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Graph/async-boundaries-misdefined

AntiPattern

Async Boundaries Misdefined

async-boundaries-misdefined

Async boundary mistakes—declaring handlers or route wrappers as coroutines when the framework expects plain callables, or performing synchronous I/O inside async endpoints—cause streaming/data processing to break or block the event loop.

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Public graph metadata

Updated7/10/2026

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30 linked nodes

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Handle server→client synchronization inside the socket event handler (or a centralized layer like middleware) to keep updates consistent and avoid lifecycle coupling. Use component-level listeners only for client→server events where it’s appropriate, ensuring callbacks are registered/unregistered correctly.

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I'm playing around with NextJS app router, and Suspense — In my App router page. Tension: this suspense boundary won't do anything.

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

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Pass `BackgroundTasks` from the route into your dependency. — when declared inside an async dependency. Tension: the background job never runs. Outcome: async def test_route(bg: BackgroundTasks, data=Depends(dependency)):.

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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:.

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Solution 1 — within an `async def` endpoint.

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Removing `async` before the function definition solved it. — class LoggingAPIRoute(APIRoute):. Outcome: def get_route_handler(self) -> Callable:.

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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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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.

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Ensure the server yields to the asyncio event loop while processing (e.g., add an `await asyncio.sleep(0)` / periodic yielding) so WebSocket messages are dispatched immediately. Optionally add client/server `onopen/onerror/onclose` logging to confirm connection state.

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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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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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bound it with a "progress ceiling" — a process emitting bytes but making NO semantic progress. Tension: a process emitting bytes but making NO semantic progress (livelock) would be shielded forever. Outcome: re-arms recovery after a long no-progress window even while bytes flow.

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An async function was passed to useActionState.

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Or you can use `async/await` and `try/catch` — createAsyncThunk. Outcome: return data.

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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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Async-iterate the Result instead of awaiting `.records` — The driver's Result is async-iterable. Tension: Combined with a streaming HTTP response, records flow end-to-end without any single buffer. Outcome: records flow end-to-end without any single buffer.

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depending on your latency budget. Outcome: Same idea, but the state lives outside the process boundary.

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Stage 2 (startup). Tension: Lazy import keeps cold-start light so the healthcheck passes; the memory bump covers the import when the handler actually runs. Outcome: functions-framework can bind the port.

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case "CallbackRouteError": return { error: "Invalid credentials!" } — in my `login.ts` server action in the switch case clause. Tension: to return the correct errors. Outcome: bypass this error.

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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.

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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?

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With buffered channels, is there a risk of the goroutine sending one more item after cancellation? — With buffered channels. Tension: the goroutine might send one more item after cancellation.

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Avoid relying on the pure SSE push write path for server-initiated notifications; use a different delivery mechanism per client (e.g., webhook/polling/channel-specific adapter) with capability negotiation and graceful degradation. Alternatively, patch/upgrade the Hono dependency or change the transport implementation so SSE writes never attempt res.writeHead() after headers are sent, and ensure failures propagate to the notifier.

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Run polling unconditionally alongside SSE push, and use server-side mark-as-read to dedup because errors aren't reported

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Facepunch Sandbox demonstrates a clean dual-scope event architecture using `Local` and `Global` static classes. Tension: Difficulty implementing cancellable events.

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Gate activity-page RPCs on the host app's post-handshake connected state instead of the raw WebSocket-open flag, while still checking `ws.readyState === 1` — activity-page RPCs. Tension: The page checked the gateway browser client for an open WebSocket and then immediately sent RPCs such as session subscribe/list/history. Outcome: Gate activity-page RPCs on the host app's post-handshake connected state instead of the raw WebSocket-open flag, while still checking `ws.readyState === 1`.

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