## The normal **FastAPI** app

Let's first see how the normal API app would look like before adding the callback.

It will have a *path operation* that will receive an `Invoice` body, and a query parameter `callback_url` that will contain the URL for the callback.

This part is pretty normal, most of the code is probably already familiar to you:

# 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

But if you go with your browser to <a href="http://127.0.0.1:8000" class="external-link" target="_blank">http://127.0.0.1:8000/app</a> you will see the normal response:

```JSON
{
    "message": "Hello World",
    "root_path": "/api/v1"
}
```

So, it won't expect to be accessed at `http://127.0.0.1:8000/api/v1/app`.

            isResolutionError &&
                    reportedPsi is KtValueArgumentList || reportedPsiParent is KtValueArgumentList && reportedPsi == reportedPsiParent.rightParenthesis -> true
            // errors on call args for normal function calls
            isCallArgError &&
                    reportedPsiParent is KtValueArgument &&

It will be called by **FastAPI** whenever it receives a request to the URL "`/`" using a `GET` operation.

In this case, it is an `async` function.

---

You could also define it as a normal function instead of `async def`:

```Python hl_lines="7"
{!../../../docs_src/first_steps/tutorial003.py!}
```

!!! note

    ```Python hl_lines="5  11-12"
    {!> ../../../docs_src/settings/app02/main.py!}
    ```

!!! tip
    We'll discuss the `@lru_cache` in a bit.

    For now you can assume `get_settings()` is a normal function.

And then we can require it from the *path operation function* as a dependency and use it anywhere we need it.

=== "Python 3.9+"

    ```Python hl_lines="17  19-21"

#!/bin/sh -e
set -x

ruff check fastapi tests docs_src scripts --fix

    * This allows adding extra code after a dependency is done. It can be used, for example, to close database connections.
    * Dependencies with `yield` can be normal or `async`, **FastAPI** will run normal dependencies in a threadpool.
    * They can be combined with normal dependencies.
    * It's possible to have arbitrary trees/levels of dependencies with `yield` and exit steps are handled in the correct order automatically.

	format := &formatErasureV3{}
	format.Version = formatMetaVersionV1
	format.Format = formatBackendErasure
	if setLen == 1 {
		format.Format = formatBackendErasureSingle
	}
	format.ID = mustGetUUID()
	format.Erasure.Version = formatErasureVersionV3
	format.Erasure.DistributionAlgo = formatErasureVersionV3DistributionAlgoV3
	format.Erasure.Sets = make([][]string, numSets)

`tf.keras`: