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FastAPI Mount Demonstration

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A minimal application for prototyping mounting the main FastAPI at a sub path.

Rationale

Typically, an application expects to be seated at the root of a domain , for example, this application defines two endpoints /hello and /bye which you can hit locally (after a make start command) at localhost:8000/hello and localhost:8000/bye. However, sometimes we might want to 'mount' an application at a sub path. This may be the case in a microservice setting where many services are sitting behind a reverse proxy and should be addressed at the same domain (see the figure below from https://traefik.io). The FastAPI documentation explains some of the problems that occur in such a situation.

traefik proxy schema

As an example, let's assume you have two services user-service and shopping-cart. Both applications were created in a way that they expect to be at the root, however, now we want to address them at different paths on the same domain. This could be api.domain.com/user-service and api.domain.com/shopping-cart. The application itself should not really be involved in this. Where it is being deployed is outside of its scope and we certainly don't want to edit all our route definitions.

It turns out that the SCRIPT_NAME HTTP header was created for exactly this purpose. Instead of letting the proxy manipulate this header with every request, it is fairly standard to let it be configurable by an environment variable. A popular option is gunicorn in combination with Flask. In Flask this is internally handled by werkzeug as shown here.

How do we do this with FastAPI (or Starlette)?

  1. If you start this service without further actions you can visit the exposed routes and documentation at:

    • localhost:8000/hello
    • localhost:8000/bye
    • localhost:8000/docs
    • localhost:8000/redoc

  2. In order to switch it up, we can now use a reverse proxy. One has already been defined for you in the docker-compose configuration. When you now try to access your service through the proxy at http://localhost/demo-service/hello you will see that this fails with a 404.

  3. We can rescue the situation by rewriting the URL in the proxy instead. You can try this by instead hitting http://localhost/demo2-service/hello. This will give you the correct response but if you now visit http://localhost/demo2-service/docs that fails because the app itself is unaware of the rewritten path.

  4. To try and remedy the situation, you can create a .env file with the content:

    SCRIPT_NAME=/demo2-service

    This will automatically be used by the docker-compose configuration so just restart the service (make clean && make start). Setting SCRIPT_NAME to a non-empty string will affect how the application is initialized.

    app = FastAPI(
    title="FastAPI Mount Demo",
    description="A prototype of mounting the main FastAPI app under "
                "SCRIPT_NAME.",
    openapi_prefix=settings.SCRIPT_NAME,
)

    When you now visit the docs at http://localhost/demo2-service/docs it works as expected!

This is a working strategy but it is rather tedious. We had to rewrite the URL in the reverse proxy and modify our application's source code (we still made it configurable) to manually set the openapi_prefix. There should be a better way.

Solution

Please take a look at the root-path branch on GitHub and the version of the README there for a proper solution implementation.

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