10 CloudFormation Template Interview Questions and Answers

CloudFormation Templates are a key component of AWS infrastructure management, enabling users to define and provision resources in a systematic and repeatable manner. By using JSON or YAML scripts, CloudFormation allows for the automation of resource deployment, reducing the potential for human error and ensuring consistency across environments. This makes it an essential tool for anyone working with AWS services, from developers to system administrators.

This article offers a curated selection of interview questions designed to test your understanding and proficiency with CloudFormation Templates. Reviewing these questions will help you solidify your knowledge, identify areas for improvement, and ultimately boost your confidence for any upcoming technical interviews.

CloudFormation Template Interview Questions and Answers

1. Describe the structure of a CloudFormation template.

A CloudFormation template is a JSON or YAML formatted text file that outlines the infrastructure and resources for your application. Its structure includes several sections:

• AWSTemplateFormatVersion: Specifies the template format version (optional).
• Description: Describes the template (optional).
• Metadata: Provides additional template information (optional).
• Parameters: Defines input values for the template (optional).
• Mappings: Creates simple “lookup tables” for use in the template (optional).
• Conditions: Controls resource creation or property assignment (optional).
• Resources: Specifies stack resources and properties. This is the only required section.
• Outputs: Describes values returned when viewing stack properties (optional).

2. Write a simple CloudFormation template to create an S3 bucket.

A simple CloudFormation template to create an S3 bucket in YAML:

AWSTemplateFormatVersion: '2010-09-09'
Resources:
  MyS3Bucket:
    Type: 'AWS::S3::Bucket'
    Properties:
      BucketName: 'my-unique-bucket-name'

3. Write a CloudFormation template snippet to create an EC2 instance with a specific AMI ID passed as a parameter.

AWSTemplateFormatVersion: '2010-09-09'
Parameters:
  AmiId:
    Description: AMI ID for the EC2 instance
    Type: String

Resources:
  MyEC2Instance:
    Type: 'AWS::EC2::Instance'
    Properties:
      ImageId: !Ref AmiId
      InstanceType: t2.micro
      KeyName: my-key-pair

4. What are mappings in CloudFormation, and how would you use them? Provide an example.

Mappings in CloudFormation are key-value pairs used to specify conditional values. They are defined in the “Mappings” section and referenced using the Fn::FindInMap function.

Example:

AWSTemplateFormatVersion: '2010-09-09'
Description: Example of Mappings in CloudFormation

Mappings:
  RegionMap:
    us-east-1:
      AMI: ami-0ff8a91507f77f867
      InstanceType: t2.micro
    us-west-1:
      AMI: ami-0bdb828fd58c52235
      InstanceType: t2.small

Resources:
  MyEC2Instance:
    Type: 'AWS::EC2::Instance'
    Properties:
      ImageId: !FindInMap [RegionMap, !Ref 'AWS::Region', AMI]
      InstanceType: !FindInMap [RegionMap, !Ref 'AWS::Region', InstanceType]

In this example, the RegionMap mapping defines AMI IDs and instance types for different regions. The !FindInMap function retrieves values based on the deployment region.

5. Write a CloudFormation template to create an RDS instance with multi-AZ deployment.

AWSTemplateFormatVersion: '2010-09-09'
Resources:
  MyDBInstance:
    Type: 'AWS::RDS::DBInstance'
    Properties:
      DBInstanceClass: db.t2.micro
      AllocatedStorage: '20'
      DBName: MyDatabase
      Engine: MySQL
      MasterUsername: admin
      MasterUserPassword: password
      MultiAZ: true
      BackupRetentionPeriod: 7
      StorageType: gp2
      VPCSecurityGroups:
        <ul>
          <li>sg-12345678</li>
        </ul>
      DBSubnetGroupName: my-db-subnet-group

6. How do you use conditions in a CloudFormation template? Provide an example.

Conditions in a CloudFormation template control resource creation based on specific criteria. They are defined in the “Conditions” section and referenced in “Resources” and “Outputs.”

Example:

AWSTemplateFormatVersion: '2010-09-09'
Parameters:
  EnvironmentType:
    Description: Type of environment
    Type: String
    Default: dev
    AllowedValues:
      - dev
      - prod

Conditions:
  IsProduction: !Equals [ !Ref EnvironmentType, prod ]

Resources:
  MyBucket:
    Type: 'AWS::S3::Bucket'
    Condition: IsProduction
    Properties:
      BucketName: my-production-bucket

Outputs:
  BucketName:
    Condition: IsProduction
    Value: !Ref MyBucket
    Description: The name of the S3 bucket if in production

In this example, the condition “IsProduction” checks if the “EnvironmentType” parameter is set to “prod”. The S3 bucket and output are created only if the condition is true.

