How can I resolve the error that I receive when I subscribe an AWS Lambda function to a push-based event source in AWS CloudFormation?

Short description

The AWS::Lambda::EventSourceMapping resource is designed for pull-based event sources, such as Amazon DynamoDB event streams and Amazon Kinesis. With push-based event sources, such as Amazon S3 event notifications or Amazon SNS messages, the event source is responsible for invoking the Lambda function. For a push event source to invoke a Lambda function, the function’s resource policy must authorize a supported event source.

Resolution

In your AWS CloudFormation template, add a resource-based policy using the AWS::Lambda::Permission resource.

For example, the following resource-based policy allows an Amazon SNS topic to invoke a Lambda function:

"LambdaResourcePolicy": {
  "Type": "AWS::Lambda::Permission",
  "Properties": {
    "FunctionName" : { "Ref" : "MyFunction" },
    "Principal": "sns.amazonaws.com",
    "Action": "lambda:InvokeFunction",
    "SourceArn" : { "Ref" : "MySNSTopic" }
  }
}

For an Amazon SNS topic event source, you must define a topic policy with the required permissions.

For an Amazon S3 event source, you must have a notification configuration statement that subscribes the Lambda function to the Amazon S3 bucket. For example:

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Parameters": {
    "BucketPrefix": {
      "Type": "String",
      "Default": "test-bucket-name"
    }
  },
  "Resources": {
    "EncryptionServiceBucket": {
      "DependsOn": "LambdaInvokePermission",
      "Type": "AWS::S3::Bucket",
      "Properties": {
        "BucketName": {
          "Fn::Sub": "${BucketPrefix}-encryption-service"
        },
        "NotificationConfiguration": {
          "LambdaConfigurations": [
            {
              "Function": {
                "Fn::GetAtt": [
                  "AppendItemToListFunction",
                  "Arn"
                ]
              },
              "Event": "s3:ObjectCreated:*",
              "Filter": {
                "S3Key": {
                  "Rules": [
                    {
                      "Name": "suffix",
                      "Value": "zip"
                    }
                  ]
                }
              }
            }
          ]
        }
      }
    },
    "LambdaInvokePermission": {
      "Type": "AWS::Lambda::Permission",
      "Properties": {
        "FunctionName": {
          "Fn::GetAtt": [
            "AppendItemToListFunction",
            "Arn"
          ]
        },
        "Action": "lambda:InvokeFunction",
        "Principal": "s3.amazonaws.com",
        "SourceAccount": {
          "Ref": "AWS::AccountId"
        },
        "SourceArn": {
          "Fn::Sub": "arn:aws:s3:::${BucketPrefix}-encryption-service"
        }
      }
    },
    "AppendItemToListFunction": {
      "Type": "AWS::Lambda::Function",
      "Properties": {
        "Handler": "index.handler",
        "Role": {
          "Fn::GetAtt": [
            "LambdaExecutionRole",
            "Arn"
          ]
        },
        "Code": {
          "ZipFile": {
            "Fn::Join": [
              "",
              [
                "exports.handler = function(event, context) {",
                "console.log('Received event: ', JSON.stringify(event, null, 2));",
                "};"
              ]
            ]
          }
        },
        "Runtime": "nodejs8.10"
      }
    },
    "LambdaExecutionRole": {
      "Type": "AWS::IAM::Role",
      "Properties": {
        "AssumeRolePolicyDocument": {
          "Version": "2012-10-17",
          "Statement": [
            {
              "Effect": "Allow",
              "Principal": {
                "Service": [
                  "lambda.amazonaws.com"
                ]
              },
              "Action": [
                "sts:AssumeRole"
              ]
            }
          ]
        },
        "Path": "/",
        "Policies": [
          {
            "PolicyName": "root",
            "PolicyDocument": {
              "Version": "2012-10-17",
              "Statement": [
                {
                  "Effect": "Allow",
                  "Action": [
                    "logs:*"
                  ],
                  "Resource": "arn:aws:logs:*:*:log-group:/path/<log-group-name>:log-stream:<log-stream-name>"
                }
              ]
            }
          }
        ]
      }
    }
  }
}

In the preceding example, the S3 bucket and notification configuration are created at the same time. The example avoids a circular dependency by using the Fn::Join intrinsic function and the DependsOn attribute to create the resources in the following order: 1) IAM role, 2) Lambda function, 3) Lambda permission, and then 4) S3 bucket. For more information, see How do I avoid the "Unable to validate the following destination configurations" error with Lambda event notifications in AWS CloudFormation?

Related information

CloudFormation best practices

Lambda permissions

Update behaviors of stack resources