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Showing content from https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/pseudo-parameter-reference.html below:

Get AWS values using pseudo parameters

Pseudo parameters are built-in variables that provide access to important AWS environment information such as account IDs, Region names, and stack details that can change between deployments or environments.

You can use pseudo parameters instead of hard-coded values to make your templates more portable and easier to reuse across different AWS accounts and Regions.

You can reference pseudo parameters using either the Ref intrinsic function or the Fn::Sub intrinsic function.

The Ref intrinsic function uses the following general syntax. For more information, see Ref.

{ "Ref" : "AWS::PseudoParameter" }

!Ref AWS::PseudoParameter

The Fn::Sub intrinsic function uses a different format that includes the ${} syntax around the pseudo parameter. For more information, see Fn::Sub.

{ "Fn::Sub" : "${AWS::PseudoParameter}" }

!Sub '${AWS::PseudoParameter}'

Returns the AWS account ID of the account in which the stack is being created, such as 123456789012.

This pseudo parameter is commonly used when defining IAM roles, policies, and other resource policies that involve account-specific ARNs.

Returns the list of Amazon Resource Names (ARNs) for the Amazon SNS topics that receive stack event notifications. You can specify these ARNs through the --notification-arns option in the AWS CLI or through the console as you are creating or updating your stack.

Unlike other pseudo parameters that return a single value, AWS::NotificationARNs returns a list of ARNs. To access a specific ARN in the list, use the Fn::Select intrinsic function. For more information, see Fn::Select.

Removes the corresponding resource property when specified as a return value in the Fn::If intrinsic function. For more information, see Fn::If.

This pseudo parameter is particularly useful for creating conditional resource properties that should only be included under certain conditions.

Returns the partition that the resource is in. For standard AWS Regions, the partition is aws. For resources in other partitions, the partition is aws-partitionname. For example, the partition for resources in the China (Beijing and Ningxia) Regions is aws-cn and the partition for resources in the AWS GovCloud (US-West) Region is aws-us-gov.

The partition forms part of the ARN for resources. Using AWS::Partition ensures your templates work correctly across different AWS partitions.

Returns a string representing the Region in which the encompassing resource is being created, such as us-west-2.

This is one of the most commonly used pseudo parameters, as it allows templates to adapt to different AWS Regions without modification.

Returns the ID (ARN) of the stack, such as arn:aws:cloudformation:us-west-2:123456789012:stack/teststack/51af3dc0-da77-11e4-872e-1234567db123.

Returns the name of the stack, such as teststack.

The stack name is commonly used to create unique resource names that are easily identifiable as belonging to a specific stack.

Returns the suffix for the AWS domain in the AWS Region where the stack is deployed. The suffix is typically amazonaws.com, but for the China (Beijing) Region, the suffix is amazonaws.com.cn.

This parameter is particularly useful when constructing URLs for AWS service endpoints.

The following examples create two resources: an Amazon SNS topic and a CloudWatch alarm that sends notifications to that topic. They use AWS::StackName, AWS::Region, and AWS::AccountId to dynamically insert the stack name, current AWS Region, and account ID into resource names, descriptions, and ARNs.

{
    "Resources": {
        "MyNotificationTopic": {
            "Type": "AWS::SNS::Topic",
            "Properties": {
                "DisplayName": { "Fn::Sub": "Notifications for ${AWS::StackName}" }
            }
        },
        "CPUAlarm": {
            "Type": "AWS::CloudWatch::Alarm",
            "Properties": {
                "AlarmDescription": { "Fn::Sub": "Alarm for high CPU in ${AWS::Region}" },
                "AlarmName": { "Fn::Sub": "${AWS::StackName}-HighCPUAlarm" },
                "MetricName": "CPUUtilization",
                "Namespace": "AWS/EC2",
                "Statistic": "Average",
                "Period": 300,
                "EvaluationPeriods": 1,
                "Threshold": 80,
                "ComparisonOperator": "GreaterThanThreshold",
                "AlarmActions": [{ "Fn::Sub": "arn:aws:sns:${AWS::Region}:${AWS::AccountId}:${MyNotificationTopic}" }]
            }
        }
    }
}

