Modularize body of a CloudFormation Resource

Question

I have a working CloudFormation template that creates an AWS Elastic Container Service cluster. Here is a sanitized version of one of the TaskDefinitions:

Resources:
  MyTaskDefinition:
    Properties:
      ContainerDefinitions:
      - Environment:
        - Name: ENV_VAR_1
        Value: !Ref EnvVar1
        - Name: ENV_VAR_2
        Value: !Ref EnvVar2
      Image: !Ref Image1
      LogConfiguration: !Ref MyLogConfiguration
      Memory: 2048
      Name: my-container-1
      - Environment:
        - Name: ENV_VAR_1
        Value: !Ref EnvVar1
        - Name: ENV_VAR_2
        Value: !Ref EnvVar2
      Image: !Ref Image2
      LogConfiguration: !Ref MyLogConfiguration
      Memory: 2048
      Name: my-container-2
    Type: AWS::ECS::TaskDefinition

As this TaskDefinition grows to define several containers, each of which have multiple environment variables, it becomes difficult to read. Instead I would like to do the following:

Resources:
  ContainerDefinition1:
    Properties:
      Environment:
        - Name: ENV_VAR_1
        Value: !Ref EnvVar1
        - Name: ENV_VAR_2
        Value: !Ref EnvVar2
      Image: !Ref Image1
      LogConfiguration: !Ref MyLogConfiguration
      Memory: 2048
      Name: my-container-1
    Type: AWS::ECS::TaskDefinition::ContainerDefinition
  ContainerDefinition2:
    Properties:
      Environment:
        - Name: ENV_VAR_1
        Value: !Ref EnvVar1
        - Name: ENV_VAR_2
        Value: !Ref EnvVar2
      Image: !Ref Image2
      LogConfiguration: !Ref MyLogConfiguration
      Memory: 2048
      Name: my-container-2
    Type: AWS::ECS::TaskDefinition::ContainerDefinition
  MyTaskDefinition:
    Properties:
      ContainerDefinitions:
      - !Ref ContainerDefinition1
      - !Ref ContainerDefinition2
    Type: AWS::ECS::TaskDefinition

This doesn't work because Type: AWS::ECS::TaskDefinition::ContainerDefinition is not a valid Resource type in CloudFormation.

Is there another way to modularize the body of a Resource in CloudFormation? If not, how do you recommend keeping CF templates readable as they grow to specify large systems?

I am familiar with the AWS::Include Transform, but I would like to keep the entire template under version control, rather than posting snippets to S3 where they will be untracked.

Rafał Wrzeszcz Rafał Wrzeszcz · Accepted Answer · 2018-06-12T10:22:25

Unfortunately it's not so straightforward. CloudFormation can't handle complex structures as parameters. You have two options:

use Lambda-backed custom resources (if you have more cases like that it may be good choice, but for single use like here it seems to be an overkill) - custom resources are kind of exception: all of the params are just pased to them and also returned structures can be comples and still kept by CloudFormation;
upload partial templates to S3 in some CI/CD step (I recommend to version it there in URLs) before triggering CloudFormation.

Modularize body of a CloudFormation Resource

1 Answers