# 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

### Just Modern Python

It's all based on standard **Python type** declarations (thanks to Pydantic). No new syntax to learn. Just standard modern Python.

If you need a 2 minute refresher of how to use Python types (even if you don't use FastAPI), check the short tutorial: [Python Types](python-types.md){.internal-link target=_blank}.

You write standard Python with types:

```Python
from datetime import date

---

## **Typer**, le FastAPI des <abbr title="Command Line Interface">CLI</abbr>

<a href="https://typer.tiangolo.com" target="_blank"><img src="https://typer.tiangolo.com/img/logo-margin/logo-margin-vector.svg" style="width: 20%;"></a>

* <a href="https://docs.pydantic.dev/latest/usage/types/extra_types/extra_types/" target="_blank"><code>pydantic-extra-types</code></a> - for extra types to be used with Pydantic.

Used by Starlette:

* <a href="https://www.python-httpx.org" target="_blank"><code>httpx</code></a> - Required if you want to use the `TestClient`.

### `tf.lite`:

*   `TFLiteConverter`:
    *   Support optional flags `inference_input_type` and
        `inference_output_type` for full integer quantized models. This allows
        users to modify the model input and output type to integer types
        (`tf.int8`, `tf.uint8`) instead of defaulting to float type
        (`tf.float32`).
*   NNAPI

| `minio_api_requests_rejected_timestamp_total`  | `counter` | Total number of requests rejected for invalid timestamp | `type,pool_index,server`         |
| `minio_api_requests_rejected_invalid_total`    | `counter` | Total number of invalid requests                        | `type,pool_index,server`         |

---

## Komut Satırı Uygulamalarının FastAPI'ı: **Typer**

<a href="https://typer.tiangolo.com" target="_blank"><img src="https://typer.tiangolo.com/img/logo-margin/logo-margin-vector.svg" style="width: 20%;"></a>

</div>

Open your browser at <a href="http://127.0.0.1:8000" class="external-link" target="_blank">http://127.0.0.1:8000</a>.

You will see a simple page like:

<img src="/img/tutorial/websockets/image01.png">

You can type messages in the input box, and send them:

<img src="/img/tutorial/websockets/image02.png">

And your **FastAPI** application with WebSockets will respond back:

<img src="/img/tutorial/websockets/image03.png">

        for ((parameter, type) in typeArguments) {
            val ktParameter = typeParameters.getOrNull(parameter.index) ?: return emptyMap()

            // i.e. we were not able to infer some types
            if (type.contains { it: UnwrappedType -> it.constructor is TypeVariableTypeConstructor }) return emptyMap()

            result[ktParameter] = type.toKtType(analysisContext)
        }

```Python hl_lines="9-13  36-53"
{!../../../docs_src/openapi_callbacks/tutorial001.py!}
```

!!! tip
    The `callback_url` query parameter uses a Pydantic <a href="https://docs.pydantic.dev/latest/concepts/types/#urls" class="external-link" target="_blank">URL</a> type.

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