Resources:
  MyNotificationTopic:
    Type: AWS::SNS::Topic
    Properties:
      DisplayName: !Sub Notifications for ${AWS::StackName}
  CPUAlarm:
    Type: AWS::CloudWatch::Alarm
    Properties:
      AlarmDescription: !Sub Alarm for high CPU in ${AWS::Region}
      AlarmName: !Sub ${AWS::StackName}-HighCPUAlarm
      MetricName: CPUUtilization
      Namespace: AWS/EC2
      Statistic: Average
      Period: 300
      EvaluationPeriods: 1
      Threshold: 80
      ComparisonOperator: GreaterThanThreshold
      AlarmActions:
        - !Sub arn:aws:sns:${AWS::Region}:${AWS::AccountId}:${MyNotificationTopic}

The following examples configure an Auto Scaling group to send notifications for instance launch events and launch errors. The configuration uses the AWS::NotificationARNs pseudo parameter, which provides a list of Amazon SNS topic ARNs that were specified during stack creation. The Fn::Select function chooses the first ARN from that list.

"myASG": {
   "Type": "AWS::AutoScaling::AutoScalingGroup",
   "Properties": {
      "LaunchTemplate": {
         "LaunchTemplateId": { "Ref": "myLaunchTemplate" },
         "Version": { "Fn::GetAtt": [ "myLaunchTemplate", "LatestVersionNumber" ] }
       },
       "MaxSize": "1",
       "MinSize": "1",
       "VPCZoneIdentifier": [
          "subnetIdAz1",
          "subnetIdAz2",
          "subnetIdAz3"
      ],
      "NotificationConfigurations" : [{
         "TopicARN" : { "Fn::Select" : [ "0", { "Ref" : "AWS::NotificationARNs" } ] },
         "NotificationTypes" : [ "autoscaling:EC2_INSTANCE_LAUNCH", "autoscaling:EC2_INSTANCE_LAUNCH_ERROR" ]
      }]
   }
}

myASG:
  Type: AWS::AutoScaling::AutoScalingGroup
  Properties:
    LaunchTemplate:
      LaunchTemplateId: !Ref myLaunchTemplate
      Version: !GetAtt myLaunchTemplate.LatestVersionNumber
    MinSize: '1'
    MaxSize: '1'
    VPCZoneIdentifier:
      - subnetIdAz1
      - subnetIdAz2
      - subnetIdAz3
    NotificationConfigurations:
      - TopicARN:
          Fn::Select:
          - '0'
          - Ref: AWS::NotificationARNs
        NotificationTypes:
        - autoscaling:EC2_INSTANCE_LAUNCH
        - autoscaling:EC2_INSTANCE_LAUNCH_ERROR

The following examples create an Amazon RDS DB instance that uses a snapshot only if a snapshot ID is provided. If the UseDBSnapshot condition evaluates to true, CloudFormation uses the DBSnapshotName parameter value for the DBSnapshotIdentifier property. If the condition evaluates to false, CloudFormation removes the DBSnapshotIdentifier property.

"MyDB" : {
  "Type" : "AWS::RDS::DBInstance",
  "Properties" : {
    "AllocatedStorage" : "5",
    "DBInstanceClass" : "db.t2.small",
    "Engine" : "MySQL",
    "EngineVersion" : "5.5",
    "MasterUsername" : { "Ref" : "DBUser" },
    "MasterUserPassword" : { "Ref" : "DBPassword" },
    "DBParameterGroupName" : { "Ref" : "MyRDSParamGroup" },
    "DBSnapshotIdentifier" : {
      "Fn::If" : [
        "UseDBSnapshot",
        {"Ref" : "DBSnapshotName"},
        {"Ref" : "AWS::NoValue"}
      ]
    }
  }
}

MyDB:
  Type: AWS::RDS::DBInstance
  Properties:
    AllocatedStorage: '5'
    DBInstanceClass: db.t2.small
    Engine: MySQL
    EngineVersion: '5.5'
    MasterUsername:
      Ref: DBUser
    MasterUserPassword:
      Ref: DBPassword
    DBParameterGroupName:
      Ref: MyRDSParamGroup
    DBSnapshotIdentifier:
      Fn::If:
        - UseDBSnapshot
        - Ref: DBSnapshotName
        - Ref: AWS::NoValue

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