Implementing Webhook Handler in Python.

What is Webhook ?

Webhook is an asynchronous HTTP callback on an event occurrence. It is a simple server to server communication for reporting a specific event occurred on a server. The server on which event occurred will fire a HTTP POST request to another server on a URL which is provided by receiving server.

For example, whenever your colleague pushes code commits to github, an event has occurred on github’s server. Now if a webhook URL is provided in github settings, a webhook will be fired to that URL. This webhook will be a HTTP POST request with commit details inside the body in a specified format.  More details on github webhook can be found here.

In this post, I will share my experience of implementing webhook handler in python. For the readers, basic knowledge on implementing web application in python would be better.

Webhook Handler

A Webhook can be handled by simply providing a URL endpoint in a web application. Following is an example using Django. Add webhook url in urls.py

Now create view function in views.py which will parse the data and process it.  In most of the cases, webhook data is sent in JSON format. So lets load the webhook data and sent the data to process_webhook function.

Most of the web applications accept POST request after verifying CSRF token, but here we need to exempt it from this check. So put @csrf_token decorator above the view function. Also put an @require_post decorator to ensure the request is only POST.

The above implementation of URL endpoint will remain different for various other python web framework like Flask, tornado, twisted. But the below code  process_webhook function implementation will remain same irrespective of any framework.

Processing event

There may be different type events we need to handle. So, before proceeding to implement process_webhook function, lets create a python module named webhook_events.py, which will contain a single function for each type of event wherein will be the logic for that particular event. In other words, we are going to map event name with its function, which will handle the logic for that particular type of webhook event.

There are many ways to implement process_webhook function and how we map a webhook event with its function. We are going to discuss different implementation of process_webhook based on extendability. Most basic version of that is below.

A Better way

Now suppose, there are 10s of webhook to be served. We certainly don’t want to write repetitive code. So below is a better way of implementing process_webhook. Here we just replace dot in event name with underscore, so that we get the function name written in webhook_events.py for that event. If the function is not found that means event is not registered (not being served). In this way, no matter the number webhook to be served, just write the function to handle it, in webhook_events.py

Decorators

More robust and pythonic way of implementing process_webhook is by using decorators. Lets define a decorator in webhook_events.py which will map the event_name to its function. Here the EVENT_MAP is dictionary inside a setting module, which will contain event name as key and event function as its value.

In this case, the process_webhook will look like below:

This is the way which I prefer to implement webhook handler in python. How would you prefer ? Please feel free to comment below.

How to implement Websocket server using Twisted.

HTTP is a request-response type one way protocol. For the web application where continuous data is to be send, websocket was introduced. Unlike HTTP, websocket provides full duplex communication. Websocket, which can be said as an upgraded version of HTTP, is standardized to be used over TCP like HTTP. In this article I will share my experience in implementing websocket with twisted, a framework of python for internet. If you are familiar with websocket, then you can skip to twisted.web or else below is a little introduction to websocket.

WebSocket

To initiate communication using websocket, a Handshake need to be done between client and server. This procedure is backward compatible to HTTP’s request – response structure. First the client sends a handshake request to the server which looks like:

Sending Upgrade header in request with value websocket will acknowledge server about websocket communication. Now if server supports websocket with specified sub-protocols (Sec-WebSocket-Protocol) and version (Sec-WebSocket-Version), it will send adequate response . Possible response could be:

In response, server will send 101 Switching Protocols code and Sec-WebSocket-Accept whose value is calculated using Sec-WebSocket-Key. you can find more information here. After a successful handshake, any of the peer can send data to each other which must be encoded in binary format described in websocket RFC. A high-level overview of the framing is given in the following figure.

Twisted.web

websocket using twisted wire diagramAs in normal twisted.web server , at TCP level, we have HTTPChannel class (a child class of T.I.protocol.Protocol) and server.Site class (which is the child class of T.I.protocol.ServerFactory). Also a Resource instance needs to be passed to server.site class, so that it can serve GET request.

Whenever a data is received, DataReceived method of HTTPChannel is invoked. Now if data starts with ‘GET’, allow the HTTPChannel handle it, which will invoke the render function of the root resource provided to Site class. Render will set 101 response code and will compute the websocket response key. During handshake do not send any raw data, because if handshake is successful this will be considered as framed binary data. Even if you want to send, frame it and send.

If data doesn’t start with ‘GET’, that means we can assume it is a binary encoded message. Now this message can be decoded using Frame.py, which is a very simple data framing module following WebSocket specification. Data send to the client by server should be unmasked as per the websocket specification.

Below is code example of an echo websocket server.