Custom Resource (AWS)

Implement custom AWS CloudFormation resources with Python Lambda functions.

This helps you reference non-AWS resources within CloudFormation - for instance, let’s get a StatusCake alert in our stack:

Type: Custom::StatusCakeTest
Version: 1.0
Properties:
  WebsiteName: vitalwebsite.com
  WebsiteURL: http://vitalwebsite.com
  CheckRate: 300
  TestType: HTTP

(Implementation left as an exercise for the reader)

See also

• Sample code in the examples directory.
• AWS docs

Lambda Handler

Here’s a simple resource:

from custom_resource import BaseHandler

class Handler(BaseHandler):
    def create(self, event, context):
        return "CreatedId", {"AndSome": "MetaData"}

    def update(self, event, context):
        return "UpdatedId", {"AndSome": "MetaData"}

    def delete(self, event, context):
        return "DeletedId", {"AndSome": "MetaData"}

handler = Handler()

We extend the BaseHandler class, and implement create, update and delete methods. All of these methods are required.

The methods must either:

• Return a string representing a resource ID. This can be used within your CFN template via the Ref function.
• Return a resource ID string, and a dict containing strings for keys and values. These key/value pairs can be used with the GetAtt function.
• Return a custom_resource.Success or custom_resource.Failed object.
• Return a custom_resource.Defer object, signifying you’ll process this asynchronously. See async responses below.
• Raise an exception.

BaseHandler will respond to CloudFormation unless Defer is returned.

Async responses

Your Handler method can return Defer to signal asynchronous processing.

To later respond, you need a copy of the original event object. This will be passed to the Responder class, e.g:

with Responder(event) as responder:
    responder.success(physical_resource_id="123", data={"Meta": "Data"})
    # or
    responder.failure(physical_resource_id="456", reason="Something went wrong")

Using with, your resource will always respond to CloudFormation even on exception - ensuring your stack doesn’t stall and eventually timeout.