Add support for ASGI root_path for openapi docs

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

@@ -0,0 +1,281 @@
+# 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`.
+
+```mermaid
+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:
+
+```JSON hl_lines="5"
+{
+    "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:
+
+<div class="termy">
+
+```console
+$ 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)
+```
+
+</div>
+
+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.
+
+```Python hl_lines="8"
+{!../../../docs_src/behind_a_proxy/tutorial001.py!}
+```
+
+Then, if you start Uvicorn with:
+
+<div class="termy">
+
+```console
+$ 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)
+```
+
+</div>
+
+The response would be something like:
+
+```JSON
+{
+    "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:
+
+```Python hl_lines="3"
+{!../../../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 <a href="http://127.0.0.1:8000" class="external-link" target="_blank">http://127.0.0.1:8000/app</a> you will see the normal response:
+
+```JSON
+{
+    "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 <a href="https://docs.traefik.io/" class="external-link" target="_blank">Traefik</a>.
+
+<a href="https://github.com/containous/traefik/releases" class="external-link" target="_blank">Download Traefik</a>, 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:
+
+```TOML hl_lines="3"
+[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`:
+
+```TOML hl_lines="5  12  20"
+[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:
+
+<div class="termy">
+
+```console
+$ ./traefik --configFile=traefik.toml
+
+INFO[0000] Configuration loaded from file: /home/user/awesomeapi/traefik.toml
+```
+
+</div>
+
+And now start your app with Uvicorn, using the `--root-path` option:
+
+<div class="termy">
+
+```console
+$ 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)
+```
+
+</div>
+
+### Check the responses
+
+Now, if you go to the URL with the port for Uvicorn: <a href="http://127.0.0.1:8000/app" class="external-link" target="_blank">http://127.0.0.1:8000/app</a>, you will see the normal response:
+
+```JSON
+{
+    "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: <a href="http://127.0.0.1:9999/api/v1/app" class="external-link" target="_blank">http://127.0.0.1:9999/api/vi/app</a>.
+
+We get the same response:
+
+```JSON
+{
+    "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 <a href="http://127.0.0.1:8000/docs" class="external-link" target="_blank">http://127.0.0.1:8000/docs</a>:
+
+<img src="/img/tutorial/behind-a-proxy/image01.png">
+
+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 <a href="http://127.0.0.1:9999/api/v1/docs" class="external-link" target="_blank">http://127.0.0.1:9999/api/v1/docs</a>:
+
+<img src="/img/tutorial/behind-a-proxy/image02.png">
+
+## Mounting a sub-application
+
+If you need to mount a sub-application (as described in [Sub Applications - Mounts](./sub-applications.md){.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. ✨

docs/en/docs/advanced/extending-openapi.md

@@ -52,15 +52,22 @@ First, write all your **FastAPI** application as normally:
 
 Then, use the same utility function to generate the OpenAPI schema, inside a `custom_openapi()` function:
 
-```Python hl_lines="2 15 16 17 18 19 20"
+```Python hl_lines="2  15 16 17 18 19 20 21"
 {!../../../docs_src/extending_openapi/tutorial001.py!}
 ```
 
+!!! tip
+    The `openapi_prefix` will contain any prefix needed for the generated OpenAPI *path operations*.
+
+    FastAPI will automatically use the `root_path` to pass it in the `openapi_prefix`.
+
+    But the important thing is that your function should receive that parameter `openapi_prefix` and pass it along.
+
 ### Modify the OpenAPI schema
 
 Now you can add the ReDoc extension, adding a custom `x-logo` to the `info` "object" in the OpenAPI schema:
 
-```Python hl_lines="21 22 23"
+```Python hl_lines="22 23 24"
 {!../../../docs_src/extending_openapi/tutorial001.py!}
 ```
 
@@ -72,15 +79,15 @@ That way, your application won't have to generate the schema every time a user o
 
 It will be generated only once, and then the same cached schema will be used for the next requests.
 
-```Python hl_lines="13 14 24 25"
+```Python hl_lines="13 14  25 26"
 {!../../../docs_src/extending_openapi/tutorial001.py!}
 ```
 
 ### Override the method
 
 Now you can replace the `.openapi()` method with your new function.
 
-```Python hl_lines="28"
+```Python hl_lines="29"
 {!../../../docs_src/extending_openapi/tutorial001.py!}
 ```