7. Write a CloudFormation template snippet to create an Auto Scaling group with a launch configuration.

Resources:
  MyLaunchConfiguration:
    Type: AWS::AutoScaling::LaunchConfiguration
    Properties: 
      ImageId: ami-0abcdef1234567890
      InstanceType: t2.micro
      SecurityGroups: 
        <ul>
          <li>sg-0123456789abcdef0</li>
        </ul>
      KeyName: my-key-pair

  MyAutoScalingGroup:
    Type: AWS::AutoScaling::AutoScalingGroup
    Properties: 
      LaunchConfigurationName: !Ref MyLaunchConfiguration
      MinSize: 1
      MaxSize: 3
      DesiredCapacity: 2
      VPCZoneIdentifier: 
        <ul>
          <li>subnet-0123456789abcdef0</li>
          <li>subnet-abcdef0123456789</li>
        </ul>
      Tags:
        <ul>
          <li>Key: Name</li>
          <li>Value: MyAutoScalingInstance</li>
          <li>PropagateAtLaunch: true</li>
        </ul>

8. Write a CloudFormation template to create a VPC with public and private subnets, including route tables and internet gateway.

AWSTemplateFormatVersion: '2010-09-09'
Resources:
  VPC:
    Type: 'AWS::EC2::VPC'
    Properties:
      CidrBlock: '10.0.0.0/16'
      EnableDnsSupport: true
      EnableDnsHostnames: true
      Tags:
        <ul>
          <li>Key: Name</li>
          <li>Value: MyVPC</li>
        </ul>

  InternetGateway:
    Type: 'AWS::EC2::InternetGateway'
    Properties:
      Tags:
        <ul>
          <li>Key: Name</li>
          <li>Value: MyInternetGateway</li>
        </ul>

  AttachGateway:
    Type: 'AWS::EC2::VPCGatewayAttachment'
    Properties:
      VpcId: !Ref VPC
      InternetGatewayId: !Ref InternetGateway

  PublicSubnet:
    Type: 'AWS::EC2::Subnet'
    Properties:
      VpcId: !Ref VPC
      CidrBlock: '10.0.1.0/24'
      MapPublicIpOnLaunch: true
      Tags:
        <ul>
          <li>Key: Name</li>
          <li>Value: PublicSubnet</li>
        </ul>

  PrivateSubnet:
    Type: 'AWS::EC2::Subnet'
    Properties:
      VpcId: !Ref VPC
      CidrBlock: '10.0.2.0/24'
      Tags:
        <ul>
          <li>Key: Name</li>
          <li>Value: PrivateSubnet</li>
        </ul>

  PublicRouteTable:
    Type: 'AWS::EC2::RouteTable'
    Properties:
      VpcId: !Ref VPC
      Tags:
        <ul>
          <li>Key: Name</li>
          <li>Value: PublicRouteTable</li>
        </ul>

  PrivateRouteTable:
    Type: 'AWS::EC2::RouteTable'
    Properties:
      VpcId: !Ref VPC
      Tags:
        <ul>
          <li>Key: Name</li>
          <li>Value: PrivateRouteTable</li>
        </ul>

  PublicRoute:
    Type: 'AWS::EC2::Route'
    Properties:
      RouteTableId: !Ref PublicRouteTable
      DestinationCidrBlock: '0.0.0.0/0'
      GatewayId: !Ref InternetGateway

  PublicSubnetRouteTableAssociation:
    Type: 'AWS::EC2::SubnetRouteTableAssociation'
    Properties:
      SubnetId: !Ref PublicSubnet
      RouteTableId: !Ref PublicRouteTable

  PrivateSubnetRouteTableAssociation:
    Type: 'AWS::EC2::SubnetRouteTableAssociation'
    Properties:
      SubnetId: !Ref PrivateSubnet
      RouteTableId: !Ref PrivateRouteTable

9. How do you define and use outputs in a CloudFormation template? Provide an example.

In CloudFormation, outputs declare values accessible from outside the stack, such as resource IDs or URLs. Outputs can be exported for cross-stack references.

Example:

Resources:
  MyBucket:
    Type: "AWS::S3::Bucket"
    Properties:
      BucketName: "my-sample-bucket"

Outputs:
  BucketName:
    Description: "The name of the S3 bucket"
    Value: !Ref MyBucket
    Export:
      Name: "MyBucketName"

In this example, an S3 bucket is created, and its name is outputted. The Export field allows this output to be referenced in other stacks.

To use this output in another stack, import it using Fn::ImportValue:

Resources:
  AnotherBucket:
    Type: "AWS::S3::Bucket"
    Properties:
      BucketName: !ImportValue MyBucketName

10. What is a resource deletion policy in CloudFormation, and how do you implement it? Provide an example.

A resource deletion policy in CloudFormation defines what happens to a resource when its stack is deleted. Options include:

• Delete: The resource is deleted.
• Retain: The resource is kept.
• Snapshot: A snapshot is created before deletion.

To implement a deletion policy, specify the DeletionPolicy attribute in the resource definition.

Example:

Resources:
  MyS3Bucket:
    Type: "AWS::S3::Bucket"
    DeletionPolicy: Retain

In this example, the S3 bucket is retained when the stack is deleted.