Webhooks are a very simple, yet very powerful way to integrate with external systems such as email or any other system via an HTTP post request. The webhook, a simple application designed to handle incoming HTTP requests, will parse and process a payload from Scalr and then integrate with the external systems when events occur during a Terraform run, such as:

  • Run complete - When the run is completed successfully
  • Run errored - When the run is prevented from continuing due to an error in the code, cancellation, or a policy failure.
  • Run needs attention - When someone will need to review a plan based on the workspace not having auto-approval or if a policy override is required.

During any of these events, Scalr will send a payload to an endpoint that the administrator has defined.

Endpoint Configuration

An endpoint is required as it is the URL that Scalr will send the payload to. Endpoints can currently be created through the Scalr API or provider within an environment:

resource "random_string" "r" {
  length = 16

resource "scalr_endpoint" "example" {
  name           = "my-endpoint-name"
  secret_key     = random_string.r.result
  timeout        = 15
  max_attempts   = 3
  url            = "https://my-endpoint.url"
  environment_id = "env-xxxxxxxxxxxx"

A secret key must be used in the webhook code to verify that Scalr is the source of the payload. The key can be generated using the random_string resource as seen above.

Webhook Configuration

The webhook configuration is where the endpoint and trigger events are defined. Webhooks can currently be created through the Scalr API or provider within an environment. All workspaces in the environment will have the webhook linked to them:

resource "scalr_webhook" "example" {
  name           = "my-webhook-name"
  enabled        = true
  endpoint_id    = "ep-xxxxxxxxxx"
  events         = ["run:completed", "run:errored", "run:needs_attention"]
  environment_id = "env-xxxxxxxxxx"

Once an event is triggered the payload will be sent to the endpoint.

Example Webhook Server Code



This code is provided as an example only and assumes the reader has the necessary Python skills to properly implement a webhook server. This should NOT be used in production systems without full testing and validation and this code IS NOT supported by Scalr. Other languages can be used.

For those of you who want to build and maintain a webhook server, the following provides an example of doing so with Python. Instructions for installing on Centos are embedded in the code. For other operating systems please use the appropriate commands to install the packages and utilities.

This code defaults to listening on port 5018. You can change this on the last line.



Code to make onward calls to other systems must be added at line 80 where a simple example is in the comments. In general, some authentication will be needed, e.g. a bearer token.

#!/usr/bin/env python3

# Disclaimer: Please don't use the following code in the production.

# # install instructions for centos:7
# yum update -y
# yum install -y python3
# pip3 install flask python-dateutil pytz
# vim server.py
# # put content of this file in server.py
# export SCALR_SIGNING_KEY=your-secret-here
# # run a server
# python3 server.py
# #use your webhook endpoint as: http://x.x.x.x:5018/webhook-endpoint/

from flask import Flask
from flask import request
from flask import abort
from flask import jsonify

import hmac
import hashlib
import logging
import os
from datetime import datetime
import dateutil.parser
import pytz

# Configuration variables
SCALR_SIGNING_KEY = os.getenv('SCALR_SIGNING_KEY', 'this-is-secret-key')
SCALR_WEBHOOK = os.getenv('SCALR_WEBHOOK', 'webhook-endpoint')

def validate_request(request):
   """ Validate webhook authenticity """
   if "X-Signature" not in request.headers or "Date" not in request.headers:
      logging.warning("Missing signature headers")
      return False
      logging.warning("SCALR_SIGNING_KEY is not set")
      return False

   # Compute our signature
   date = request.headers["Date"]
   expected_signature = hmac.new(
      SCALR_SIGNING_KEY.encode(), request.data + date.encode(), hashlib.sha256
   if expected_signature != request.headers['X-Signature']:
      logging.warning('Signature does not match')
      return False
   # Check for freshness (this still allows rapid replay attack)
   date = dateutil.parser.parse(date)
   now = datetime.now(pytz.utc)
   delta = abs((now - date).total_seconds())
   if delta >= MAX_AGE_SIGNATURE:
      logging.warning("Signature is too old (%ds)" % delta)
      return False

   return True

app = Flask(__name__)

   logging.info('Config: %s = %s', var, globals()[var] if 'PASS' not in var else '*' * len(globals()[var]))

@app.route('/' + SCALR_WEBHOOK + '/', methods=['POST'])
def webhook_listener():
   if not validate_request(request):
   out={"Status" : "ok"}
   # your custom code that handles webhook can be here
   # This is a very simple sample of doing a POST to a 3rd party system
   # import json
   # import requests
   # api_url = 'http://localhost:5000/create-row-in-gs'
   # create_row_data = {'id': '1235','name':'Joel','created_on':'27/01/2018','modified_on':'27/01/2018','desc':'This # is Joel!!'}
   # print(create_row_data)
   # r = requests.post(url=api_url, json=create_row_data)
   # print(r.status_code, r.reason, r.text)

   return jsonify(out)

if __name__ == '__main__':
   app.run(debug=False, host='', port=5018)