Nested stacks for modularity in AWS - Commands & Configuration

Introduction

When building cloud resources, managing many parts in one big file can get confusing. Nested stacks let you break your setup into smaller pieces that fit together, making it easier to organize and update your cloud resources.

When you want to reuse common parts like networking or security settings across multiple projects.

When your cloud setup grows large and you want to keep files simple and easy to understand.

When different teams manage different parts of the cloud setup and you want to separate their work.

When you want to update one part without touching the whole setup.

When you want to share standard resource setups as templates for new projects.

Config File - parent-stack.yaml

parent-stack.yaml

AWSTemplateFormatVersion: '2010-09-09'
Description: Parent stack that includes nested stacks for modular resources
Resources:
  NetworkStack:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: https://s3.amazonaws.com/example-bucket/network-stack.yaml
  ComputeStack:
    Type: AWS::CloudFormation::Stack
    Properties:
      TemplateURL: https://s3.amazonaws.com/example-bucket/compute-stack.yaml

This is the main template called the parent stack. It includes two nested stacks: NetworkStack and ComputeStack.

Each nested stack is a separate template stored in S3, referenced by TemplateURL. This setup helps organize resources into smaller, reusable parts.

Commands

This command creates the parent stack which includes the nested stacks. The capability flag allows creating IAM roles if needed.

Terminal

aws cloudformation create-stack --stack-name my-parent-stack --template-body file://parent-stack.yaml --capabilities CAPABILITY_NAMED_IAM

Expected Output

A stack with id arn:aws:cloudformation:us-east-1:123456789012:stack/my-parent-stack/abcd1234-5678-90ef-ghij-klmnopqrstuv was created

→

--capabilities CAPABILITY_NAMED_IAM - Allows the stack to create IAM resources

This command checks the status and details of the parent stack and its nested stacks to confirm they are created successfully.

Terminal

aws cloudformation describe-stacks --stack-name my-parent-stack

Expected Output

{

    "Stacks": [

        {

            "StackName": "my-parent-stack",

            "StackStatus": "CREATE_COMPLETE",

            "StackId": "arn:aws:cloudformation:us-east-1:123456789012:stack/my-parent-stack/abcd1234-5678-90ef-ghij-klmnopqrstuv"

        }

    ]

}

This command lists all resources in the parent stack, including the nested stacks, showing how the modular parts are included.

Terminal

aws cloudformation list-stack-resources --stack-name my-parent-stack

Expected Output

StackResourceSummaries:

- LogicalResourceId: NetworkStack

  ResourceType: AWS::CloudFormation::Stack

  ResourceStatus: CREATE_COMPLETE

- LogicalResourceId: ComputeStack

  ResourceType: AWS::CloudFormation::Stack

  ResourceStatus: CREATE_COMPLETE

Key Concept

If you remember nothing else from this pattern, remember: nested stacks let you split big cloud setups into smaller, reusable parts that work together smoothly.

Common Mistakes

Using local file paths in TemplateURL instead of an accessible S3 URL

CloudFormation requires nested stack templates to be accessible via a URL, usually in S3, so it can fetch them during deployment.

Upload nested stack templates to an S3 bucket and use the HTTPS URL in TemplateURL.

Not including the CAPABILITY_NAMED_IAM flag when the nested stacks create IAM roles

CloudFormation will reject the stack creation without this flag if IAM resources are involved.

Always add --capabilities CAPABILITY_NAMED_IAM when creating stacks that include IAM resources.

Summary

Create a parent CloudFormation template that references smaller nested stack templates stored in S3.

Use AWS CLI to create the parent stack, which deploys all nested stacks together.

Check stack status and resources to confirm successful deployment of modular parts.

Practice

1. What is the main benefit of using nested stacks in AWS CloudFormation?

easy

A. They replace the need for IAM roles.

B. They automatically increase server CPU capacity.

C. They break large templates into smaller, reusable parts.

D. They encrypt all data stored in S3 buckets.

Solution

1. Step 1: Understand the purpose of nested stacks

   Nested stacks allow splitting a big CloudFormation template into smaller templates for easier management.

2. Step 2: Compare options with this purpose

   Only They break large templates into smaller, reusable parts. describes breaking large templates into reusable parts, which matches nested stacks' main benefit.

