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.
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.