A "middleware" is a function that works with every **request** before it is processed by any specific *path operation*. And also with every **response** before returning it.

* It takes each **request** that comes to your application.
* It can then do something to that **request** or run any needed code.
* Then it passes the **request** to be processed by the rest of the application (by some *path operation*).

    You could also use `from starlette.staticfiles import StaticFiles`.

    **FastAPI** provides the same `starlette.staticfiles` as `fastapi.staticfiles` just as a convenience for you, the developer. But it actually comes directly from Starlette.

### What is "Mounting"

"Mounting" means adding a complete "independent" application in a specific path, that then takes care of handling all the sub-paths.

!!! check
    Notice that the value your function received (and returned) is `3`, as a Python `int`, not a string `"3"`.

    So, with that type declaration, **FastAPI** gives you automatic request <abbr title="converting the string that comes from an HTTP request into Python data">"parsing"</abbr>.

## Data validation

The main thing you need to run a **FastAPI** application (or any other ASGI application) in a remote server machine is an ASGI server program like **Uvicorn**, this is the one that comes by default in the `fastapi` command.

There are several alternatives, including:

* <a href="https://www.uvicorn.org/" class="external-link" target="_blank">Uvicorn</a>: a high performance ASGI server.

!!! note "Technical Details"
    You could also use `from starlette.testclient import TestClient`.

    **FastAPI** provides the same `starlette.testclient` as `fastapi.testclient` just as a convenience for you, the developer. But it comes directly from Starlette.

!!! tip

We are already halfway there.

We just need to add a *path operation* for the user/client to actually send the `username` and `password`.

!!! note "Technical Details"
    You could also use `from starlette import status`.

    **FastAPI** provides the same `starlette.status` as `fastapi.status` just as a convenience for you, the developer. But it comes directly from Starlette.

## Tags

You can add tags to your *path operation*, pass the parameter `tags` with a `list` of `str` (commonly just one `str`):

=== "Python 3.10+"

    ```Python hl_lines="15  20  25"

All the same process that applied for path parameters also applies for query parameters:

* Editor support (obviously)
* Data <abbr title="converting the string that comes from an HTTP request into Python data">"parsing"</abbr>
* Data validation
* Automatic documentation

## Defaults

As query parameters are not a fixed part of a path, they can be optional and can have default values.

```

!!! note "Technical Details"
    You could also use `from starlette.websockets import WebSocket`.

    **FastAPI** provides the same `WebSocket` directly just as a convenience for you, the developer. But it comes directly from Starlette.

## Await for messages and send messages

In your WebSocket route you can `await` for messages and send messages.

```Python hl_lines="48-52"
{!../../../docs_src/websockets/tutorial001.py!}

=== "Python 3.10+"

    ```Python hl_lines="1  14-15  18-20  33"
    {!> ../../../docs_src/extra_models/tutorial003_py310.py!}
    ```

=== "Python 3.8+"