How would you organize a larger FastAPI application with `APIRouter`?

Uses routers as feature-oriented HTTP modules while keeping application logic, composition, and contract ownership clear.

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TL;DR

I group path operations into APIRouter modules around a cohesive feature or public resource, then include those routers in the application composition root. A router can own a prefix, tags, shared dependencies, and documented responses while each endpoint delegates real work to application services. This keeps the main app focused on startup, cross-cutting middleware, and router composition. It also avoids one routes file growing into a mixture of unrelated contracts and business logic.

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Practical FastAPI modularity beyond putting every endpoint in one file or mirroring technical layers mechanically.

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Routers scale best when they express cohesive API features rather than become arbitrary folders of decorators.
I group path operations into APIRouter modules around a cohesive feature or public resource, then include those routers in the application composition root. A router can own a prefix, tags, shared dependencies, and documented responses while each endpoint delegates real work to application services. This keeps the main app focused on startup, cross-cutting middleware, and router composition. It also avoids one routes file growing into a mixture of unrelated contracts and business logic.
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For example, an orders router may own /orders, request and response models, and HTTP-to-use-case mapping, while order rules remain outside the router. Shared authentication can be attached at a router level, but resource authorization still belongs where the concrete order is known. I keep imports one-directional so routers do not import the app object or each other for side effects. Prefixes and versioning are composed explicitly, and operation IDs remain unique for client generation.

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Putting all path operations in the application entry file.
Moving business logic into router modules to keep services small.
Importing the global app from routers for registration side effects.
Applying resource authorization only as a broad router dependency.

How would you organize a larger FastAPI application with `APIRouter`?

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