Why IaC

• Scaling
• Version Management
• Accountability
• Reusability

• Divide stacks into smaller chunks
• Reuse standard templates
• Mapped variables
• conditions
• Nested stacks
• cross stack ref instead of nested stack but check limitations

• The Fn::GetAtt intrinsic function returns the value of an attribute from a resource in the template
• Fn::Join appends a set of values into a single value, separated by the specified delimiter. Please note AWS::Partition Partition is a group of AWS Region and Service objects.

!Join
  - ''
  - - 'arn:'
    - !Ref AWS::Partition
    - ':s3:::elasticbeanstalk-*-'
    - !Ref AWS::AccountId
# output - arn:aws:s3:::elasticbeanstalk-*-xxxyyyxxx

• The intrinsic function Fn::Select returns a single object from a list of objects by index.

!Select [ 0, !Ref DbSubnetIpBlocks ]

• Fn::Split intrinsic function, split a string into a list of string values so that you can select an element from the resulting string list.
• The intrinsic function Fn::ImportValue returns the value of an output exported by another stack. For each AWS account, Export names must be unique within a region. Use Fn::Sub substitutes variables in an input string with values that you specify. You can use AWS defined params AWS::StackName for usability.

Outputs:
  PublicSubnet:
    Description: The subnet ID to use for public web servers
    Value:
      Ref: PublicSubnet
    Export:
      Name:
        'Fn::Sub': '${AWS::StackName}-SubnetID'

Resources:
  WebServerInstance:
    Type: 'AWS::EC2::Instance'
    Properties:
      InstanceType: t2.micro
      ImageId: ami-a1b23456
      NetworkInterfaces:
          AssociatePublicIpAddress: 'true'
          DeviceIndex: '0'
          DeleteOnTermination: 'true'
          SubnetId: !ImportValue 
            'Fn::Sub': '${PublicSubnet}-SubnetID'

### CloudFormation Support for external resources Custom resources (CRs) are the resources that don't fall under the official support of CloudFormation. CRs can be declared in a long (AWS::CloudFormation::CustomResource) or short form (Custom::MyResourceName)

The SAM template, on its own, is nothing but a macro template that will be processed during deployment by CloudFormation.

Transform: AWS::Serverless-2016-10-31
Resources:
  DynamoDb:
    Type: AWS::Serverless::SimpleTable
    Properties:
      TableName: myTable
      PrimaryKey:
        Name: id
        Type: string

• QUERY AMI

aws ec2 describe-images --owners amazon --filters "Name=name,Values=amzn*" --query 'sort_by(Images, &CreationDate)[].Name'

https://aws.amazon.com/blogs/compute/query-for-the-latest-amazon-linux-ami-ids-using-aws-systems-manager-parameter-store/