Custom Resource Helper

Simplify best practice Custom Resource creation, sending responses to CloudFormation and providing exception, timeout trapping, and detailed configurable logging.

Features

• Dead simple to use, reduces the complexity of writing a CloudFormation custom resource
• Guarantees that CloudFormation will get a response even if an exception is raised
• Returns meaningful errors to CloudFormation Stack events in the case of a failure
• Polling enables run times longer than the lambda 15 minute limit
• JSON logging that includes request id's, stack id's and request type to assist in tracing logs relevant to a particular CloudFormation event
• Catches function timeouts and sends CloudFormation a failure response
• Static typing (mypy) compatible

Installation

Install into the root folder of your lambda function

cd my-lambda-function/
pip install crhelper -t .

Example Usage

This blog covers usage in more detail.

from __future__ import print_function
from crhelper import CfnResource
import logging

logger = logging.getLogger(__name__)
# Initialise the helper, all inputs are optional, this example shows the defaults
helper = CfnResource(json_logging=False, log_level='DEBUG', boto_level='CRITICAL', sleep_on_delete=120, ssl_verify=None)

try:
    ## Init code goes here
    pass
except Exception as e:
    helper.init_failure(e)


@helper.create
def create(event, context):
    logger.info("Got Create")
    # Optionally return an ID that will be used for the resource PhysicalResourceId, 
    # if None is returned an ID will be generated. If a poll_create function is defined 
    # return value is placed into the poll event as event['CrHelperData']['PhysicalResourceId']
    #
    # To add response data update the helper.Data dict
    # If poll is enabled data is placed into poll event as event['CrHelperData']
    helper.Data.update({"test": "testdata"})

    # To return an error to cloudformation you raise an exception:
    if not helper.Data.get("test"):
        raise ValueError("this error will show in the cloudformation events log and console.")
    
    return "MyResourceId"


@helper.update
def update(event, context):
    logger.info("Got Update")
    # If the update resulted in a new resource being created, return an id for the new resource. 
    # CloudFormation will send a delete event with the old id when stack update completes


@helper.delete
def delete(event, context):
    logger.info("Got Delete")
    # Delete never returns anything. Should not fail if the underlying resources are already deleted.
    # Desired state.


@helper.poll_create
def poll_create(event, context):
    logger.info("Got create poll")
    # Return a resource id or True to indicate that creation is complete. if True is returned an id 
    # will be generated
    return True


def handler(event, context):
    helper(event, context)

Polling

If you need longer than the max runtime of 15 minutes, you can enable polling by adding additional decorators for poll_create, poll_update or poll_delete. When a poll function is defined for create/update/delete the function will not send a response to CloudFormation and instead a CloudWatch Events schedule will be created to re-invoke the lambda function every 2 minutes. When the function is invoked the matching @helper.poll_ function will be called, logic to check for completion should go here, if the function returns None then the schedule will run again in 2 minutes. Once complete either return a PhysicalResourceID or True to have one generated. The schedule will be deleted and a response sent back to CloudFormation. If you use polling the following additional IAM policy must be attached to the function's IAM role:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "lambda:AddPermission",
        "lambda:RemovePermission",
        "events:PutRule",
        "events:DeleteRule",
        "events:PutTargets",
        "events:RemoveTargets"
      ],
      "Resource": "*"
    }
  ]
}

Certificate Verification

To turn off certification verification, or to use a custom CA bundle path for the underlying boto3 clients used by this library, override the ssl_verify argument with the appropriate values. These can be either:

• False - do not validate SSL certificates. SSL will still be used, but SSL certificates will not be verified.
• path/to/cert/bundle.pem - A filename of the CA cert bundle to uses. You can specify this argument if you want to use a different CA cert bundle than the one used by botocore.

Use CDK to depoy a Custom Resource that uses Custom Resource Helper

You can use the AWS Cloud Development Kit (AWS CDK) to deploy a Custom Resource that uses Custom Resource Helper. AWS CDK is an open-source software development framework for defining cloud infrastructure in code and provisioning it through AWS CloudFormation.

Note: crhelper is not intended to be used with AWS CDK using the Provider construct.

AWS CDK template example

from aws_cdk import (
    ...
    aws_lambda as _lambda,
    CustomResource,
)

crhelperSumResource = _lambda.Function(...)

customResource = CustomResource(
  self, 
  'MyCustomResource'
  serviceToken = crhelperSumResource.function_arn,
  properties = {
    'No1': 1,
    'No2': 2
  },
)

Credits

Decorator implementation inspired by https://github.com/ryansb/cfn-wrapper-python

Log implementation inspired by https://gitlab.com/hadrien/aws_lambda_logging

License

This library is licensed under the Apache 2.0 License.