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

The process normally is that you define in your code what is the message that you will send, the body of the request.

You also define in some way at which moments your app will send those requests or events.

And your users define in some way (for example in a web dashboard somewhere) the URL where your app should send those requests.

All the logic about how to register the URLs for webhooks and the code to actually send those requests is up to you. You write it however you want to in your own code.

Documenting webhooks with FastAPI and OpenAPI

With FastAPI, using OpenAPI, you can define the names of these webhooks, the types of HTTP operations that your app can send (e.g. POST, PUT, etc.) and the request bodies that your app would send.

This can make it a lot easier for your users to implement their APIs to receive your webhook requests, they might even be able to autogenerate some of their own API code.

Info

Webhooks are available in OpenAPI 3.1.0 and above, supported by FastAPI 0.99.0 and above.

An app with webhooks

When you create a FastAPI application, there is a webhooks attribute that you can use to define webhooks, the same way you would define path operations, for example with @app.webhooks.post().

from datetime import datetime

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()


class Subscription(BaseModel):
    username: str
    monthly_fee: float
    start_date: datetime


@app.webhooks.post("new-subscription")
def new_subscription(body: Subscription):
    """
    When a new user subscribes to your service we'll send you a POST request with this
    data to the URL that you register for the event `new-subscription` in the dashboard.
    """


@app.get("/users/")
def read_users():
    return ["Rick", "Morty"]

The webhooks that you define will end up in the OpenAPI schema and the automatic docs UI.

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.

This is because it is expected that your users would define the actual URL path where they want to receive the webhook request in some other way (e.g. a web dashboard).

Check the docs

Now you can start your app and go to http://127.0.0.1:8000/docs.

You will see your docs have the normal path operations and now also some webhooks: