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8 minFastAPI Async Patterns: When to Use Background Tasks vs Real Concurrency
FastAPI is fast, but speed is not the same as correctness. Learn where async, background jobs, and bounded concurrency matter in Python services.
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python
8 minFastAPI Backpressure: Protecting Your Service from Unbounded Fan-Out
A practical guide to request concurrency, connection pools, timeouts, and overload behavior in async Python APIs.
Q&A
Q&AHow do async def and standard def routes differ in FastAPI?
Async routes run on the event loop; sync routes are generally dispatched to a worker thread pool.
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An async route can yield during nonblocking I/O, allowing the event loop to serve other work. A normal def route is typically run in a thread pool. Calling blocking I/O inside async def blocks the event loop; using async syntax alone does not improve CPU-bound work. Choose an execution model based on the library and bottleneck.
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Q&AWhy is one global SQLAlchemy Session unsafe in an async FastAPI service?
A Session carries mutable identity-map and transaction state and is not safe for concurrent requests.
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Concurrent coroutines can interleave operations on the same Session, mixing transaction state and identity-map contents. Create a session per request or task, scope its transaction clearly, and size the connection pool across all worker processes and replicas. A global Engine/pool is different from a shared mutable Session.
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17 minPython Async Runtime Advanced: GIL, ASGI Lifecycles, and Safe Concurrency
Understand event-loop scheduling, CPU versus I/O concurrency, ASGI worker lifecycles, per-request sessions, and cancellation-safe background work.
Q&A
Q&AHow do you prevent an async API from overwhelming dependencies?
Bound concurrency and queue depth at the scarce resource, and enforce an end-to-end deadline.
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Use bounded connection pools, semaphores or worker queues, request deadlines, and deliberate overload responses. Local limits must account for replica count because each replica has its own pool. Avoid unbounded in-memory tasks and retries that amplify load during an incident.
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Q&AHow do you preserve exit failures through a shell pipeline?
Use pipefail where supported and handle expected no-match statuses separately from actual errors.
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Many shells report only the last pipeline command status. In Bash, `set -o pipefail` makes a pipeline fail when a stage fails. grep status 1 often means no match rather than a tool error, so scripts should distinguish that expected result from status 2 or an upstream failure.