--- title: Our FastAPI async def Route Froze Every Other Request... description: A production incident where a FastAPI async def route froze all other requests for up to 90 seconds was caused by two synchronous blocking calls inside an... canonical: https://daily.dev/posts/our-fastapi-async-def-route-froze-every-other-request-for-90-seconds-rt72tpj7m twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Our FastAPI async def Route Froze Every Other Request for 90 Seconds | daily.dev og:description: A production incident where a FastAPI async def route froze all other requests for up to 90 seconds was caused by two synchronous blocking calls inside an... og:url: https://daily.dev/posts/our-fastapi-async-def-route-froze-every-other-request-for-90-seconds-rt72tpj7m og:image: https://api.daily.dev/og/posts/Rt72TpJ7m.png og:image:alt: Our FastAPI async def Route Froze Every Other Request for 90 Seconds og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Our FastAPI async def Route Froze Every Other Request for 90 Seconds **[Medium](https://daily.dev/sources/medium_js)** · 8 min read · 0 upvotes · 0 comments ## Summary A production incident where a FastAPI async def route froze all other requests for up to 90 seconds was caused by two synchronous blocking calls inside an async handler: weasyprint's render_pdf (CPU-bound, ~420ms) and requests.post (blocking I/O, up to 1.4s). Because asyncio is cooperative, these calls held the event loop hostage, causing health check timeouts, pod cycling, and cascading failures across workers. The fix was wrapping CPU-bound work in run_in_threadpool and replacing requests with httpx.AsyncClient. The team also audited the codebase with ripgrep, found four more offending routes, added flake8-async linting to block PRs, and implemented an event loop lag alert at 250ms drift. Key lesson: async def routes get no automatic threadpool safety net — any blocking call is a full-worker outage waiting for enough traffic to trigger it. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why did my FastAPI async def route freeze all other requests on the same worker? A synchronous, blocking call inside an async def handler (such as requests.post or a CPU-bound function like weasyprint's render_pdf) has no await point, so it holds the single event loop hostage for its full duration. During that window no other coroutine on that worker can run, including health checks, which can cause the load balancer to mark the pod unhealthy and reroute traffic, cascading load onto remaining workers. _Developers hardening FastAPI services against event-loop stalls can find fixes like this one on daily.dev._ ### How do I fix a blocking call like requests.post inside an async def FastAPI route? Either wrap the blocking call with starlette.concurrency.run_in_threadpool so it runs off the event loop's thread, or replace the blocking client with an async one like httpx.AsyncClient and await it directly. Awaiting a non-coroutine such as requests.post() raises a TypeError, so adding 'await' alone does not work; the call itself must move to a thread or be swapped for an async-native library. _Teams migrating blocking HTTP clients to async equivalents can track patterns like this via daily.dev._ ### How can I detect blocking calls hiding inside async def routes before they cause an outage? Use a lint rule such as flake8-async to flag known-synchronous calls (requests.*, time.sleep, blocking file opens) inside any async def function, and add a periodic heartbeat task that measures asyncio event-loop lag, alerting when scheduled callback delay exceeds a threshold like 250ms. A load test that fires 50+ concurrent requests at the same route, using locust or asyncio.gather, also exposes this bug class in seconds. _Engineers building guardrails against event-loop stalls can surface tooling like this through daily.dev._ ## Similar posts on daily.dev - [Invisible Failures: The Python Server Mistake We Made in Production](https://daily.dev/posts/invisible-failures-the-python-server-mistake-we-made-in-production-ujme9h4jg) · ITNEXT · 1 upvotes · 0 comments - [Why Adding More AI Agents Made Our System Slower](https://daily.dev/posts/why-adding-more-ai-agents-made-our-system-slower-vbauklndp) · Towards Data Science · 1 upvotes · 1 comments - [Python SQLAlchemy AsyncSession Shared Across Tasks Quietly Exhausted Our Pool](https://daily.dev/posts/python-sqlalchemy-asyncsession-shared-across-tasks-quietly-exhausted-our-pool-2z5sq7oiy) · Medium · 2 upvotes · 0 comments - [Engineering at Babylist](https://daily.dev/posts/engineering-at-babylist-eu2yhxdsr) · Ruby Flow · 0 upvotes · 0 comments - [How to Scale Long-Running API Requests](https://daily.dev/posts/how-to-scale-long-running-api-requests-aj4mhj66o) · Milan Jovanović · 57 upvotes · 3 comments --- Tags: [#python](https://daily.dev/tags/python), [#fastapi](https://daily.dev/tags/fastapi) [View this post on daily.dev](https://daily.dev/posts/our-fastapi-async-def-route-froze-every-other-request-for-90-seconds-rt72tpj7m) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Our FastAPI async def Route Froze Every Other Request for 90 Seconds","url":"https://daily.dev/posts/our-fastapi-async-def-route-froze-every-other-request-for-90-seconds-rt72tpj7m","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/our-fastapi-async-def-route-froze-every-other-request-for-90-seconds-rt72tpj7m"},"datePublished":"2026-07-23T14:59:28.691Z","dateModified":"2026-09-14T06:52:07.531Z","description":"A production incident where a FastAPI async def route froze all other requests for up to 90 seconds was caused by two synchronous blocking calls inside an...","image":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/7ec080e172dbe466bc1098ed92ab1461?_a=AQAEuop","thumbnailUrl":"https://media.daily.dev/image/upload/f_auto,q_auto/v1/posts/7ec080e172dbe466bc1098ed92ab1461?_a=AQAEuop","isAccessibleForFree":true,"articleSection":"Medium","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Medium","logo":"https://media.daily.dev/image/upload/t_logo,f_auto/v1/logos/medium","url":"https://daily.dev/sources/medium_js"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/our-fastapi-async-def-route-froze-every-other-request-for-90-seconds-rt72tpj7m","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"python,fastapi","timeRequired":"PT8M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Medium","item":"https://daily.dev/sources/medium_js"},{"@type":"ListItem","position":3,"name":"Our FastAPI async def Route Froze Every Other Request for 90 Seconds"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/our-fastapi-async-def-route-froze-every-other-request-for-90-seconds-rt72tpj7m#faq","mainEntity":[{"@type":"Question","name":"Why did my FastAPI async def route freeze all other requests on the same worker?","acceptedAnswer":{"@type":"Answer","text":"A synchronous, blocking call inside an async def handler (such as requests.post or a CPU-bound function like weasyprint's render_pdf) has no await point, so it holds the single event loop hostage for its full duration. 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