CloudFormation Template Anatomy

Every CloudFormation template follows a well-defined structure. Understanding the anatomy of a template is essential for writing, reading, and debugging infrastructure definitions. In this lesson we will examine each section of a template, what it does, and when to use it.

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The Six Sections of a CloudFormation Template

A CloudFormation YAML template can contain the following top-level sections:

AWSTemplateFormatVersion: '2010-09-09'
Description: A description of what this template does

Metadata:
  # Optional information about the template

Parameters:
  # Input values supplied at stack creation time

Mappings:
  # Static key-value lookup tables

Conditions:
  # Logic to control whether resources are created

Resources:
  # The AWS resources to create (REQUIRED)

Outputs:
  # Values to return after the stack is created

Only the Resources section is required. All other sections are optional, but most real-world templates use several of them.

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AWSTemplateFormatVersion

This declares the template format version. The only valid value is '2010-09-09', which has been the version since CloudFormation launched. You should always include it for clarity:

AWSTemplateFormatVersion: '2010-09-09'

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Description

A human-readable string that describes the purpose of the template. It is displayed in the CloudFormation console when you view a stack:

Description: >
  This template provisions a three-tier web application
  with an Application Load Balancer, EC2 instances,
  and an RDS PostgreSQL database.

Descriptions have a maximum length of 1,024 bytes. Keep them concise but informative.

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Metadata

The Metadata section provides additional information about the template. It is most commonly used to:

• Control the order and grouping of parameters in the console
• Store configuration for the cfn-init helper

Metadata:
  AWS::CloudFormation::Interface:
    ParameterGroups:
      - Label:
          default: Network Configuration
        Parameters:
          - VpcCidr
          - SubnetCidr
      - Label:
          default: Instance Configuration
        Parameters:
          - InstanceType
          - KeyPairName

This tells the CloudFormation console to group the parameters under meaningful headings instead of listing them alphabetically.

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Parameters

Parameters let you pass values into a template at stack creation time. This makes your templates reusable — the same template can create different environments by supplying different parameter values.

Parameters:
  EnvironmentType:
    Type: String
    Default: dev
    AllowedValues:
      - dev
      - staging
      - production
    Description: The environment type for this stack

  InstanceType:
    Type: String
    Default: t3.micro
    AllowedValues:
      - t3.micro
      - t3.small
      - t3.medium
    Description: The EC2 instance type

  VpcCidr:
    Type: String
    Default: '10.0.0.0/16'
    AllowedPattern: '^(\d{1,3}\.){3}\d{1,3}/\d{1,2}$'
    Description: The CIDR block for the VPC

Parameter Properties