3. Final Answer:

   They break large templates into smaller, reusable parts. -> Option C

4. Quick Check:

   Nested stacks = modular templates [OK]

Hint: Think modular: nested stacks split big templates [OK]

Common Mistakes:

• Confusing nested stacks with scaling features
• Assuming nested stacks handle encryption automatically
• Mixing nested stacks with IAM role functions

2. Which syntax correctly defines a nested stack resource in a CloudFormation template?

easy

A. "Type": "AWS::Lambda::Function",
"Properties": {"Handler": "index.handler"}

B. "Type": "AWS::S3::Bucket",
"Properties": {"BucketName": "nested-stack"}

C. "Type": "AWS::EC2::Instance",
"Properties": {"ImageId": "ami-123456"}

D. "Type": "AWS::CloudFormation::Stack",
"Properties": {"TemplateURL": "https://s3.amazonaws.com/mybucket/mystack.yaml"}

Solution

1. Step 1: Identify nested stack resource type

   The resource type for nested stacks is AWS::CloudFormation::Stack.

2. Step 2: Check the properties for nested stack

   Nested stacks require a TemplateURL property pointing to the child template location.

3. Final Answer:

   "Type": "AWS::CloudFormation::Stack", "Properties": {"TemplateURL": "https://s3.amazonaws.com/mybucket/mystack.yaml"} -> Option D

4. Quick Check:

   Nested stack resource = AWS::CloudFormation::Stack [OK]

Hint: Nested stack resource type is AWS::CloudFormation::Stack [OK]

Common Mistakes:

• Using S3 bucket or EC2 instance types for nested stacks
• Omitting TemplateURL property
• Confusing Lambda function syntax with nested stacks

3. Given a parent stack that passes a parameter VpcId to a nested stack, what will happen if the nested stack template does NOT declare VpcId as a parameter?

medium

A. The nested stack deployment will fail with a parameter validation error.

B. The nested stack will ignore the parameter and deploy successfully.

C. The parent stack will automatically create the missing parameter.

D. The nested stack will use a default VPC ID automatically.

Solution

1. Step 1: Understand parameter passing in nested stacks

   Parameters passed from parent to nested stack must be declared in the nested stack template.

2. Step 2: Consequence of missing parameter declaration

   If the nested stack does not declare the parameter, CloudFormation cannot map the value, causing deployment failure.

3. Final Answer:

   The nested stack deployment will fail with a parameter validation error. -> Option A

4. Quick Check:

   Missing nested stack parameter = deployment error [OK]

Hint: Nested stack must declare all passed parameters [OK]

Common Mistakes:

• Assuming parameters are optional in nested stacks
• Thinking parent stack auto-creates missing parameters
• Believing nested stacks use default values without declaration

4. You have a nested stack that fails to update because the child stack template URL is incorrect. What is the best way to fix this error?

medium

A. Delete the parent stack and recreate it from scratch.

B. Update the parent stack template to use the correct TemplateURL for the nested stack.

C. Change the nested stack resource type to AWS::S3::Bucket.

D. Remove the nested stack resource from the parent template.

Solution

1. Step 1: Identify cause of failure

   The error is due to an incorrect TemplateURL in the parent stack referencing the nested stack template.

2. Step 2: Correct the TemplateURL

   Fixing the TemplateURL in the parent stack template ensures CloudFormation can find and deploy the nested stack correctly.

3. Final Answer:

   Update the parent stack template to use the correct TemplateURL for the nested stack. -> Option B

4. Quick Check:

   Fix TemplateURL in parent stack to fix nested stack update [OK]

Hint: Fix TemplateURL in parent template to fix nested stack errors [OK]

Common Mistakes:

• Deleting entire parent stack unnecessarily
• Changing resource type incorrectly
• Removing nested stack resource instead of fixing URL

5. You want to create a modular infrastructure with nested stacks where a VPC stack outputs a VpcId and a subnet stack needs this VpcId as input. How should you connect these stacks to ensure modularity and correct parameter passing?

hard

A. Use Outputs in the VPC stack and pass them as Parameters to the subnet nested stack in the parent template.

B. Hardcode the VPC ID in the subnet stack template directly.

C. Deploy the subnet stack separately without any parameters.

D. Merge both VPC and subnet resources into a single large template.

Solution

1. Step 1: Use Outputs to share data from VPC stack

   The VPC nested stack should declare an Output for VpcId to share it externally.

2. Step 2: Pass Output as Parameter to subnet stack

   The parent stack template passes the VPC stack's VpcId Output as a Parameter to the subnet nested stack, maintaining modularity.

3. Final Answer:

   Use Outputs in the VPC stack and pass them as Parameters to the subnet nested stack in the parent template. -> Option A

4. Quick Check:

   Outputs to Parameters connect nested stacks modularly [OK]

Hint: Connect stacks via Outputs and Parameters in parent template [OK]

Common Mistakes:

• Hardcoding values breaking modularity
• Deploying stacks separately without parameter passing
• Combining all resources in one big template losing modularity