Build a reliable FastAPI integration by matching async def to the I/O library you use, managing request-level database sessions with dependencies, validating credentials beyond extracting a bearer token, and initializing shared resources through application lifespan. Test those same lifecycles rather than only checking that an endpoint returns the expected response.
Choose async or synchronous endpoints from the library you call
Start with the API of your database, HTTP, or other I/O library—not with a goal of making every function asynchronous. If the library requires await, use async def for the endpoint or dependency that awaits it. If the library is blocking and has no awaitable interface, FastAPI recommends a normal def path operation. Its guidance is: If you just don’t know, use normal
See FastAPI’s concurrency and async/await guide.def.
- Awaitable I/O: an
async defendpoint can await the library call. - Blocking I/O: a normal
defendpoint or dependency is run by FastAPI in an external threadpool.
That threadpool behavior applies to normal path operations and dependencies managed by FastAPI. It does not automatically protect an ordinary utility function that your code calls directly. For example, calling a blocking database function directly from an async endpoint can still block the event loop. Changing def to async def does not make a blocking library non-blocking; use the library’s supported async interface or keep the blocking call on an appropriate synchronous path. FastAPI supports mixing async and synchronous endpoints and dependencies, with performance benefits possible when the choice matches the library.
Use dependencies as the integration seam
FastAPI dependencies let an endpoint declare the resources and checks it needs. The official dependencies guide identifies shared logic, database connections, and security requirements as uses. A dependency can itself depend on other dependencies, so a small, visible chain can provide a database session and then resolve an authenticated user. FastAPI incorporates dependency and sub-dependency declarations, validation, and requirements into OpenAPI.
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Prefer Annotated aliases when they make repeated dependencies clearer while retaining type information for editors and tools:
from typing import Annotated
from fastapi import Depends
SessionDep = Annotated[Session, Depends(get_session)]
@app.get("/items/")
def read_items(session: SessionDep):
...Keep the responsibilities legible: one dependency acquires a resource, the endpoint or another dependency consumes it, and the lifecycle code releases it. Add endpoint-specific security requirements at the layer that needs them rather than concealing important behavior in an opaque chain.
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Give each database session a clear lifetime
The FastAPI SQL (Relational) Databases tutorial demonstrates SQLModel and one session per request, provided through a dependency using yield. This is an example integration, not a requirement to use SQLModel or a relational database:
def get_session():
with Session(engine) as session:
yield sessionThe endpoint receives the yielded session; leaving the context manager closes it. FastAPI’s dependencies with yield guide describes setup before yield and cleanup afterward. A try/finally structure is useful when writing cleanup explicitly, because it makes the release path clear even when an exception is propagated back through the dependency.
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Keep three different concerns distinct: a request-scoped session, a process-wide connection pool, and transaction policy. FastAPI’s examples demonstrate request-level session delivery and application-wide shared-resource lifecycle; they do not prescribe universal commit or rollback rules for every database library. Follow the documentation for your chosen database and driver for transaction semantics and async-specific behavior.
Do not confuse bearer-token extraction with authentication
OAuth2PasswordBearer is a dependency that reads a Bearer value from the Authorization header, returns the token as a string, and declares a security scheme in OpenAPI. If the expected header or token form is missing, it returns an unauthorized response. But the FastAPI Security – First Steps guide is explicit: “We are not verifying the validity of the token yet, but that’s a start already.”
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A route parameter typed as token: str therefore proves only that extraction produced a string. It does not prove the token is genuine, unexpired, correctly scoped, or tied to a user who may access the requested resource. Add application-appropriate validation and identity resolution in a downstream dependency or equivalent layer before treating the request as authenticated.
Authentication answers “who is this?” Authorization answers “may this identity perform this action?” For scope-based authorization, FastAPI’s OAuth2 scopes guide shows how Security extends Depends to declare scopes and how SecurityScopes can collect requirements through dependencies. Those requirements can also be represented in OpenAPI. The tutorial’s illustrative password flow is not, on its own, a production security design; assess the application’s identity provider and security model separately.
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Initialize shared resources once with lifespan
A connection pool or loaded model shared among requests belongs to the application’s lifespan, not in a function that runs for every request. FastAPI’s lifespan documentation describes setup before the application starts receiving requests and cleanup after it finishes. Its async context-manager pattern places startup work before yield and shutdown cleanup after it.
from contextlib import asynccontextmanager
from fastapi import FastAPI
@asynccontextmanager
async def lifespan(app: FastAPI):
app.state.resource = await create_shared_resource()
try:
yield
finally:
await app.state.resource.close()
app = FastAPI(lifespan=lifespan)The function names here illustrate the lifecycle shape; use the setup and close operations supported by your actual resource. Keep the pool’s application-wide lifetime separate from a request’s session or unit of work: lifespan creates and disposes of the shared resource, while a dependency provides the appropriate request-level access.
Test async behavior and lifespan explicitly
Use a test style that matches what the test needs to do. The FastAPI async tests guide shows synchronous pytest functions with TestClient for ordinary request tests. When the test itself must await async database or other functions, it demonstrates pytest.mark.anyio, HTTPX AsyncClient, and ASGITransport.
One lifecycle detail is easy to miss: creating an AsyncClient with ASGITransport does not trigger application lifespan events. If the application creates a pool or other resource during lifespan, wrap the test application with LifespanManager so startup and shutdown actually run. The guide also identifies event-loop attachment errors as a reason to create loop-dependent objects during async setup rather than at import time.
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- Check endpoint behavior and validation: use a request test to verify the response and input handling.
- Isolate database integration: use the project’s chosen database test strategy, and verify dependency behavior as well as persistence.
- Exercise async persistence when needed: make the test async and await both the request and the persistence assertion.
- Exercise startup and cleanup: use a lifespan-aware test when resources are initialized or released by lifespan.
The FastAPI documentation pages cited here were available when checked on October 4, 2026, and do not state publication or revision dates in the retrieved content. Check the documentation for the FastAPI and integration-library versions installed in your project before relying on an example in a release-specific implementation.
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