These examples run the server program (e.g Uvicorn), starting **a single process**, listening on all the IPs (`0.0.0.0`) on a predefined port (e.g. `80`).

This is the basic idea. But you will probably want to take care of some additional things, like:

* Security - HTTPS
* Running on startup
* Restarts
* Replication (the number of processes running)
* Memory
* Previous steps before starting

# 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

So we are going to use that same knowledge to document how the *external API* should look like... by creating the *path operation(s)* that the external API should implement (the ones your API will call).

!!! tip
    When writing the code to document a callback, it might be useful to imagine that you are that *external developer*. And that you are currently implementing the *external API*, not *your API*.

* This also allows using automatic **client code generation** in many languages.

### Automatic docs

Interactive API documentation and exploration web user interfaces. As the framework is based on OpenAPI, there are multiple options, 2 included by default.

a[data-terminal-control] {
    text-align: right;
    display: block;
    color: #aebbff;
}

[data-ty] {
    display: block;
    line-height: 2;
}

[data-ty]:before {
    /* Set up defaults and ensure empty lines are displayed. */
    content: '';
    display: inline-block;
    vertical-align: middle;
}

[data-ty="input"]:before,
[data-ty-prompt]:before {
    margin-right: 0.75em;
    color: var(--color-text-subtle);
}

It is also built to work as a future reference.

So you can come back and see exactly what you need.

## Run the code

All the code blocks can be copied and used directly (they are actually tested Python files).

To run any of the examples, copy the code to a file `main.py`, and start `fastapi dev` with:

<div class="termy">

```console