What is a Webhook?

A Webhook is a way for one application to automatically send information to another application when something happens.

Instead of continuously asking a server whether something has changed, the application simply waits for an event and receives a notification when it happens.

Simple Example

Imagine you ordered something online.

Instead of repeatedly asking:

"Has my order been delivered?"

your delivery company automatically sends you a notification when the order is delivered.

That notification is similar to how a webhook works.

How Does a Webhook Work?

A webhook usually follows these steps:

1. Event happens — for example, a payment is completed.
2. Service detects the event.
3. Service sends an HTTP request to a URL provided by another application.
4. The receiving application processes the data.
5. A response is returned to confirm that the request was received.

For example:

Payment Service
      ↓
Payment Completed
      ↓
Webhook Request
      ↓
Your Server
      ↓
Process Payment

Webhook vs API

They may look similar, but their purpose is different.

Example

With an API:

"Give me the latest payment status."

With a webhook:

"Tell me automatically when a payment is completed."

Webhooks in FastAPI

FastAPI can create an endpoint that receives webhook requests.

from fastapi import FastAPI, Request

app = FastAPI()

@app.post("/webhook")
async def webhook(request: Request):
    data = await request.json()
    print(data)

    return {"status": "received"}

When another service sends a POST request to /webhook, FastAPI receives the data and processes it.

For example, the incoming data might look like:

{
    "event": "payment_success",
    "amount": 500,
    "user": "charan"
}

Your FastAPI application can then perform an action such as updating a database, sending an email, or creating an order.

Where Are Webhooks Used?

Webhooks are commonly used for:

• Payment notifications
• Order and delivery updates
• Authentication events
• Messaging systems
• CI/CD pipelines
• Email events
• E-commerce applications
• Real-time notifications
• Connecting different applications

Important Security

A webhook endpoint is exposed to the internet, so you should not blindly trust incoming requests.

Professional webhook systems commonly use:

• Signature verification to confirm the request came from the expected service
• HTTPS to protect data during transmission
• Authentication or secret tokens
• Input validation
• Idempotency to prevent processing the same event twice
• Logging and monitoring for failed requests

Webhook in One Sentence

A webhook is an event-based HTTP callback where one application automatically sends data to another application when a specific event occurs.