So, you could add additional data to the automatically generated schema.

For example, you could decide to read and validate the request with your own code, without using the automatic features of FastAPI with Pydantic, but you could still want to define the request in the OpenAPI schema.

You could do that with `openapi_extra`:

```Python hl_lines="20-37  39-40"

 files for and see if we can't incorporate it all into the POM. Or if we do
 have properties file (something I would like to avoid) say they don't
 contribute in any meaningful way to information in the POM. For example a
 properties file could be used the specify $ so it can be interpolated in
 <developerConnection/> but you couldn't use a properties file to specify the
 version of your project say. Anyway, food for thought to begin with.

This client could be a browser with a frontend, a code from someone else, an IoT device, etc.

You could need to tell the client that:

* The client doesn't have enough privileges for that operation.
* The client doesn't have access to that resource.
* The item the client was trying to access doesn't exist.
* etc.

# OpenAPI Callbacks

You could create an API with a *path operation* that could trigger a request to an *external API* created by someone else (probably the same developer that would be *using* your API).

<img src="/img/tutorial/extending-openapi/image03.png">

## Change the Theme

The same way you could set the syntax highlighting theme with the key `"syntaxHighlight.theme"` (notice that it has a dot in the middle):

```Python hl_lines="3"
{!../../../docs_src/configure_swagger_ui/tutorial002.py!}
```

That configuration would change the syntax highlighting color theme:

<img src="/img/tutorial/extending-openapi/image04.png">

            final InputStreamThread ist = jobProcess.getInputStreamThread();
            try {
                ist.interrupt();
            } catch (final Exception e) {
                logger.warn("Could not interrupt a thread of an input stream.", e);
            }

            final CountDownLatch latch = new CountDownLatch(3);
            final Process process = jobProcess.getProcess();
            new Thread(() -> {

     */
    @Nonnull
    SettingsBuilderResult build(@Nonnull SettingsBuilderRequest request);

    /**
     * Builds the effective settings of the specified settings sources.
     *
     * @return the result of the settings building, never {@code null}
     * @throws SettingsBuilderException if the effective settings could not be built
     */
    @Nonnull

     * @return The result of the project building, never {@code null}.
     * @throws ProjectBuildingException If the project descriptor could not be successfully built.
     */
    ProjectBuildingResult build(File projectFile, ProjectBuildingRequest request) throws ProjectBuildingException;

    /**
     * Builds a project descriptor for the specified artifact.

│   └── main.py
```

In the file `main.py` you have your **FastAPI** app:


```Python
{!../../../docs_src/app_testing/main.py!}
```

### Testing file

Then you could have a file `test_main.py` with your tests. It could live on the same Python package (the same directory with a `__init__.py` file):

``` hl_lines="5"
.
├── app
│   ├── __init__.py
│   ├── main.py
│   └── test_main.py
```

# OpenAPI Webhooks

There are cases where you want to tell your API **users** that your app could call *their* app (sending a request) with some data, normally to **notify** of some type of **event**.

This means that instead of the normal process of your users sending requests to your API, it's **your API** (or your app) that could **send requests to their system** (to their API, their app).

This is normally called a **webhook**.

## Webhooks steps