fastapi/docs/en/docs/advanced/behind-a-proxy.md

Behind a Proxy

In some situations, you might need to use a proxy server like Traefik or Nginx with a configuration that adds an extra path prefix that is not seen by your application.

In these cases you can use root_path to configure your application.

The root_path is a mechanism provided by the ASGI specification (that FastAPI is built on, through Starlette).

The root_path is used to handle these specific cases.

And it's also used internally when mounting sub-applications.

Proxy with a stripped path prefix

Having a proxy with a stripped path prefix, in this case, means that you could declare a path at /app in your code, but then, you add a layer on top (the proxy) that would put your FastAPI application under a path like /api/v1.

In this case, the original path /app would actually be served at /api/v1/app.

Even though all your code is written assuming there's just /app.

And the proxy would be "stripping" the path prefix on the fly before transmitting the request to Uvicorn, keep your application convinced that it is serving at /app, so that you don't have to update all your code to include the prefix /api/v1.

Up to here, everything would work as normally.

But then, when you open the integrated docs UI (the frontend), it would expect to get the OpenAPI schema at /openapi.json, instead of /api/v1/openapi.json.

So, the frontend (that runs in the browser) would try to reach /openapi.json and wouldn't be able to get the OpenAPI schema.

Because we have a proxy with a path prefix of /api/v1 for our app, the frontend needs to fetch the OpenAPI schema at /api/v1/openapi.json.

graph LR

browser("Browser")
proxy["Proxy on http://0.0.0.0:9999/api/v1/app"]
server["Server on http://127.0.0.1:8000/app"]

browser --> proxy
proxy --> server

!!! tip The IP 0.0.0.0 is commonly used to mean that the program listens on all the IPs available in that machine/server.

The docs UI would also need that the JSON payload with the OpenAPI schema has the path defined as /api/v1/app (behind the proxy) instead of /app. For example, something like:

{
    "openapi": "3.0.2",
    // More stuff here
    "paths": {
        "/api/v1/app": {
            // More stuff here
        }
    }
}

In this example, the "Proxy" could be something like Traefik. And the server would be something like Uvicorn, running your FastAPI application.

Providing the root_path

To achieve this, you can use the command line option --root-path like:

$ uvicorn main:app --root-path /api/v1

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

If you use Hypercorn, it also has the option --root-path.

!!! note "Technical Details" The ASGI specification defines a root_path for this use case.

And the `--root-path` command line option provides that `root_path`.

Checking the current root_path

You can get the current root_path used by your application for each request, it is part of the scope dictionary (that's part of the ASGI spec).

Here we are including it in the message just for demonstration purposes.

{!../../../docs_src/behind_a_proxy/tutorial001.py!}

Then, if you start Uvicorn with:

$ uvicorn main:app --root-path /api/v1

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

The response would be something like:

{
    "message": "Hello World",
    "root_path": "/api/v1"
}

Setting the root_path in the FastAPI app

Alternatively, if you don't have a way to provide a command line option like --root-path or equivalent, you can set the root_path parameter when creating your FastAPI app:

{!../../../docs_src/behind_a_proxy/tutorial002.py!}

Passing the root_path to FastAPI would be the equivalent of passing the --root-path command line option to Uvicorn or Hypercorn.

About root_path

Have in mind that the server (Uvicorn) won't use that root_path for anything else than passing it to the app.

But if you go with your browser to http://127.0.0.1:8000/app you will see the normal response:

{
    "message": "Hello World",
    "root_path": "/api/v1"
}

So, it won't expect to be accessed at http://127.0.0.1:8000/api/v1/app.

Uvicorn will expect the proxy to access Uvicorn at http://127.0.0.1:8000/app, and then it would be the proxy's responsibility to add the extra /api/v1 prefix on top.

About proxies with a stripped path prefix

Have in mind that a proxy with stripped path prefix is only one of the ways to configure it.

