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# Constants
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Pre-defined constants for AWS regions, instance types, ports, pseudo parameters, and CloudFormation limits to improve template readability and reduce errors.
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## Capabilities
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### Pseudo Parameters
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CloudFormation pseudo parameters that provide runtime values about the stack and AWS environment.
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```python { .api }
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# Pseudo Parameter String Constants
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AWS_ACCOUNT_ID: str = "AWS::AccountId"
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AWS_NOTIFICATION_ARNS: str = "AWS::NotificationARNs"  
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AWS_NO_VALUE: str = "AWS::NoValue"
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AWS_PARTITION: str = "AWS::Partition"
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AWS_REGION: str = "AWS::Region"
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AWS_STACK_ID: str = "AWS::StackId"
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AWS_STACK_NAME: str = "AWS::StackName"
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AWS_URL_SUFFIX: str = "AWS::URLSuffix"
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# Pre-defined Ref Objects for Pseudo Parameters
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AccountId: Ref = Ref(AWS_ACCOUNT_ID)
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NotificationARNs: Ref = Ref(AWS_NOTIFICATION_ARNS)
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NoValue: Ref = Ref(AWS_NO_VALUE)
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Partition: Ref = Ref(AWS_PARTITION)
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Region: Ref = Ref(AWS_REGION)
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StackId: Ref = Ref(AWS_STACK_ID)
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StackName: Ref = Ref(AWS_STACK_NAME)
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URLSuffix: Ref = Ref(AWS_URL_SUFFIX)
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```
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### CloudFormation Policy Constants
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Constants for DeletionPolicy and UpdateReplacePolicy values.
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```python { .api }
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Delete: str = "Delete"
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Retain: str = "Retain"
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RetainExceptOnCreate: str = "RetainExceptOnCreate"
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Snapshot: str = "Snapshot"
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```
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### Template Limits
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CloudFormation template limits for resources, parameters, outputs, and mappings.
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```python { .api }
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MAX_MAPPINGS: int = 200
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MAX_OUTPUTS: int = 200  
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MAX_PARAMETERS: int = 200
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MAX_RESOURCES: int = 500
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PARAMETER_TITLE_MAX: int = 255
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```
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### AWS Regions
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All AWS regions with standardized constant names.
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```python { .api }
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# US Regions
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US_EAST_1 = "us-east-1"
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US_EAST_2 = "us-east-2"
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US_WEST_1 = "us-west-1"
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US_WEST_2 = "us-west-2"
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# Europe Regions
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EU_CENTRAL_1 = "eu-central-1"
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EU_NORTH_1 = "eu-north-1"
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EU_SOUTH_1 = "eu-south-1"
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EU_WEST_1 = "eu-west-1"
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EU_WEST_2 = "eu-west-2"
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EU_WEST_3 = "eu-west-3"
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# Asia Pacific Regions
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AP_EAST_1 = "ap-east-1"
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AP_NORTHEAST_1 = "ap-northeast-1"
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AP_NORTHEAST_2 = "ap-northeast-2"
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AP_NORTHEAST_3 = "ap-northeast-3"
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AP_SOUTH_1 = "ap-south-1"
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AP_SOUTHEAST_1 = "ap-southeast-1"
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AP_SOUTHEAST_2 = "ap-southeast-2"
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AP_SOUTHEAST_3 = "ap-southeast-3"
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# Other Regions
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AF_SOUTH_1 = "af-south-1"
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CA_CENTRAL_1 = "ca-central-1"
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CN_NORTH_1 = "cn-north-1"
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CN_NORTHWEST_1 = "cn-northwest-1"
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ME_CENTRAL_1 = "me-central-1"
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ME_SOUTH_1 = "me-south-1"
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SA_EAST_1 = "sa-east-1"
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US_GOV_EAST_1 = "us-gov-east-1"
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US_GOV_WEST_1 = "us-gov-west-1"
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```
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### Availability Zones
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Availability zone constants for all AWS regions.
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```python { .api }
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# US East 1 Availability Zones
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US_EAST_1A = "us-east-1a"
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US_EAST_1B = "us-east-1b"
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US_EAST_1C = "us-east-1c"
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US_EAST_1D = "us-east-1d"
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US_EAST_1E = "us-east-1e"
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US_EAST_1F = "us-east-1f"
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# US West 2 Availability Zones  
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US_WEST_2A = "us-west-2a"
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US_WEST_2B = "us-west-2b"
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US_WEST_2C = "us-west-2c"
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US_WEST_2D = "us-west-2d"
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# Additional zones available for all regions...
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```
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### EC2 Instance Types
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Constants for all EC2 instance types across different families.
