AWS CloudFormation CodePipeline、ParameterOverrides、将列表传递给嵌套堆栈

Question

The scope of the question:

AWS CodeBuild, ParametersOverrides section

Using Parameter Override Functions with CodePipeline Pipelines

1) I can pass a string to ParameterOverrides, and then pass them to a nested stack, as it is described below

-create a string parameter

-pass it to ParameterOverrides section of the AWS CodeBuild project

-parse parameter in the nested stack

Quoting the official documentation: AWS::CloudFormation::Stack

If you use the Ref function to pass a parameter value to a nested stack, comma-delimited list parameters must be of type String. In other words, you cannot pass values that are of type CommaDelimitedList to nested stacks.

2) I can not figure out on how to transform a List to a string inside a ParameterOverrides section with the configuration below:

-define a Parameter with type of list ServiceSubnets: Type: List Description: Subnets associated with the service

-try to pass a parameter inside ParameterOverrides section as a value to a nested stack, apply Join function to transform it to a string

 ParameterOverrides: !Sub | { "ImageURI" : { "Fn::GetParam" : [ "BuildOutput", "imageDetail.json", "ImageURI" ] }, "ApplicationRepoName": "${ApplicationRepoName}", "Cluster": "${Cluster}", "ListenerArn": "${ListenerArn}", "TargetGroup": "${TargetGroup}", "ServiceDesiredCount": "${ServiceDesiredCount}", "ServiceLoadBalancerPath": "${ServiceLoadBalancerPath}", "ServiceContainerPort": "${ServiceContainerPort}", "ServiceSecurityGroups": { "Fn::Join" : [ ",", "${ServiceSecurityGroups}"] ] }, "ServiceSubnets": { "Fn::Join" : [ ",", "${ServiceSubnets}" ] }, "TaskContainerPort": "${TaskContainerPort}", "TaskCpu": "${TaskCpu}", "TaskMemory": "${TaskMemory}", "TaskExecutionRoleArn": "${TaskExecutionRoleArn}" }

So I expect that the List should be transformed to a String and String should be passed and then used in the nested stack, however, attempt to deploy such stack returned an error:

Template error: variable ServiceSecurityGroups in Fn::Sub expression does not resolve to a string

My question:

Is it possible to use Join function inside ParameterOverrides section to transform a List to a String?

If yes, I would much appreciate if you share with me some example that illustrates how to do this.

Thank you.

Answer 1

Unfortunately you cannot use intrinsic functions within the Fn::Sub Syntax Reference

You can still use other intrinsic functions such as Fn::Join within Fn::Sub variable map. So your ParameterOverrides will be as follows instead:

ParameterOverrides: !Sub 
  - |
    {
      "ImageURI" : { "Fn::GetParam" : [ "BuildOutput", "imageDetail.json", "ImageURI" ] },
      "ApplicationRepoName": "${ApplicationRepoName}",
      "Cluster": "${Cluster}",
      "ListenerArn": "${ListenerArn}",
      "TargetGroup": "${TargetGroup}",
      "ServiceDesiredCount": "${ServiceDesiredCount}",
      "ServiceLoadBalancerPath": "${ServiceLoadBalancerPath}",
      "ServiceContainerPort": "${ServiceContainerPort}",
      "ServiceSecurityGroups": "${KEY_NAME_1}",
      "ServiceSubnets": "${KEY_NAME_2}",
      "TaskContainerPort": "${TaskContainerPort}",
      "TaskCpu": "${TaskCpu}",
      "TaskMemory": "${TaskMemory}",
      "TaskExecutionRoleArn": "${TaskExecutionRoleArn}"
    }
 - KEY_NAME_1: !Join [ ",", [ !Ref ServiceSecurityGroups ] ]
   KEY_NAME_2: !Join [ ",", [ !Ref ServiceSubnets ] ]

If your ServiceSecurityGroups and ServiceSubnets are already lists then remove the square braces around the !Ref statements.