Behind a Proxy

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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.

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.

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 the OpenAPI schema to declare that this API server is located at /api/v1 (behind the proxy). For example:

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

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

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

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.

from fastapi import FastAPI, Request
app = FastAPI()
@app.get("/app")
def read_main(request: Request):
    return {"message": "Hello World", "root_path": request.scope.get("root_path")}

Then, if you start Uvicorn with:

The response would be something like:

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

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:

from fastapi import FastAPI, Request
app = FastAPI(root_path="/api/v1")
@app.get("/app")
def read_main(request: Request):
    return {"message": "Hello World", "root_path": request.scope.get("root_path")}

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:

So, it won’t expect to be accessed at .

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.

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.

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

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.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:

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

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: .

We get the same response:

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 :

But if we access the docs UI at the “official” URL using the proxy with port 9999, at /api/v1/docs, it works correctly! 🎉

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

Right as we wanted it. ✔️

This is because FastAPI uses this root_path to create the default server in OpenAPI with the URL provided by root_path.

Warning

This is a more advanced use case. Feel free to skip it.

By default, FastAPI will create a server in the OpenAPI schema with the URL for the .

But you can also provide other alternative servers, for example if you want the same docs UI to interact with a staging and production environments.

If you pass a custom list of servers and there’s a root_path (because your API lives behind a proxy), FastAPI will insert a “server” with this root_path at the beginning of the list.

For example:

from fastapi import FastAPI, Request
app = FastAPI(
    servers=[
        {"url": "https://stag.example.com", "description": "Staging environment"},
        {"url": "https://prod.example.com", "description": "Production environment"},
    ],
    root_path="/api/v1",
)
@app.get("/app")
def read_main(request: Request):
    return {"message": "Hello World", "root_path": request.scope.get("root_path")}

Will generate an OpenAPI schema like:

{
    "openapi": "3.0.2",
    // More stuff here
    "servers": [
        {
            "url": "/api/v1"
        },
        {
            "url": "https://stag.example.com",
            "description": "Staging environment"
        },
        {
            "url": "https://prod.example.com",
            "description": "Production environment"
        }
    ],
    "paths": {
            // More stuff here
    }
}

Tip

Notice the auto-generated server with a url value of /api/v1, taken from the root_path.

In the docs UI at http://127.0.0.1:9999/api/v1/docs it would look like:

Tip

The docs UI will interact with the server that you select.

If you don’t want FastAPI to include an automatic server using the root_path, you can use the parameter root_path_in_servers=False:

from fastapi import FastAPI, Request
app = FastAPI(
    servers=[
        {"url": "https://stag.example.com", "description": "Staging environment"},
        {"url": "https://prod.example.com", "description": "Production environment"},
    ],
    root_path="/api/v1",
    root_path_in_servers=False,
)
@app.get("/app")
    return {"message": "Hello World", "root_path": request.scope.get("root_path")}

and then it won’t include it in the OpenAPI schema.

If you need to mount a sub-application (as described in ) while also using a proxy with , you can do it normally, as you would expect.