OAuth2 was designed so that the backend or API could be independent of the server that authenticates the user.

But in this case, the same **FastAPI** application will handle the API and the authentication.

So, let's review it from that simplified point of view:

* The user types the `username` and `password` in the frontend, and hits `Enter`.

This can be very useful for setting up **resources** that you need to use for the whole app, and that are **shared** among requests, and/or that you need to **clean up** afterwards. For example, a database connection pool, or loading a shared machine learning model.

## Use Case { #use-case }

Let's start with an example **use case** and then see how to solve it with this.

    /** Array of fields that can be highlighted in search results */
    protected String[] highlightedFields;

    /** Array of fields that can be searched against */
    protected String[] searchFields;

    /** Set of fields that can be searched against for O(1) lookup */
    protected Set<String> searchFieldSet;

    /** Array of fields that can be used for faceted search */

OpenAPI requires that each operation ID is unique across all the *path operations*, so FastAPI uses the **function name**, the **path**, and the **HTTP method/operation** to generate that operation ID, because that way it can make sure that the operation IDs are unique.

But I'll show you how to improve that next. 🤓

### Required, can be `None` { #required-can-be-none }

You can declare that a parameter can accept `None`, but that it's still required. This would force clients to send a value, even if the value is `None`.

To do that, you can declare that `None` is a valid type but simply do not declare a default value:

{* ../../docs_src/query_params_str_validations/tutorial006c_an_py310.py hl[9] *}

   * The code in FinalizableReferenceQueue goes to considerable trouble to ensure that there are
   * no such references and the tests here check that that trouble has not been in vain.
   *
   * When we reference FinalizableReferenceQueue in this test, we are referencing a class that is
   * loaded by this test and that will obviously remain loaded for as long as the test is running.

That way, the original `APIRouter` will stay unmodified, so we can still share that same `app/internal/admin.py` file with other projects in the organization.

The result is that in our app, each of the *path operations* from the `admin` module will have:

* The prefix `/admin`.
* The tag `admin`.
* The dependency `get_token_header`.
* The response `418`. 🍵

But now that FastAPI 0.99.0 and above uses OpenAPI 3.1.0, that uses JSON Schema 2020-12, and Swagger UI 5.0.0 and above, everything is more consistent and the examples are included in JSON Schema.

We want FastAPI to keep **filtering** the data using the response model. So that even though the function returns more data, the response will only include the fields declared in the response model.

That's why the current versions are still `0.x.x`, this reflects that each version could potentially have breaking changes. This follows the [Semantic Versioning](https://semver.org/) conventions.

You can create production applications with **FastAPI** right now (and you have probably been doing it for some time), you just have to make sure that you use a version that works correctly with the rest of your code.