for those updates.

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Change the engine async fixture from session scope to function scope (remove scope="session") so all async fixtures/tests run on the same event loop, preventing the cross-loop Future/Task error.

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Create a lifespan-managed shared state (e.g., instantiate a state manager/model in FastAPI lifespan) and store the loaded model in that state so request handlers can access it via Request/app state or a dependency that reads from lifespan.

MATCHESSolution

Using `asyncio.sleep()` in a loop is perfectly fine. Tension: every time you `await` the sleep, you effectively yield control back to the event loop. Outcome: calling `asyncio.sleep(0)` does not sleep at all, but only yields control back to the event loop.

MATCHESSolution

use `pytest_asyncio` and the async fixture — If you don't want to utilize an `AsyncClient` like the one by `httpx`. Tension: ensuring that the FastAPI app's lifespan context correctly works.

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ASGIMiddleware. Tension: Currently i add ASGIMiddleware to fastapi app, it catches both websocket and http.

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I have solved it using the lifespan of FastAPI. — use it inside the routes. Tension: I couldn't identify where the problem is. Outcome: I have just initialized the Pubsub client in LifeSpan.

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any time you have an I/O bound process with lots of idle time — waiting for user input, waiting for a TLS handshake. Tension: wrapping it in an async task can let other code run.

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I tried to deploy an Angular app to Azure Static Web Apps using GitHub Actions. Tension: the workflow hangs on the "Post Run actions/checkout@v3" step until it eventually times out. Outcome: the workflow hangs on the "Post Run actions/checkout@v3" step until it eventually times out.

MATCHESSolution

Ensure the S3 download runs in a way that doesn’t block the event loop (i.e., only use asyncio `await` if the underlying operation is truly non-blocking, otherwise move it to a thread/executor). Use `asyncio.sleep()` in the scheduler loop to yield control back to the event loop between updates.

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An async function was passed to useActionState.

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Use myAsyncAction.fulfilled as the listener actionCreator and prepend listenerMiddleware.middleware in configureStore

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Avoid running multiple DB operations concurrently on the same session by removing asyncio.gather and iterating sequentially, and/or use an async_scoped_session so each task uses its own session.

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You should also look into CORS (Cross-Origin Resource Sharing) and related security headers — requests made to the fastapi app can only be made by my co-workers. Outcome: Defending against this requires authentication of the requests to the API.

MATCHESSolution

Use asynchronous sending with callbacks. — If you're using KafkaTemplate.send() without handling the ListenableFuture returned. Tension: you're effectively blocking the calling thread until the message is acknowledged by the Kafka broker. Outcome: The addCallback method allows you to execute code when the send operation succeeds or fails, without blocking the main thread.

MATCHESSolution

Rewrite the test as an async test and run it with an async test runner (e.g., pytest.mark.asyncio), using await on the async ORM methods (acreate/acount) instead of asyncio.run().

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the goroutine blocks forever on the channel send because nobody is reading — When the parent context is cancelled, the goroutine blocks forever on the channel send because nobody is reading. Tension: if the result is ready at the exact moment the context cancels, which case fires?

MATCHESSolution

Use Component.Task (TaskSource) for all async work inside components — inside components. Tension: it auto-cancels when the component is destroyed.

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