I'll show you some concrete examples in the next chapters.

---

Then the next concepts to consider are all about the program running your actual API (e.g. Uvicorn).

## Program and Process

* Memory
* Previous steps before starting

The only new thing is the `callbacks=invoices_callback_router.routes` as an argument to the *path operation decorator*. We'll see what that is next.

## Documenting the callback

The actual callback code will depend heavily on your own API app.

And it will probably vary a lot from one app to the next.

It could be just one or two lines of code, like:

```Python
callback_url = "https://example.com/api/v1/invoices/events/"

```Python
{!../../../docs_src/first_steps/tutorial001.py!}
```

Copy that to a file `main.py`.

Run the live server:

<div class="termy">

```console
$ <font color="#4E9A06">fastapi</font> dev <u style="text-decoration-style:single">main.py</u>
<font color="#3465A4">INFO    </font> Using path <font color="#3465A4">main.py</font>

Hello World from Python
```

</div>

!!! tip
    You can read more about it at <a href="https://12factor.net/config" class="external-link" target="_blank">The Twelve-Factor App: Config</a>.

### Types and validation

    Copy **only** the file with the requirements first, not the rest of the code.

    As this file **doesn't change often**, Docker will detect it and use the **cache** for this step, enabling the cache for the next step too.

4. Install the package dependencies in the requirements file.

So, a request with a content type of `text/plain` containing JSON data would be accepted and the JSON data would be extracted.

 * @author Ines Montani <******@****.***>
 * @version 0.0.1
 * @license MIT
 */

:root {
    --color-bg: #252a33;
    --color-text: #eee;
    --color-text-subtle: #a2a2a2;
}

[data-termynal] {
    width: 750px;
    max-width: 100%;
    background: var(--color-bg);
    color: var(--color-text);
    /* font-size: 18px; */
    font-size: 15px;

In your WebSocket route you can `await` for messages and send messages.

```Python hl_lines="48-52"
{!../../../docs_src/websockets/tutorial001.py!}
```

You can receive and send binary, text, and JSON data.

## Try it

If your file is named `main.py`, run your application with:

<div class="termy">

```console
$ fastapi dev main.py

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

Notice that with webhooks you are actually not declaring a *path* (like `/items/`), the text you pass there is just an **identifier** of the webhook (the name of the event), for example in `@app.webhooks.post("new-subscription")`, the webhook name is `new-subscription`.