Sub Applications - Mounts

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If you need to have two independent FastAPI applications, with their own independent OpenAPI and their own docs UIs, you can have a main app and "mount" one (or more) sub-application(s).

Mounting a FastAPI application

"Mounting" means adding a completely "independent" application in a specific path, that then takes care of handling everything under that path, with the path operations declared in that sub-application.

Top-level application

First, create the main, top-level, FastAPI application, and its path operations:

Python 3.8+

from fastapi import FastAPI

app = FastAPI()


@app.get("/app")
def read_main():
    return {"message": "Hello World from main app"}


subapi = FastAPI()


@subapi.get("/sub")
def read_sub():
    return {"message": "Hello World from sub API"}


app.mount("/subapi", subapi)

Sub-application

Then, create your sub-application, and its path operations.

This sub-application is just another standard FastAPI application, but this is the one that will be "mounted":

Python 3.8+

from fastapi import FastAPI

app = FastAPI()


@app.get("/app")
def read_main():
    return {"message": "Hello World from main app"}


subapi = FastAPI()


@subapi.get("/sub")
def read_sub():
    return {"message": "Hello World from sub API"}


app.mount("/subapi", subapi)

Mount the sub-application

In your top-level application, app, mount the sub-application, subapi.

In this case, it will be mounted at the path /subapi:

Python 3.8+

from fastapi import FastAPI

app = FastAPI()


@app.get("/app")
def read_main():
    return {"message": "Hello World from main app"}


subapi = FastAPI()


@subapi.get("/sub")
def read_sub():
    return {"message": "Hello World from sub API"}


app.mount("/subapi", subapi)

Check the automatic API docs

Now, run the fastapi command with your file:

$ fastapi dev main.py

<span style="color: green;">INFO</span>:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)

And open the docs at http://127.0.0.1:8000/docs.

You will see the automatic API docs for the main app, including only its own path operations:

And then, open the docs for the sub-application, at http://127.0.0.1:8000/subapi/docs.

You will see the automatic API docs for the sub-application, including only its own path operations, all under the correct sub-path prefix /subapi:

If you try interacting with any of the two user interfaces, they will work correctly, because the browser will be able to talk to each specific app or sub-app.

Technical Details: root_path

When you mount a sub-application as described above, FastAPI will take care of communicating the mount path for the sub-application using a mechanism from the ASGI specification called a root_path.

That way, the sub-application will know to use that path prefix for the docs UI.

And the sub-application could also have its own mounted sub-applications and everything would work correctly, because FastAPI handles all these root_paths automatically.

You will learn more about the root_path and how to use it explicitly in the section about Behind a Proxy.