AWSTemplateFormatVersion: '2010-09-09'
Description: 'My Application Stack'

Parameters:                      # User inputs at deploy time
  Environment:
    Type: String
    Default: staging
    AllowedValues: [dev, staging, prod]
  
  InstanceType:
    Type: String
    Default: t3.medium

Mappings:                        # Static lookup tables
  InstanceTypes:
    dev:     { Type: t3.micro }
    staging: { Type: t3.medium }
    prod:    { Type: m5.large }

Conditions:                      # Conditional resource creation
  IsProd: !Equals [!Ref Environment, prod]

Resources:                       # Required — what to create
  WebServer:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: !FindInMap [InstanceTypes, !Ref Environment, Type]
      ImageId: !Sub '{{resolve:ssm:/aws/service/ami-amazon-linux-latest/al2023-ami-kernel-default-x86_64}}'
      Tags:
        - Key: Name
          Value: !Sub '${Environment}-web-server'

  Database:
    Type: AWS::RDS::DBInstance
    Condition: IsProd              # Only create in prod
    DeletionPolicy: Snapshot       # Take snapshot before delete
    Properties:
      DBInstanceClass: db.r6g.large
      Engine: postgres
      MasterUsername: !Sub '{{resolve:secretsmanager:${Environment}/db:username}}'
      MasterUserPassword: !Sub '{{resolve:secretsmanager:${Environment}/db:password}}'

Outputs:                          # Values to export
  WebServerIP:
    Value: !GetAtt WebServer.PublicIp
    Export:
      Name: !Sub '${AWS::StackName}-WebIP'

!Ref Resource           # Reference a resource's default value
!GetAtt Resource.Attr   # Get specific attribute
!Sub 'prefix-${Param}'  # String substitution
!Join ['-', [a, b, c]]  # Join array: a-b-c
!Select [0, [a,b,c]]    # Select by index: a
!If [Condition, yes, no] # Conditional value
!FindInMap [Map,Key1,Key2] # Lookup in Mappings
!ImportValue ExportName  # Cross-stack reference

# Condition functions
!Equals [!Ref Env, prod]
!And [!Condition C1, !Condition C2]
!Or  [!Condition C1, !Condition C2]
!Not [!Condition C1]

# Deploy/update stack
aws cloudformation deploy \
    --stack-name my-app-prod \
    --template-file template.yaml \
    --parameter-overrides Environment=prod InstanceType=m5.large \
    --capabilities CAPABILITY_IAM \
    --region ap-south-1

# Validate template
aws cloudformation validate-template --template-body file://template.yaml

# Describe stack and outputs
aws cloudformation describe-stacks --stack-name my-app-prod
aws cloudformation describe-stack-outputs --stack-name my-app-prod

# List events (debug failures)
aws cloudformation describe-stack-events --stack-name my-app-prod \
    --query 'StackEvents[?ResourceStatus==`CREATE_FAILED`]'

# Delete stack (careful!)
aws cloudformation delete-stack --stack-name my-app-dev

# Change sets (preview changes before applying)
aws cloudformation create-change-set \
    --stack-name my-app-prod \
    --change-set-name my-changes \
    --template-body file://template.yaml
aws cloudformation describe-change-set --stack-name my-app-prod --change-set-name my-changes
aws cloudformation execute-change-set --stack-name my-app-prod --change-set-name my-changes