@subrouter.post("/invoices/", callbacks=invoices_callback_router.routes)
def create_invoice(invoice: Invoice, callback_url: Optional[HttpUrl] = None):
    """
    Create an invoice.

    This will (let's imagine) let the API user (some external developer) create an
    invoice.

    And this path operation will:

    * Send the invoice to the client.
    * Collect the money from the client.

For "synchronous" (contrary to "asynchronous") they commonly also use the term "sequential", because the computer / program follows all the steps in sequence before switching to a different task, even if those steps involve waiting.

### Concurrency and Burgers

This idea of **asynchronous** code described above is also sometimes called **"concurrency"**. It is different from **"parallelism"**.

        "paths": {
            "/invoices/": {
                "post": {
                    "summary": "Create Invoice",

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

The user of your API (an external developer) will create an invoice in your API with a POST request.

Then your API will (let's imagine):

* Send the invoice to some customer of the external developer.
* Collect the money.
* Send a notification back to the API user (the external developer).

（假设）您的 API 将：

* 把发票发送至外部开发者的消费者
* 归集现金
* 把通知发送至 API 的用户（外部开发者）
    * 通过（从您的 API）发送 POST 请求至外部 API （即**回调**）来完成

## 常规 **FastAPI** 应用

添加回调前，首先看下常规 API 应用是什么样子。

常规 API 应用包含接收 `Invoice` 请求体的*路径操作*，还有包含回调 URL 的查询参数 `callback_url`。

这部分代码很常规，您对绝大多数代码应该都比较熟悉了：

```Python hl_lines="10-14  37-54"
{!../../../docs_src/openapi_callbacks/tutorial001.py!}
```

!!! tip "提示"

# Help FastAPI - Get Help

Do you like **FastAPI**?

Would you like to help FastAPI, other users, and the author?

Or would you like to get help with **FastAPI**?

There are very simple ways to help (several involve just one or two clicks).

And there are several ways to get help too.

## Subscribe to the newsletter

)
def invoice_notification(body: InvoiceEvent):
    pass


@app.post("/invoices/", callbacks=invoices_callback_router.routes)
def create_invoice(invoice: Invoice, callback_url: Union[HttpUrl, None] = None):
    """
    Create an invoice.

    This will (let's imagine) let the API user (some external developer) create an
    invoice.

    And this path operation will:

        return wrapped_app

    return middleware_constructor


def test_depend_validation():
    """
    Verify that a validation in a dependency invokes the correct exception handler
    """
    caught = []

    @websocket_middleware
    async def catcher(websocket, call_next):
        try:
            return await call_next()

## Die normale **FastAPI**-Anwendung

Sehen wir uns zunächst an, wie die normale API-Anwendung aussehen würde, bevor wir den Callback hinzufügen.

Sie verfügt über eine *Pfadoperation*, die einen `Invoice`-Body empfängt, und einen Query-Parameter `callback_url`, der die URL für den Callback enthält.

Dieser Teil ist ziemlich normal, der größte Teil des Codes ist Ihnen wahrscheinlich bereits bekannt:

* 📈 💸.
* 📨 📨 🔙 🛠️ 👩‍💻 (🔢 👩‍💻).
    * 👉 🔜 🔨 📨 🏤 📨 (⚪️➡️ *👆 🛠️*) *🔢 🛠️* 🚚 👈 🔢 👩‍💻 (👉 "⏲").

## 😐 **FastAPI** 📱

➡️ 🥇 👀 ❔ 😐 🛠️ 📱 🔜 👀 💖 ⏭ ❎ ⏲.

⚫️ 🔜 ✔️ *➡ 🛠️* 👈 🔜 📨 `Invoice` 💪, & 🔢 🔢 `callback_url` 👈 🔜 🔌 📛 ⏲.

👉 🍕 📶 😐, 🌅 📟 🎲 ⏪ 😰 👆:

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

!!! tip