Probably in many cases the default will be that the proxy doesn't have a stripped path prefix.

In a case like that (without a stripped path prefix), the proxy would listen on something like https://myawesomeapp.com, and then if the browser goes to https://myawesomeapp.com/api/v1/app and your server (e.g. Uvicorn) listens on http://127.0.0.1:8000 the proxy (without a stripped path prefix) would access Uvicorn at the same path: http://127.0.0.1:8000/api/v1/app.

Testing locally with Traefik

You can easily run the experiment locally with a stripped path prefix using Traefik.

Download Traefik, it's a single binary, you can extract the compressed file and run it directly from the terminal.

Then create a file traefik.toml with:

[entryPoints]
  [entryPoints.http]
    address = ":9999"

[providers]
  [providers.file]
    filename = "routes.toml"

This tells Traefik to listen on port 9999 and to use another file routes.toml.

!!! tip We are using port 9999 instead of the standard HTTP port 80 so that you don't have to run it with admin (sudo) privileges.

Now create that other file routes.toml:

[http]
  [http.middlewares]

    [http.middlewares.api-stripprefix.stripPrefix]
      prefixes = ["/api/v1"]

  [http.routers]

    [http.routers.app-http]
      entryPoints = ["http"]
      service = "app"
      rule = "PathPrefix(`/api/v1`)"
      middlewares = ["api-stripprefix"]

  [http.services]

    [http.services.app]
      [http.services.app.loadBalancer]
        [[http.services.app.loadBalancer.servers]]
          url = "http://127.0.0.1:8000"

This file configures Traefik to use the path prefix /api/v1.

And then it will redirect its requests to your Uvicorn running on http://127.0.0.1:8000.

Now start Traefik:

$ ./traefik --configFile=traefik.toml

INFO[0000] Configuration loaded from file: /home/user/awesomeapi/traefik.toml

And now start your app with Uvicorn, using the --root-path option:

$ uvicorn main:app --root-path /api/v1

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

Check the responses

Now, if you go to the URL with the port for Uvicorn: http://127.0.0.1:8000/app, you will see the normal response:

{
    "message": "Hello World",
    "root_path": "/api/v1"
}

!!! tip Notice that even though you are accessing it at http://127.0.0.1:8000/app it shows the root_path of /api/v1, taken from the option --root-path.

And now open the URL with the port for Traefik, including the path prefix: http://127.0.0.1:9999/api/vi/app.

We get the same response:

{
    "message": "Hello World",
    "root_path": "/api/v1"
}

but this time at the URL with the prefix path provided by the proxy: /api/v1.

Of course, the idea here is that everyone would access the app through the proxy, so the version with the path prefix /app/v1 is the "correct" one.

And the version without the path prefix (http://127.0.0.1:8000/app), provided by Uvicorn directly, would be exclusively for the proxy (Traefik) to access it.

That demonstrates how the Proxy (Traefik) uses the path prefix and how the server (Uvicorn) uses the root_path from the option --root-path.

Check the docs UI

But here's the fun part. ✨

The "official" way to access the app would be through the proxy with the path prefix that we defined. So, as we would expect, if you try the docs UI served by Uvicorn directly, without the path prefix in the URL, it won't work, because it expects to be accessed through the proxy.

You can check it at http://127.0.0.1:8000/docs:

But if we access the docs UI at the "official" URL using the proxy, at /api/v1/docs, it works correctly! 🎉

Right as we wanted it. ✔️

This is because FastAPI uses this root_path internally to tell the docs UI to use the URL for OpenAPI with the path prefix provided by root_path.

You can check it at http://127.0.0.1:9999/api/v1/docs:

Mounting a sub-application

If you need to mount a sub-application (as described in Sub Applications - Mounts{.internal-link target=_blank}) while also using a proxy with root_path, you can do it normally, as you would expect.

FastAPI will internally use the root_path smartly, so it will just work. ✨