# 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

   * documentation for {@link Collection}:
   *
   * <blockquote>
   *
   * "Some collection implementations have restrictions on the elements that they may contain. For
   * example, some implementations prohibit null elements, and some have restrictions on the types
   * of their elements."
   *
   * </blockquote>
   */
  RESTRICTS_ELEMENTS,

  /**

* Be careful when using features introduced in Java 1.7 or later. Some parts of Gradle still need to run on Java 6.
* Normalize file paths in tests. The `org.gradle.util.internal.TextUtil` class has some useful functions for this purpose.

You can consult the [Architecture documentation](architecture) to learn about some of the architecture of Gradle.

    entries.add(entry(null, v3()));

    resetContainer(getSubjectGenerator().create(entries.toArray()));
    assertTrue(
        "A Map should equal any other Map containing the same entries,"
            + " even if some keys are null.",
        getMap().equals(newHashMap(entries)));
  }

  @CollectionSize.Require(absent = CollectionSize.ZERO)
  public void testEquals_otherContainsNullKey() {

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

Let's imagine that you have some **machine learning models** that you want to use to handle requests. 🤖

The same models are shared among requests, so, it's not one model per request, or one per user or something similar.

Let's imagine that loading the model can **take quite some time**, because it has to read a lot of **data from disk**. So you don't want to do it for every request.

///

It will:

* Return that status code in the response.
* Document it as such in the OpenAPI schema (and so, in the user interfaces):

<img src="/img/tutorial/response-status-code/image01.png">

/// note

Some response codes (see the next section) indicate that the response does not have a body.

FastAPI knows this, and will produce OpenAPI docs that state there is no response body.

///

## About HTTP status codes

   * @see IntStream#findFirst()
   * @throws NullPointerException if the last element of the stream is null
   */
  public static OptionalInt findLast(IntStream stream) {
    // findLast(Stream) does some allocation, so we might as well box some more
    java.util.Optional<Integer> boxedLast = findLast(stream.boxed());
    return boxedLast.map(OptionalInt::of).orElse(OptionalInt.empty());
  }

  /**

You can use virtually **any cloud provider** to deploy your FastAPI application.

In most of the cases, the main cloud providers have guides to deploy FastAPI with them.

## Cloud Providers - Sponsors

Some cloud providers ✨ [**sponsor FastAPI**](../help-fastapi.md#sponsor-the-author){.internal-link target=_blank} ✨, this ensures the continued and healthy **development** of FastAPI and its **ecosystem**.

 * reference. It allows you to represent "a {@code T} that must be present" and a "a {@code T} that
 * might be absent" as two distinct types in your program, which can aid clarity.
 *
 * <p>Some uses of this class include
 *
 * <ul>
 *   <li>As a method return type, as an alternative to returning {@code null} to indicate that no
 *       value was available

# Custom Docs UI Static Assets (Self-Hosting)

The API docs use **Swagger UI** and **ReDoc**, and each of those need some JavaScript and CSS files.

By default, those files are served from a <abbr title="Content Delivery Network: A service, normally composed of several servers, that provides static files, like JavaScript and CSS. It's commonly used to serve those files from the server closer to the client, improving performance.">CDN</abbr>.