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Simplify best practice Custom Resource creation, sending responses to CloudFormation and providing exception, timeout trapping, and detailed configurable logging.
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Using AWS Partition instead of hardcoding aws in targets
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README.md

Custom Resource Helper

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

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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

Installation

Install into the root folder of your lambda function

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

Example Usage

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')

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"})
    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": "*"
    }
  ]
}

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.

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