# Extending OpenAPI

There are some cases where you might need to modify the generated OpenAPI schema.

In this section you will see how.

## The normal process

The normal (default) process, is as follows.

A `FastAPI` application (instance) has an `.openapi()` method that is expected to return the OpenAPI schema.

## O aplicativo **FastAPI** normal

Vamos primeiro ver como o aplicativo da API normal se pareceria antes de adicionar o callback.

Ele terá uma *operação de rota* que receberá um corpo `Invoice`, e um parâmetro de consulta `callback_url` que conterá a URL para o callback.

Essa parte é bastante normal, a maior parte do código provavelmente já é familiar para você:

/**
 * Maven encrypt CLI "new-gen".
 */
public class MavenEncCling extends ClingSupport<EncryptOptions, EncryptInvokerRequest> {
    /**
     * "Normal" Java entry point. Note: Maven uses ClassWorld Launcher and this entry point is NOT used under normal
     * circumstances.
     */
    public static void main(String[] args) throws IOException {
        int exitCode = new MavenEncCling().run(args);

# 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

/**
 * Maven CLI "new-gen".
 */
public class MavenCling extends ClingSupport<MavenOptions, MavenInvokerRequest<MavenOptions>> {
    /**
     * "Normal" Java entry point. Note: Maven uses ClassWorld Launcher and this entry point is NOT used under normal
     * circumstances.
     */
    public static void main(String[] args) throws IOException {
        int exitCode = new MavenCling().run(args);

  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <executions>
          <execution>
            <id>normal</id>

            <goals>
              <goal>compile</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>

## HTTPX

Even if your **FastAPI** application uses normal `def` functions instead of `async def`, it is still an `async` application underneath.

    protected final ClassWorld classWorld;
    protected final boolean classWorldManaged;

    /**
     * Ctor that creates "managed" ClassWorld. This constructor is not used in "normal" circumstances.
     */
    public ClingSupport() {
        this(new ClassWorld(CORE_CLASS_REALM_ID, Thread.currentThread().getContextClassLoader()), true);
    }

    /**

It is just a standard function that can receive parameters.

It can be an `async def` or normal `def` function, **FastAPI** will know how to handle it correctly.

In this case, the task function will write to a file (simulating sending an email).

And as the write operation doesn't use `async` and `await`, we define the function with normal `def`:

{* ../../docs_src/background_tasks/tutorial001.py hl[6:9] *}

## Add the background task

 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.maven;

import org.apache.maven.plugin.MojoFailureException;

/**
 * Exception which occurs when a normal (i.e. non-aggregator) mojo fails to
 * execute. In this case, the mojo failed while executing against a particular
 * project instance, so we can wrap the {@link MojoFailureException} with context