Developing a Self Destructing Cloudformation Stack

In this tutorial, we will develop a self destructing cloudformation stack. The stack can ben named harakiri since it will delete not only the resources created but also itself after a specified duration. The stack is best suited for timely

For example, a potential customer wants to try your SaaS before paid subscription and you do not want to be charged for the unused resources.

It would be better have a basic knowledge about CloudFormation.

AWS Cloud Formation is a infrastructure as a code that creates, updates and deletes resources with JSON or YAML configuration files.

Defining Stack

AWS Cloud Formation template has five sections:

Mapping
Parameters
Conditions
Resources
Outputs

In the resources section we define the AWS Resources that will be created. I will keep it simple and focus on only the resources section.

Trivial Section

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AWSTemplateFormatVersion: "2010-09-09"

Mappings:
  RegionMap:
    us-east-1:
      AMI: "ami-0ff8a91507f77f867"

Parameters: {}

Conditions: {}

There is nothing fancy here. Although we are not going to create and EC2 instance, at least one region (the region you will be creating the stack) is required. Also AMI (Amazon Machine Image) can be wrong. Just skip it.

IAM Role for The Lambda

We use lambda for triggering the stack delete process. This IAM role must be priviled to delete all resources created with stack. You would like give full privilege this role but be safe by giving fine grained privilege by giving privilege to this role for only the resources created with stack.

We gave privilege to this role:

Deleting all cloud formation stacks
Deleting all IAM roles. (This role will able to delete itself)
Deleting all lambda functions
Deleting all cloudwatch events and rules

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Resources:
  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:*:*:*
          
          - Effect: Allow
            Action:
              - cloudformation:DeleteStack
            Resource: "*"
          
          - Effect: Allow
            Action: 
              - iam:DeleteRolePolicy
              - iam:DeleteRole
            Resource: "*"
          
          - Effect: Allow
            Action: 
              - lambda:DeleteFunction
            Resource: "*"
          
          - Effect: Allow
            Action:
              - events:RemoveTargets
            Resource: "*"

          - Effect: Allow
            Action:
              - events:DeleteRule
            Resource: "*"

          - Effect: Allow
            Action: 
              - lambda:RemovePermission
            Resource: "*"

Lambda Function

This lambda function will start stack delete process.

How does this lambda function trigger?
Cloud Watch Rules.

We will define a cloud watch timer later. The cloud watch timer will trigger every X times but since the lambda function will delete all resources created with stack, probably there would be no second execution.

Boto is a Python package that provides interfaces to Amazon Web Services.

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import boto3
client = boto3.client('cloudformation')

def handler(event, context):
return client.delete_stack(
    StackName=event.StackName
)

Source code of lambda function is simple. It requires a simple JSON object with a StackName property which is the name of the Cloud Formation stack.

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  HarakiriLambda:
    Type: AWS::Lambda::Function
    Properties:
      Handler: index.handler
      Role: !GetAtt LambdaExecutionRole.Arn
      Code:
        ZipFile: |
          import boto3
          client = boto3.client('cloudformation')

          def handler(event, context):
            return client.delete_stack(
                StackName=event.StackName
            )

      Runtime: python3.6
  PermissionForEventsToInvokeLambda: 
    Type: AWS::Lambda::Permission
    Properties: 
      FunctionName: 
        Ref: "HarakiriLambda"
      Action: "lambda:InvokeFunction"
      Principal: "events.amazonaws.com"
      SourceArn: 
        Fn::GetAtt: 
          - "HarakiriRule"
          - "Arn"  

CloudWatch Timer (Rule)

We will define a cloud watch cron rule that will be executed every thirty minutes but we expect it does not executed twice about five or ten minutes after the first execution, all resources including this rule will be deleted.

We prepare a simple JSON object at line 107, for the lambda by substituting the stack name.

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  HarakiriRule:
    Type: AWS::Events::Rule
    Properties: 
      Description: "ScheduledRule"
      ScheduleExpression: "cron(0/30 * * * ? *)"
      State: "ENABLED"
      Targets: 
        - 
          Arn: 
            Fn::GetAtt: 
              - "HarakiriLambda"
              - "Arn"
          Id: "HarakiriLambdaV1"
          Input: !Sub
          - "{\"StackName\": \"${Stack}\"}"
          - { Stack: !Ref "AWS::StackName" }   

Final

Go to AWS Cloud Formation Console: console.aws.amazon.com/cloudformation
Download the full template from template.yml and change the 30 minutes section to 2-3 minutes.
Click “Create Stack” button and upload the template.
Hit the “Next” button and give a name to your stack.
Watch the events tabs at the bottom.

You will see the stack create process, just after 2-3 minutes, “stack delete in progress” event will appear.

From the CLI you can create the stack by typing:

aws cloudformation deploy \
		--stack-name myteststack \
		--capabilities CAPABILITY_IAM \
--template-file template.yml

Conclusion

I hope you enjoyed and learnt a lot.

Happy coding.