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```python { .api }
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# General Purpose - T2 Family
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T2_NANO = "t2.nano"
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T2_MICRO = "t2.micro"
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T2_SMALL = "t2.small"
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T2_MEDIUM = "t2.medium"
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T2_LARGE = "t2.large"
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T2_XLARGE = "t2.xlarge"
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T2_2XLARGE = "t2.2xlarge"
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# General Purpose - T3 Family
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T3_NANO = "t3.nano"
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T3_MICRO = "t3.micro"
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T3_SMALL = "t3.small"
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T3_MEDIUM = "t3.medium"
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T3_LARGE = "t3.large"
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T3_XLARGE = "t3.xlarge"
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T3_2XLARGE = "t3.2xlarge"
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# General Purpose - M5 Family
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M5_LARGE = "m5.large"
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M5_XLARGE = "m5.xlarge"
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M5_2XLARGE = "m5.2xlarge"
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M5_4XLARGE = "m5.4xlarge"
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M5_8XLARGE = "m5.8xlarge"
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M5_12XLARGE = "m5.12xlarge"
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M5_16XLARGE = "m5.16xlarge"
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M5_24XLARGE = "m5.24xlarge"
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# Compute Optimized - C5 Family
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C5_LARGE = "c5.large"
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C5_XLARGE = "c5.xlarge"
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C5_2XLARGE = "c5.2xlarge"
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C5_4XLARGE = "c5.4xlarge"
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C5_9XLARGE = "c5.9xlarge"
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C5_12XLARGE = "c5.12xlarge"
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C5_18XLARGE = "c5.18xlarge"
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C5_24XLARGE = "c5.24xlarge"
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# Memory Optimized - R5 Family
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R5_LARGE = "r5.large"
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R5_XLARGE = "r5.xlarge"
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R5_2XLARGE = "r5.2xlarge"
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R5_4XLARGE = "r5.4xlarge"
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R5_8XLARGE = "r5.8xlarge"
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R5_12XLARGE = "r5.12xlarge"
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R5_16XLARGE = "r5.16xlarge"
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R5_24XLARGE = "r5.24xlarge"
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# Additional instance families available...
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```
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### RDS Instance Classes
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Constants for RDS database instance classes.
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```python { .api }
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# General Purpose - db.t3 Family
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DB_T3_MICRO = "db.t3.micro"
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DB_T3_SMALL = "db.t3.small"
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DB_T3_MEDIUM = "db.t3.medium"
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DB_T3_LARGE = "db.t3.large"
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DB_T3_XLARGE = "db.t3.xlarge"
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DB_T3_2XLARGE = "db.t3.2xlarge"
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# General Purpose - db.m5 Family
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DB_M5_LARGE = "db.m5.large"
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DB_M5_XLARGE = "db.m5.xlarge"
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DB_M5_2XLARGE = "db.m5.2xlarge"
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DB_M5_4XLARGE = "db.m5.4xlarge"
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DB_M5_8XLARGE = "db.m5.8xlarge"
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DB_M5_12XLARGE = "db.m5.12xlarge"
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DB_M5_16XLARGE = "db.m5.16xlarge"
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DB_M5_24XLARGE = "db.m5.24xlarge"
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# Memory Optimized - db.r5 Family
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DB_R5_LARGE = "db.r5.large"
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DB_R5_XLARGE = "db.r5.xlarge"
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DB_R5_2XLARGE = "db.r5.2xlarge"
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DB_R5_4XLARGE = "db.r5.4xlarge"
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DB_R5_8XLARGE = "db.r5.8xlarge"
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DB_R5_12XLARGE = "db.r5.12xlarge"
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DB_R5_16XLARGE = "db.r5.16xlarge"
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DB_R5_24XLARGE = "db.r5.24xlarge"
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```
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### Network Constants
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Common networking constants for CIDR blocks, ports, and protocols.
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```python { .api }
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# CIDR Blocks
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QUAD_ZERO = "0.0.0.0/0"
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VPC_CIDR_16 = "10.0.0.0/16"
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VPC_CIDR_20 = "10.0.0.0/20"
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PRIVATE_SUBNET_CIDR_24 = "10.0.1.0/24"
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PUBLIC_SUBNET_CIDR_24 = "10.0.2.0/24"
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# Common Ports
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SSH_PORT = 22
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TELNET_PORT = 23
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SMTP_PORT = 25
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DNS_PORT = 53
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HTTP_PORT = 80
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POP3_PORT = 110
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NTP_PORT = 123
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IMAP_PORT = 143
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SNMP_PORT = 161
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HTTPS_PORT = 443
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SMTPS_PORT = 465
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IMAPS_PORT = 993
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POP3S_PORT = 995
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MYSQL_PORT = 3306
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POSTGRESQL_PORT = 5432
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REDIS_PORT = 6379
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HTTP_ALT_PORT = 8080
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HTTPS_ALT_PORT = 8443
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# Protocol Constants
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TCP = "tcp"
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UDP = "udp" 
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ICMP = "icmp"
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ALL_PROTOCOLS = "-1"
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```
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### CloudWatch Logs Retention
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Valid retention periods for CloudWatch Logs.
