FastAPI / Chapter 8 of 8

Patterns, pitfalls, and reference

Look up FastAPI terms and common mistakes: blocking async handlers, leaking database sessions, response validation, and background jobs.

2 topicsUpdated 22 Oct 2026

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Common mistakes and quick patterns#

Common causes of failures under load or during deployment.

Mistake	Safer pattern
Calling sync I/O inside `async def`	Use an async client or a sync `def` handler.
One global ORM Session	Create one Session per request/task and close it.
Returning ORM objects with unplanned lazy relations	Eager load fields the response model needs.
Using the same schema for create/read/update	Separate server-owned and client-owned fields.
Using `PUT` with `exclude_unset=True`	Use PATCH for partial changes; PUT should represent full replacement.
Trusting user IDs from URLs	Compare row owner to authenticated identity.
CORS wildcard as a security rule	Authenticate and authorize on the server.
`create_all()` in every production startup	Migrate with Alembic once per release.
Passing request DB Session to background work	Open a new session inside the task/worker.
Multiple workers but tiny Postgres connection limit	Budget pools across all workers and services.

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Terms used by FastAPI and its supporting libraries.

Term	Meaning
HTTP method	Action requested: GET, POST, PUT, PATCH, DELETE.
Path parameter	Value in a URL segment such as `/books/7`.
Query parameter	Value after `?`, e.g. `?limit=20`.
JSON body	Structured request data submitted by the client.
422	Input did not satisfy the endpoint schema.
Pydantic model	Typed data contract for parsing and serialization.
Dependency	Reusable callable run by FastAPI before a route.
ASGI	Protocol between FastAPI and an async server like Uvicorn.
Session	Per-operation SQLAlchemy unit of work/transaction.
JWT	Signed token carrying claims; readable, not encrypted.
OpenAPI	Machine-readable API contract at `/openapi.json`.
CORS	Browser permission mechanism for cross-origin requests.
Lifespan	Process startup/shutdown setup and cleanup.