cfn-response module - AWS CloudFormation

cfn-response module

In your CloudFormation template, you can specify a Lambda function as the target of a custom resource. When you use the ZipFile property to specify your function's source code, you can load the cfn-response module to send responses from your Lambda function to a custom resource. The cfn-response module is a library that simplifies sending responses to the custom resource that invoked your Lambda function. The module has a send method that sends a response object to a custom resource by way of an Amazon S3 presigned URL (the ResponseURL).

The cfn-response module is available only when you use the ZipFile property to write your source code. It isn't available for source code that's stored in Amazon S3 buckets. For code in buckets, you must write your own functions to send responses.


After executing the send method, the Lambda function terminates, so anything you write after that method is ignored.

Loading the cfn-response module

For Node.js functions, use the require() function to load the cfn-response module. For example, the following code example creates a cfn-response object with the name response:

var response = require('cfn-response');

For Python, use the import statement to load the cfnresponse module, as shown in the following example:


Use this exact import statement. If you use other variants of the import statement, CloudFormation doesn't include the response module.

import cfnresponse

send method parameters

You can use the following parameters with the send method.


The fields in a custom resource request.


An object, specific to Lambda functions, that you can use to specify when the function and any callbacks have completed execution, or to access information from within the Lambda execution environment. For more information, see Programming model (Node.js) in the AWS Lambda Developer Guide.


Whether the function successfully completed. Use the cfnresponse module constants to specify the status: SUCCESS for successful executions and FAILED for failed executions.


The Data field of a custom resource response object. The data is a list of name-value pairs.


Optional. The unique identifier of the custom resource that invoked the function. By default, the module uses the name of the Amazon CloudWatch Logs log stream that's associated with the Lambda function.

The value returned for a PhysicalResourceId can change custom resource update operations. If the value returned is the same, it's considered a normal update. If the value returned is different, CloudFormation recognizes the update as a replacement and sends a delete request to the old resource. For more information, see AWS::CloudFormation::CustomResource.


Optional. Indicates whether to mask the output of the custom resource when it's retrieved by using the Fn::GetAtt function. If set to true, all returned values are masked with asterisks (*****), except for information stored in the locations specified below. By default, this value is false.


Using the NoEcho attribute does not mask any information stored in the following:

  • The Metadata template section. CloudFormation does not transform, modify, or redact any information you include in the Metadata section. For more information, see Metadata.

  • The Outputs template section. For more information, see Outputs.

  • The Metadata attribute of a resource definition. For more information, see Metadata attribute.

We strongly recommend you do not use these mechanisms to include sensitive information, such as passwords or secrets.

For more information about using NoEcho to mask sensitive information, see the Do not embed credentials in your templates best practice.



In the following Node.js example, the inline Lambda function takes an input value and multiplies it by 5. Inline functions are especially useful for smaller functions because they allow you to specify the source code directly in the template, instead of creating a package and uploading it to an Amazon S3 bucket. The function uses the cfn-response send method to send the result back to the custom resource that invoked it.


"ZipFile": { "Fn::Join": ["", [ "var response = require('cfn-response');", "exports.handler = function(event, context) {", " var input = parseInt(event.ResourceProperties.Input);", " var responseData = {Value: input * 5};", " response.send(event, context, response.SUCCESS, responseData);", "};" ]]}


ZipFile: > var response = require('cfn-response'); exports.handler = function(event, context) { var input = parseInt(event.ResourceProperties.Input); var responseData = {Value: input * 5}; response.send(event, context, response.SUCCESS, responseData); };


In the following Python example, the inline Lambda function takes an integer value and multiplies it by 5.


"ZipFile" : { "Fn::Join" : ["\n", [ "import json", "import cfnresponse", "def handler(event, context):", " responseValue = int(event['ResourceProperties']['Input']) * 5", " responseData = {}", " responseData['Data'] = responseValue", " cfnresponse.send(event, context, cfnresponse.SUCCESS, responseData, \"CustomResourcePhysicalID\")" ]]}


ZipFile: | import json import cfnresponse def handler(event, context): responseValue = int(event['ResourceProperties']['Input']) * 5 responseData = {} responseData['Data'] = responseValue cfnresponse.send(event, context, cfnresponse.SUCCESS, responseData, "CustomResourcePhysicalID")

Module source code

The following is the response module source code for the Node.js functions. Review it to understand what the module does and for help with implementing your own response functions.

// Copyright, Inc. or its affiliates. All Rights Reserved. // SPDX-License-Identifier: MIT-0 exports.SUCCESS = "SUCCESS"; exports.FAILED = "FAILED"; exports.send = function(event, context, responseStatus, responseData, physicalResourceId, noEcho) { var responseBody = JSON.stringify({ Status: responseStatus, Reason: "See the details in CloudWatch Log Stream: " + context.logStreamName, PhysicalResourceId: physicalResourceId || context.logStreamName, StackId: event.StackId, RequestId: event.RequestId, LogicalResourceId: event.LogicalResourceId, NoEcho: noEcho || false, Data: responseData }); console.log("Response body:\n", responseBody); var https = require("https"); var url = require("url"); var parsedUrl = url.parse(event.ResponseURL); var options = { hostname: parsedUrl.hostname, port: 443, path: parsedUrl.path, method: "PUT", headers: { "content-type": "", "content-length": responseBody.length } }; var request = https.request(options, function(response) { console.log("Status code: " + parseInt(response.statusCode)); context.done(); }); request.on("error", function(error) { console.log("send(..) failed executing https.request(..): " + error); context.done(); }); request.write(responseBody); request.end(); }

The following is the response module source code for Python 2 and 3 functions:

# Copyright, Inc. or its affiliates. All Rights Reserved. # SPDX-License-Identifier: MIT-0 from __future__ import print_function import urllib3 import json SUCCESS = "SUCCESS" FAILED = "FAILED" http = urllib3.PoolManager() def send(event, context, responseStatus, responseData, physicalResourceId=None, noEcho=False, reason=None): responseUrl = event['ResponseURL'] print(responseUrl) responseBody = { 'Status' : responseStatus, 'Reason' : reason or "See the details in CloudWatch Log Stream: {}".format(context.log_stream_name), 'PhysicalResourceId' : physicalResourceId or context.log_stream_name, 'StackId' : event['StackId'], 'RequestId' : event['RequestId'], 'LogicalResourceId' : event['LogicalResourceId'], 'NoEcho' : noEcho, 'Data' : responseData } json_responseBody = json.dumps(responseBody) print("Response body:") print(json_responseBody) headers = { 'content-type' : '', 'content-length' : str(len(json_responseBody)) } try: response = http.request('PUT', responseUrl, headers=headers, body=json_responseBody) print("Status code:", response.status) except Exception as e: print("send(..) failed executing http.request(..):", e)