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```python { .api }
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LOGS_ALLOWED_RETENTION_DAYS = [
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    1, 3, 5, 7, 14, 30, 60, 90, 120, 150, 180, 365, 400, 545, 731, 1827, 3653
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]
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```
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## Usage Examples
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### Using Pseudo Parameters
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```python
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from troposphere import Template, Output, Sub
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from troposphere import Region, AccountId, StackName
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template = Template()
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# Use pseudo parameters in string substitution
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template.add_output(Output(
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    "StackInfo",
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    Value=Sub("Stack ${StackName} in region ${Region} for account ${AccountId}", {
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        "StackName": StackName,
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        "Region": Region,
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        "AccountId": AccountId
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    }),
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    Description="Stack information"
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))
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# Direct usage
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bucket_name = Sub("${AWS::StackName}-${AWS::AccountId}-bucket")
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```
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### Using Instance Type Constants
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```python
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from troposphere import Template, Parameter
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from troposphere.constants import T2_MICRO, T2_SMALL, T2_MEDIUM, M5_LARGE
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from troposphere.ec2 import Instance
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template = Template()
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# Parameter with instance type constants
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instance_type = template.add_parameter(Parameter(
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    "InstanceType",
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    Type="String",
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    Default=T2_MICRO,
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    AllowedValues=[T2_MICRO, T2_SMALL, T2_MEDIUM, M5_LARGE],
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    Description="EC2 instance type"
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))
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# Use in conditional logic
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instance = template.add_resource(Instance(
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    "MyInstance",
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    ImageId="ami-0abcdef1234567890",
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    InstanceType=If("IsProduction", M5_LARGE, T2_MICRO)
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))
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```
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### Using Network Constants
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```python
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from troposphere import Template
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from troposphere.constants import HTTP_PORT, HTTPS_PORT, SSH_PORT, QUAD_ZERO
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from troposphere.ec2 import SecurityGroup, SecurityGroupRule
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template = Template()
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# Security group with port constants
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web_sg = template.add_resource(SecurityGroup(
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    "WebServerSecurityGroup",
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    GroupDescription="Security group for web servers",
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    SecurityGroupIngress=[
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        {
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            "IpProtocol": "tcp",
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            "FromPort": HTTP_PORT,
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            "ToPort": HTTP_PORT,
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            "CidrIp": QUAD_ZERO
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        },
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        {
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            "IpProtocol": "tcp", 
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            "FromPort": HTTPS_PORT,
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            "ToPort": HTTPS_PORT,
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            "CidrIp": QUAD_ZERO
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        },
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        {
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            "IpProtocol": "tcp",
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            "FromPort": SSH_PORT,
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            "ToPort": SSH_PORT,
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            "CidrIp": "10.0.0.0/8"
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        }
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    ]
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))
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```
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### Using Region Constants
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```python
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from troposphere import Template, FindInMap
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from troposphere.constants import US_EAST_1, US_WEST_2, EU_WEST_1
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template = Template()
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# Regional mappings with constants
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template.add_mapping("RegionMap", {
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    US_EAST_1: {"AMI": "ami-0ff8a91507f77f867"},
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    US_WEST_2: {"AMI": "ami-0bdf93799014acdc4"},
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    EU_WEST_1: {"AMI": "ami-047bb4163c506cd98"}
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})
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# Use in resources
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ami_id = FindInMap("RegionMap", Region, "AMI")
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```
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### Using Policy Constants
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```python
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from troposphere import Template
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from troposphere.constants import Retain, Snapshot
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from troposphere.ec2 import Instance, Volume
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template = Template()
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# Critical instance with retain policy
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critical_instance = template.add_resource(Instance(
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    "CriticalInstance",
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    ImageId="ami-0abcdef1234567890",
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    InstanceType="m5.large"
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))
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critical_instance.DeletionPolicy = Retain
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# Database volume with snapshot policy
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db_volume = template.add_resource(Volume(
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    "DatabaseVolume",
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    Size=100,
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    VolumeType="gp3",
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    AvailabilityZone="us-east-1a"
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))
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db_volume.DeletionPolicy = Snapshot
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```