Webhooks and Endpoints

Definition and Scope


Webhooks are very simple, yet very powerful way to integrate with external systems such as a CMDB, IPAM, ITSM or Monitoring tools. Using Webhooks, Scalr delivers HTTP notifications to external services of your choosing when lifecycle events occur on your infrastructure, such as a Farm being launched, a server being provisioned or decommissioned. While Scalr Orchestration could be used to integrate with external systems, you may wish to trigger integration events at the Farm/Service Catalog level, e.g. for Approval workflows. For this you should use Webhooks. There are also a few constraints with orchestration as a script runs on the server you are provisioning rather than a central location. This may also mean you need to use a Webhook under the following circumstances:

  • Those external systems may be firewalled off, and not accessible from your instances. This is commonplace for CMDBs.
  • Those external systems may require credentials that you wouldn’t feel comfortable distributing to all your instances.
  • Those external systems may need to be notified when instances go down or crash, in which case there may not be an instance to run the action on. This is typical for an alerting system.

Unlike Orchestration, Webhook Notifications are delivered by Scalr itself to those external systems, which effectively lifts those limitations.



Webhooks and generally one way, in that the webhook payload is sent to the Endpoint and a final response is expected immediately. However Webhooks can be asynchronously bi-directional in the context of Approvals. If Scalr receives a response from an Approval Endpoint indicating it is processing the request (HTTP 102), then Scalr will pause the execution of the triggering event until a final reply is received from the Endpoint. This caters for external approvals through systems like ServiceNow where there is an inherent delay in the process whilst the approval request is processed. In these circumstances the affected Farm will show a status of “PENDING LAUNCH” with an appropriate message showing the reason for the delay.

To setup an integration with an external system the following will be needed:

  • A Webhook App
  • An Endpoint that is serving the Webhook App
  • A Webhook configured in Scalr


Scalr can provide example webhooks for a variety of use cases, including Approvals, sending emails, logging to a CMDB, IPAM integration etc. Please contact Scalr Support for more details or browse the Scalr Github NEWWIN repository. Note that code used from the repository is NOT deemed to be production ready and must be modified and fully tested by customers before using in a production environment.

Configuring Endpoints

Before creating a Webhook, an Endpoint is needed for the Webhook to contact. The Endpoint is the URL of the application that is running the Webhook. To create an Endpoint at the Account or Scalr scopes, click on the Scalr icon on the top left MENU_ACC and then click on Integration Hub > Endpoints.

Field Description
Name The name of the endpoint.
URL The URL the endpoint can be reached at.
Status A radio button to turn make the endpoint active/ inactive within Scalr.
Shared Determines if the endpoint can be used at a lower level. This is only seen at the Account and Scalr scopes.

Configuring Webhooks


To use an Endpoint for Farm Approvals you DO NOT need to configure a Webhook. Approvals are set up when configuring an Environment and they use the Endpoint directly. Please see Approvals for more details.

The next step is to configure the Webhook, to do this at the Account or Scalr scope, click on the Scalr icon on the top left MENU_ACC and then click on Integration Hub > Webhooks.

Field Description
Name Name of the webhook
Endpoints The endpoint URL which should be used for this webhook.
Events The event at which the webhook call should be triggered. This can be Server or Farm lifecycle event such as BeforeInstanceLaunch and FarmLaunched. See Event Descriptions.
Farms Which farm(s) the webhook should trigger on. If left blank it will be trigger on all farms within the scope that the webhook is set.
Advanced Settings - Timeout The length of time in which Scalr should get a response back from the webhook before timing out.
Advance Settings - Max. Delivery Attempts How many times Scalr should attempt to send a payload if it initially fails to send.
Do not expose private GlobalVariables in webhook payload If a Global Variable is marked as hidden and this is checked then the Global Variable will not be passed in the payload
User Data Webhook user data can be used to add extra information to Webhook Notifications. Webhook User Data is passed as-is in the userData field of the Webhook Notification’s payload so if you configure it in a specific format (i.e. JSON) you will need to manually decode it in your webhook handler.

Webhook Payloads

A Webhook payload will dump all data it knows about a server built in Scalr to the endpoint application. This includes the Built-In Variables and Global Variables, which you can find out more about here: Global Variables. The application that is running on the endpoint should be configured in a way that is parses for the relevant data.

