AWS CloudFormation: Understanding the DependsOn Attribute

• Purpose of DependsOn:
  • The DependsOn attribute is used to define the creation order of AWS resources in a CloudFormation template.
  • It ensures that certain resources are created only after the specified dependencies are successfully created.
• Example Scenario:
  • An EC2 instance and an RDS database instance are defined in a CloudFormation template.
  • Without DependsOn, both resources would be created simultaneously.
  • By adding DependsOn: DBInstance to the EC2 instance resource, the RDS database instance (DBInstance) will be created first, followed by the EC2 instance.

Resources:
  MyDBInstance:
    Type: 'AWS::RDS::DBInstance'
    Properties:
      AllocatedStorage: '5'
      DBInstanceClass: 'db.t2.small'
      Engine: 'mysql'
      EngineVersion: '5.6'
      MasterUsername: 'admin'
      MasterUserPassword: 'password'
      VPCSecurityGroups: 
        - !Ref MyDBSecurityGroup

  MyDBSecurityGroup:
    Type: 'AWS::RDS::DBSecurityGroup'
    Properties:
      GroupDescription: 'My database security group'
      EC2VpcId: 'vpc-1a2b3c4d'

  MyEC2Instance:
    Type: 'AWS::EC2::Instance'
    DependsOn: MyDBInstance
    Properties:
      InstanceType: 't2.micro'
      ImageId: 'ami-0abcdef1234567890'
      KeyName: 'my-key-pair'
      NetworkInterfaces:
        - AssociatePublicIpAddress: 'true'
          DeviceIndex: '0'
          SubnetId:
            - !Ref MyEC2SecurityGroup

  MyEC2SecurityGroup:
    Type: 'AWS::EC2::SecurityGroup'
    Properties:
      GroupDescription: 'My EC2 security group'
      VpcId: 'vpc-1a2b3c4d'

• When to Use DependsOn:
  • Use DependsOn when there is no direct reference (using Ref or Fn::GetAtt) between resources that should have a creation order.
  • It is applicable to any resource in the template.
• Template Example: 5-dependson.yml
  • Contains two resources: an EC2 instance (MyEC2Instance) and an S3 bucket (MyBucket).
  • The S3 bucket has a DependsOn attribute set to MyEC2Instance, ensuring the bucket is created after the EC2 instance.

Resources:
  MyS3Bucket:
    Type: 'AWS::S3::Bucket'
    Properties:
      BucketName: 'my-s3-bucket'

  MyEC2Instance:
    Type: 'AWS::EC2::Instance'
    DependsOn: MyS3Bucket
    Properties:
      InstanceType: 't2.micro'
      ImageId: 'ami-0abcdef1234567890'
      KeyName: 'my-key-pair'
      NetworkInterfaces:
        - AssociatePublicIpAddress: 'true'
          DeviceIndex: '0'
          GroupSet:
            - !Ref MyEC2SecurityGroup
          SubnetId: 'subnet-1a2b3c4d'

  MyEC2SecurityGroup:
    Type: 'AWS::EC2::SecurityGroup'
    Properties:
      GroupDescription: 'My EC2 security group'
      VpcId: 'vpc-1a2b3c4d'

• Observing DependsOn in Action:
  • Upon uploading the template and initiating the stack creation, the EC2 instance is created first.
  • The S3 bucket creation begins only after the EC2 instance reaches the CREATE_COMPLETE status.
• DependsOn with Stack Deletion:
  • The deletion order is also influenced by DependsOn.
  • The S3 bucket will be deleted first, followed by the termination of the EC2 instance.

Summary

• DependsOn is crucial for managing resource creation and deletion order in AWS CloudFormation.
• It is particularly useful when there are no intrinsic function references between resources.
• The attribute ensures that dependencies are respected, avoiding potential issues during stack creation and deletion.

Next Steps

• Understand when to use DependsOn vs. intrinsic functions like Ref and Fn::GetAtt.
• Practice creating templates with various dependencies to see how DependsOn affects the creation and deletion processes.