<img src="/img/async/concurrent-burgers/concurrent-burgers-02.png" class="illustration">

The cashier says something to the cook in the kitchen so they know they have to prepare your burgers (even though they are currently preparing the ones for the previous clients).

<img src="/img/async/concurrent-burgers/concurrent-burgers-03.png" class="illustration">

You pay. 💸

The cashier gives you the number of your turn.

                """
                The version string of OpenAPI.

                FastAPI will generate OpenAPI version 3.1.0, and will output that as
                the OpenAPI version. But some tools, even though they might be
                compatible with OpenAPI 3.1.0, might not recognize it as a valid.

                So you could override this value to trick those tools into using

    ```

### Return values

And they can return values or not, the values won't be used.

So, you can re-use a normal dependency (that returns a value) you already use somewhere else, and even though the value won't be used, the dependency will be executed:

=== "Python 3.9+"

    ```Python hl_lines="11  16"
    {!> ../../../docs_src/dependencies/tutorial006_an_py39.py!}
    ```

=== "Python 3.8+"

        Prefer to use the `Annotated` version if possible.

    ```Python hl_lines="13"
    {!> ../../../docs_src/dependencies/tutorial005.py!}
    ```

Let's focus on the parameters declared:

* Even though this function is a dependency ("dependable") itself, it also declares another dependency (it "depends" on something else).
    * It depends on the `query_extractor`, and assigns the value returned by it to the parameter `q`.

    ```

!!! tip
    Keep in mind that JSON only supports `str` as keys.

    But Pydantic has automatic data conversion.

    This means that, even though your API clients can only send strings as keys, as long as those strings contain pure integers, Pydantic will convert them and validate them.

    The `Union` in `Union[str, None]` will allow your editor to give you better support and detect errors.

## Additional validation

We are going to enforce that even though `q` is optional, whenever it is provided, **its length doesn't exceed 50 characters**.

### Import `Query` and `Annotated`

To achieve that, first import:

* `Query` from `fastapi`

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`.

```Python hl_lines="6"
{!../../../docs_src/behind_a_proxy/tutorial001.py!}
```

    ```

=== "Python 3.8+"

    ```Python hl_lines="9  11  16"
    {!> ../../../docs_src/response_model/tutorial003.py!}
    ```

Here, even though our *path operation function* is returning the same input user that contains the password:

=== "Python 3.10+"

    ```Python hl_lines="24"
    {!> ../../../docs_src/response_model/tutorial003_py310.py!}
    ```

        "price": 42.0,
        "tax": 3.2
    },
    "user": {
        "username": "dave",
        "full_name": "Dave Grohl"
    }
}
```

!!! note
    Notice that even though the `item` was declared the same way as before, it is now expected to be inside of the body with a key `item`.

<img src="/img/tutorial/metadata/image02.png">

### Order of tags

The order of each tag metadata dictionary also defines the order shown in the docs UI.

For example, even though `users` would go after `items` in alphabetical order, it is shown before them, because we added their metadata as the first dictionary in the list.

## OpenAPI URL

By default, the OpenAPI schema is served at `/openapi.json`.