Webhook Payloads are dispatched as an HTTP POST request to your Endpoint. Notable features of the request include:

  • A JSON body containing the Webhook Notification
  • A Date HTTP Header, containing the timestamp when the Webhook Notification was dispatched
  • A X-Signature HTTP Header, containing a cryptographic signature of the Webhook Notification
  • A X-Scalr-Webhook-Id HTTP Header, containing the ID of the Webhook Notification

The JSON body contains the following keys:

Parameter Description
eventName The name of the event that triggered this Webhook Notification
eventId The ID of the event that triggered this Webhook Notification
timestamp The UTC time (formatted as an HTTP date) when the the event that triggered this Webhook Notification fired.
data View Webhook User Data above.
userData The list of Global Variables that are in scope for the element that fired the Event that triggered this Webhook Notification. The list may vary depending on the Event.
configurationId The internal ID of the Webhook configuration that triggered this Webhook Notification.
endpointId The internal ID of the webhook endpoint the Webhook Notification is sent to.

Sample Payload:

  "configurationId": "d6e3a602-4a86-4871-9c02-931b543c6df8",
  "data": {
    "SCALR_ACCOUNT_ID": "1",
    "SCALR_ACCOUNT_NAME": "ACME Operations",
    "SCALR_BEHAVIORS": "base",
    "SCALR_CLOUD_LOCATION": "europe-west1-b",
    "SCALR_CLOUD_LOCATION_ZONE": "europe-west1-b",
    "SCALR_CLOUD_SERVER_ID": "bdc02d5b-613f-4c34-b4fc-04f01db4f118",
    "SCALR_COST_CENTER_ID": "3f54770e-bf1a-11e3-92c5-000feae9c516",
    "SCALR_COST_CENTER_NAME": "ACME Operations",
    "SCALR_ENV_ID": "2",
    "SCALR_ENV_NAME": "IT Labs US",
    "SCALR_EVENT_CLOUD_LOCATION": "europe-west1-b",
    "SCALR_EVENT_CLOUD_LOCATION_ZONE": "europe-west1-b",
    "SCALR_EVENT_CLOUD_SERVER_ID": "bdc02d5b-613f-4c34-b4fc-04f01db4f118",
    "SCALR_EVENT_COST_CENTER_ID": "3f54770e-bf1a-11e3-92c5-000feae9c516",
    "SCALR_EVENT_ENV_ID": "2",
    "SCALR_EVENT_FARM_HASH": "501a9f8cab6b5f",
    "SCALR_EVENT_FARM_ID": "1824",
    "SCALR_EVENT_FARM_NAME": "Temp Scalr sandbox",
    "SCALR_EVENT_FARM_OWNER_EMAIL": "aloys@scalr.com",
    "SCALR_EVENT_FARM_ROLE_ALIAS": "base-ubuntu1604",
    "SCALR_EVENT_ID": "333d5bfa",
    "SCALR_EVENT_IMAGE_ID": "scalr-demo/images/ubuntu1604-gce-scalr-1497530014",
    "SCALR_EVENT_LAUNCHED_BY_EMAIL": "aloys@scalr.com",
    "SCALR_EVENT_NAME": "HostUp",
    "SCALR_EVENT_PROJECT_ID": "30c59dba-fc9b-4d0f-83ec-4b5043b12f72",
    "SCALR_EVENT_ROLE_NAME": "base-ubuntu1604",
    "SCALR_EVENT_SERVER_HOSTNAME": "scalrserver01",
    "SCALR_EVENT_SERVER_ID": "bdc02d5b-613f-4c34-b4fc-04f01db4f118",
    "SCALR_EVENT_SERVER_TYPE": "n1-standard-4",
    "SCALR_FARM_HASH": "501a9f8cab6b5f",
    "SCALR_FARM_ID": "1824",
    "SCALR_FARM_NAME": "Temp Scalr sandbox",
    "SCALR_FARM_OWNER_EMAIL": "aloys@scalr.com",
    "SCALR_FARM_ROLE_ALIAS": "base-ubuntu1604",
    "SCALR_FARM_ROLE_ID": "2682",
    "SCALR_FARM_TEAM": "",
    "SCALR_ID": "333d5bfa",
    "SCALR_IMAGE_ID": "scalr-demo/images/ubuntu1604-gce-scalr-1497530014",
    "SCALR_LAUNCHED_BY_EMAIL": "aloys@scalr.com",
    "SCALR_PROJECT_BC": "PR-01",
    "SCALR_PROJECT_ID": "30c59dba-fc9b-4d0f-83ec-4b5043b12f72",
    "SCALR_ROLE_NAME": "base-ubuntu1604",
    "SCALR_SERVER_HOSTNAME": "scalrserver01",
    "SCALR_SERVER_ID": "bdc02d5b-613f-4c34-b4fc-04f01db4f118",
    "SCALR_SERVER_TYPE": "n1-standard-4",
    "endpointId": "4dd66e35-9955-45cc-a6fb-f24e945bf16a",
    "eventId": "417692b9-2f1e-4d08-a7a5-fbb8954a288d",
    "eventName": "HostUp",
    "timestamp": "Wed 06 Sep 2017 12:21:17 UTC",
    "userData": ""

