You could create an API with a *path operation* that could trigger a request to an *external API* created by someone else (probably the same developer that would be *using* your API).
The process that happens when your API app calls the *external API* is named a "callback". Because the software that the external developer wrote sends a request to your API and then your API *calls back*, sending a request to an *external API* (that was probably created by the same developer).
In this case, you could want to document how that external API *should* look like. What *path operation* it should have, what body it should expect, what response it should return, etc.
## An app with callbacks
Let's see all this with an example.
Imagine you develop an app that allows creating invoices.
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).
* This will be done by sending a POST request (from *your API*) to some *external API* provided by that external developer (this is the "callback").
## 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:
```Python hl_lines="8 9 10 11 12 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53"
{!./src/openapi_callbacks/tutorial001.py!}
```
!!! tip
The `callback_url` query parameter uses a Pydantic URL type.
The only new thing is the `callbacks=messages_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/"
requests.post(callback_url, json={"description": "Invoice paid", "paid": True})
```
But possibly the most important part of the callback is making sure that your API user (the external developer) implements the *external API* correctly, according to the data that *your API* is going to send in the request body of the callback, etc.
So, what we will do next is add the code to document how that *external API* should look like to receive the callback from *your API*.
That documentation will show up in the Swagger UI at `/docs` in your API, and it will let external developers know how to build the *external API*.
This example doesn't implement the callback itself (that could be just a line of code), only the documentation part.
!!! tip
The actual callback is just an HTTP request.
When implementing the callback yourself, you could use something like HTTPX or Requests.
## Write the callback documentation code
This code won't be executed in your app, we only need it to *document* how that *external API* should look like.
But, you already know how to easily create automatic documentation for an API with **FastAPI**.
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*.
Temporarily adopting this point of view (of the *external developer*) can help you feel like it's more obvious where to put the parameters, the Pydantic model for the body, for the response, etc. for that *external API*.
### Create a callback `APIRouter`
First create a new `APIRouter` that will contain one or more callbacks.
This router will never be added to an actual `FastAPI` app (i.e. it will never be passed to `app.include_router(...)`).
Because of that, you need to declare what will be the `default_response_class`, and set it to `JSONResponse`.
!!! Note "Technical Details"
The `response_class` is normally set by the `FastAPI` app during the call to `app.include_router(some_router)`.
But as we are never calling `app.include_router(some_router)`, we need to set the `default_response_class` during creation of the `APIRouter`.
```Python hl_lines="3 24"
{!./src/openapi_callbacks/tutorial001.py!}
```
### Create the callback *path operation*
To create the callback *path operation* use the same `APIRouter` you created above.
It should look just like a normal FastAPI *path operation*:
* It should probably have a declaration of the body it should receive, e.g. `body: InvoiceEvent`.
* And it could also have a declaration of the response it should return, e.g. `response_model=InvoiceEventReceived`.
```Python hl_lines="15 16 17 20 21 27 28 29 30 31"
{!./src/openapi_callbacks/tutorial001.py!}
```
There are 2 main differences from a normal *path operation*:
* It doesn't need to have any actual code, because your app will never call this code. It's only used to document the *external API*. So, the function could just have `pass`.
* The *path* can contain an OpenAPI 3 expression (see more below) where it can use variables with parameters and parts of the original request sent to *your API*.
### The callback path expression
The callback *path* can have an OpenAPI 3 expression that can contain parts of the original request sent to *your API*.
In this case, it's the `str`:
```Python
"{$callback_url}/invoices/{$request.body.id}"
```
So, if your API user (the external developer) sends a request to *your API* to:
```
https://yourapi.com/invoices/?callback_url=https://www.external.org/events
```
with a JSON body of:
```JSON
{
"id": "2expen51ve",
"customer": "Mr. Richie Rich",
"total": "9999"
}
```
Then *your API* will process the invoice, and at some point later, send a callback request to the `callback_url` (the *external API*):
```
https://www.external.org/events/invoices/2expen51ve
```
with a JSON body containing something like:
```JSON
{
"description": "Payment celebration",
"paid": true
}
```
and it would expect a response from that *external API* with a JSON body like:
```JSON
{
"ok": true
}
```
!!! tip
Notice how the callback URL used contains the URL received as a query parameter in `callback_url` (`https://www.external.org/events`) and also the invoice `id` from inside of the JSON body (`2expen51ve`).
### Add the callback router
At this point you have the *callback path operation(s)* needed (the one(s) that the *external developer* should implement in the *external API*) in the callback router you created above.
Now use the parameter `callbacks` in *your API's path operation decorator* to pass the attribute `.routes` (that's actually just a `list` of routes/*path operations*) from that callback router:
```Python hl_lines="34"
{!./src/openapi_callbacks/tutorial001.py!}
```
!!! tip
Notice that you are not passing the router itself (`invoices_callback_router`) to `callback=`, but the attribute `.routes`, as in `invoices_callback_router.routes`.
### Check the docs
Now you can start your app with Uvicorn and go to http://127.0.0.1:8000/docs.
You will see your docs including a "Callback" section for your *path operation* that shows how the *external API* should look like:
