AWS Cloudformation-如何依赖另一个嵌套堆栈中的DependsOn资源

Question

I have a CF parent template with nested stacks in it. What I'm trying to do is set DependsOn attribute in one of the nested stacks, to check for the resource from another nested stack.

Here is my setup:

Parent stack: (Passing the resource reference between nested stacks)

  RDS:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: https://cf-app-stack.s3.eu-west-2.amazonaws.com/infrastructure/rds.yaml
      Parameters:
        EnvironmentName: !Ref AWS::StackName
        DBVPCSecurityGroup: !GetAtt SecurityGroups.Outputs.DBVPCSecurityGroup
        PrivateSubnet1: !GetAtt VPC.Outputs.PrivateSubnet1
        PrivateSubnet2: !GetAtt VPC.Outputs.PrivateSubnet2

  ECS:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: https://cf-app-stack.s3.eu-west-2.amazonaws.com/infrastructure/ecs-cluster.yaml
      Parameters:
        EnvironmentName: !Ref AWS::StackName
        MasterDB: !GetAtt RDS.Outputs.MasterDB
        InstanceType: t2.micro
        ClusterSize: 1
        VPC: !GetAtt VPC.Outputs.VPC
        SecurityGroup: !GetAtt SecurityGroups.Outputs.ECSHostSecurityGroup
        Subnets: !GetAtt VPC.Outputs.PrivateSubnets

nested RDS stack: (exports the DB resource ref)

 MasterDB:
    Type: AWS::RDS::DBInstance
    Properties:
      DBSnapshotIdentifier: arn:aws:rds:eu-west-2:731152906121:snapshot:db-starter-image
      AllocatedStorage: !Ref DBAllocatedStorage
      DBInstanceClass: !Ref DBInstanceClass
      Engine: MySQL
      # Some DB instance properties aren't valid when you restore from a snapshot, such as the MasterUsername and MasterUserPassword properties. 
      #MasterUsername: !Ref DBUser
      #MasterUserPassword: !Ref DBPassword
      MultiAZ: !Ref 'MultiAZ'
      Tags:
      - Key: Name
        Value: !Sub ${EnvironmentName}-Database
      DBSubnetGroupName: !Ref myDBSubnetGroup
      VPCSecurityGroups: [ !Ref DBVPCSecurityGroup ]
    DeletionPolicy: Snapshot

Outputs:
  MasterDB:
    Description: A reference to the created DB
    Value: MasterDB

nested ECS stack: (I want this one to depend on the RDS instance from the above nested stack)

Parameters:
  MasterDB:
    Description: A reference to the created DB
    Type: String

Resources:
  ECSCluster:
    Type: AWS::ECS::Cluster
    Properties:
      ClusterName: !Ref EnvironmentName

  ECSAutoScalingGroup:
    DependsOn: [ECSCluster, !Ref MasterDB]
    Type: AWS::AutoScaling::AutoScalingGroup
    Properties:
      VPCZoneIdentifier: !Ref Subnets
      LaunchConfigurationName: !Ref ECSLaunchConfiguration
      MinSize: !Ref ClusterSize
      MaxSize: !Ref ClusterSize
      DesiredCapacity: !Ref ClusterSize
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName} ECS host
          PropagateAtLaunch: true

See "DependsOn: [ECSCluster, !Ref MasterDB]" in the above code. Am I doing this wrong? I tried other variations trying to satisfy DependsOn but so far no luck.

Answer 1

You don't really need to use DependsOn for you specific Scenario and I think this attribute doesn't even support referring to resources outside of the stack. The reason is that in order to reference a value in a nested stack, it needs to be passed in from Output attributes from another stack. And just passing an Output parameter to a nested stack makes this stack dependent on the other nested stack it was exported from - and that alone achieves your goal.

Taking your code,

nested ECS stack:

Parameters:

  MasterDB:
    Description: Make this stack dependent on RDS resource
    Type: String

That's all you need to do, the parameter does not even need to be used anywhere in the nested stack.

So if one stack is dependent on another, they can only be executed and completed top to bottom, one after another.

For example if:

Stack A: accepts Attr1 Output from stack B

and

Stack B: accepts Attr2 Output from stack A

The above will always fail, because regardless of which stack will be executed first, the Attr param it is dependent on will not be ready.