Webhook Security and Authentication

Scalr adds a cryptographic signature to Webhook Notifications in order for you to be able to validate that they were indeed generated by Scalr.

To validate Webhook Notifications, follow this procedure:

  • Obtain the Webhook Endpoint Signing Key for your Endpoint, this is shown as soon as the Webhook is created. Note that the Signing Key is unique to an Endpoint.
  • When receiving a Webhook message, compute its signature using the following algorithm (see below for code samples)
    • Concatenate the JSON message payload and the Date header
    • Compute a HMAC digest of the concatenated message, using the SHA-1 algorithm
    • Retrieve the hexadecimal value of the HMAC digest (this is the signature)
  • Compare the signature you computed to the one that is found in the X-Signature HTTP Header. For maximum security, ensure you perform a constant-time comparison.
    • If the signatures match, the message is authentic, and was indeed sent by Scalr
    • If the signatures do not match, the message is a forgery, and was not sent by Scalr

To compute a Webhook Notification signature in PHP, use the following algorithm. We assume that the $payload variable contains the request JSON payload, and that timestamp contains the Date Header.

$canonical_string = $payload . $timestamp;
$signature = hash_hmac('SHA1', $canonical_string, $endpoint->securityKey);

Webhook History

To track Webhook calls as well as troubleshoot if you suspect an integration is failing, there is a Webhook History section that will provide this information. To look at the Webhook History, go to the Account or Scalr scope, click on the Scalr icon on the top left MENU_ACC and then click on Integration Hub > Webhooks > History.


Highly Available Webhooks

Highly-available Webhooks can be simply deployed using DNS-based high availability. To achieve this, you should ensure that:

  • A DNS name is assigned to the webhook service (e.g. webhook.scalr.corp.com).
  • The Webhook Endpoint is Scalr is configured with this DNS name.
  • The Webhook server itself can be replicated to make it highly available. Since most webhook handlers are stateless, this should not be an issue.
  • Several instances of the webhook server are deployed.
  • The DNS server returns several A records for the webhook service, one for each webhook server instance.
; <<>> DiG 9.10.6 <<>> webhook.scalr.corp.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 60289
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

; EDNS: version: 0, flags:; udp: 4096
;webhook.scalr.corp.com. IN A

webhook.scalr.corp.com. 3600 IN A
webhook.scalr.corp.com. 3600 IN A

;; Query time: 15 msec
;; WHEN: Mon Jun 25 20:17:15 CEST 2018
;; MSG SIZE rcvd: 90

If your webhook server is deployed from Scalr, and your Scalr installation is integrated with your DNS system, steps 4 and 5 can be as simple as scaling up the Farm Role of the webhook.

If all these conditions are met, Scalr will correctly and quickly retry webhook requests if one of the servers becomes unavailable, making the webhook integration highly available. As long as one server is reachable, you will not see any errors in the Scalr webhook log.

Naturally, this should be coupled with a monitoring and alerting system to ensure that this doesn’t just hide failures.

Example Webhooks

To see examples of Webhooks please visit our Github site: Scalr Github NEWWIN