This example doesn't implement the callback itself (that could be just a line of code), only the documentation part.

!!! tip
    The actual callback is just an HTTP request.

![ReDoc](https://fastapi.tiangolo.com/img/index/index-06-redoc-02.png)

### Just Modern Python

It's all based on standard **Python type** declarations (thanks to Pydantic). No new syntax to learn. Just standard modern Python.

### Single Container

!!! info
    The `app.webhooks` object is actually just an `APIRouter`, the same type you would use when structuring your app with multiple files.

        The `model_config` attribute is used just for Pydantic configuration. You can read more at <a href="https://docs.pydantic.dev/latest/usage/model_config/" class="external-link" target="_blank">Pydantic Model Config</a>.

=== "Pydantic v1"

    ```Python hl_lines="9-10"
    {!> ../../../docs_src/settings/app03_an/config_pv1.py!}
    ```

    !!! tip

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!}
```

   * @throws NullPointerException if any element in the stream is {@code null}.
   * @return {@code Optional.of(onlyElement)} if the stream has exactly one element (must not be
   *     {@code null}) and returns {@code Optional.empty()} if it has none.
   */
  @SuppressWarnings("unchecked")
  public static <T> Collector<T, ?, Optional<T>> toOptional() {
    return (Collector) TO_OPTIONAL;

/**
 * Request used to build a {@link org.apache.maven.api.Project} using
 * the {@link ProjectBuilder} service.
 *
 * TODO: replace ModelRepositoryHolder with just the enum for the strategy
 * TODO: replace validation level with an enum (though, we usually need just a boolean)
 *
 * @since 4.0.0
 */
@Experimental
@Immutable
public interface ModelBuilderRequest {

    /**

		return err
	}

	log.Debug("notifying subscribed node proxies")
	if err := s.sendPodToZtunnelAndWaitForAck(ctx, pod, openNetns); err != nil {
		// we must return PartialAdd error here. the pod was injected with iptables rules,
		// so it should be annotated, so if it is removed from the mesh, the rules will be removed.

And if the server is restarted (for example after updates, or migrations from the cloud provider) you probably **won't notice it**. And because of that, you won't even know that you have to restart the process manually. So, your API will just stay dead. 😱

### Run Automatically on Startup