CloudFormation Template Basics

Structure of a template

A CloudFormation template is made up of several sections:

AWSTemplateFormatVersion: '2010-09-09'
Description: Description du template

Metadata:
  # Informations additionnelles

Parameters:
  # Paramètres d'entrée

Mappings:
  # Correspondances clé-valeur

Conditions:
  # Logique conditionnelle

Transform:
  # Macros et transformations

Resources:
  # OBLIGATOIRE - Ressources AWS

Outputs:
  # Valeurs de sortie

Sections in detail

AWSTemplateFormatVersion

The template format version. Currently, only 2010-09-09 exists.

AWSTemplateFormatVersion: '2010-09-09'

Description

A textual description of the template (max 1024 characters).

Description: |
  Ce template crée une infrastructure web complète
  incluant VPC, EC2, et RDS.
  Version: 1.0.0

Metadata

Additional information for the template.

Metadata:
  # Interface pour la console AWS
  AWS::CloudFormation::Interface:
    ParameterGroups:
      - Label:
          default: "Configuration Réseau"
        Parameters:
          - VPCCidr
          - SubnetCidr
      - Label:
          default: "Configuration Instance"
        Parameters:
          - InstanceType
          - KeyName
    ParameterLabels:
      VPCCidr:
        default: "CIDR du VPC"

YAML vs JSON

CloudFormation supports both formats. YAML is recommended for its readability.

YAML

Resources:
  MyBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: my-bucket
      Tags:
        - Key: Environment
          Value: Production

Equivalent JSON

{
  "Resources": {
    "MyBucket": {
      "Type": "AWS::S3::Bucket",
      "Properties": {
        "BucketName": "my-bucket",
        "Tags": [
          {
            "Key": "Environment",
            "Value": "Production"
          }
        ]
      }
    }
  }
}

Comparison

Aspect	YAML	JSON
Readability	✅ Excellent	⚠️ Average
Comments	✅ Supported	❌ Not supported
Verbosity	Concise	Verbose
Common errors	Indentation	Commas, quotes

YAML syntax for CloudFormation

Data types

# String
Name: "ma-valeur"

# Number
Port: 443

# Boolean
Enabled: true

# List
AvailabilityZones:
  - eu-west-1a
  - eu-west-1b
  - eu-west-1c

# Map
Tags:
  Environment: Production
  Team: DevOps

# Multi-line string (literal)
UserData: |
  #!/bin/bash
  yum update -y
  yum install -y httpd

# Multi-line string (folded)
Description: >
  Ceci est une description
  sur plusieurs lignes
  qui sera jointe en une seule.

Anchors and aliases

# Définir une ancre
CommonTags: &common-tags
  Environment: Production
  ManagedBy: CloudFormation

Resources:
  Bucket1:
    Type: AWS::S3::Bucket
    Properties:
      Tags:
        <<: *common-tags  # Réutiliser l'ancre
        Name: bucket-1
  
  Bucket2:
    Type: AWS::S3::Bucket
    Properties:
      Tags:
        <<: *common-tags
        Name: bucket-2

Pseudo-parameters

CloudFormation provides predefined pseudo-parameters:

Resources:
  MyBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Sub 'app-${AWS::Region}-${AWS::AccountId}'
      Tags:
        - Key: StackName
          Value: !Ref AWS::StackName
        - Key: StackId
          Value: !Ref AWS::StackId

Pseudo-parameter	Description
`AWS::AccountId`	AWS account ID
`AWS::Region`	Current region
`AWS::StackName`	Stack name
`AWS::StackId`	Stack ARN ID
`AWS::URLSuffix`	URL suffix (amazonaws.com)
`AWS::Partition`	Partition (aws, aws-cn, aws-us-gov)
`AWS::NoValue`	Removes a property

Template validation

Via CLI

# Validation locale
aws cloudformation validate-template \
    --template-body file://template.yaml

# Validation avec template S3
aws cloudformation validate-template \
    --template-url https://s3.amazonaws.com/bucket/template.yaml

Via cfn-lint

# Installer cfn-lint
pip install cfn-lint

# Valider un template
cfn-lint template.yaml

# Avec règles spécifiques
cfn-lint -t template.yaml --ignore-checks W3002

Common errors

# ❌ ERREUR: Indentation incorrecte
Resources:
MyBucket:
  Type: AWS::S3::Bucket

# ✅ CORRECT
Resources:
  MyBucket:
    Type: AWS::S3::Bucket

# ❌ ERREUR: Type invalide
Resources:
  MyBucket:
    Type: AWS::S3::Buckett  # Typo!

# ✅ CORRECT
Resources:
  MyBucket:
    Type: AWS::S3::Bucket

Complete template example

AWSTemplateFormatVersion: '2010-09-09'
Description: Infrastructure web basique avec VPC, Subnet et EC2

Metadata:
  AWS::CloudFormation::Interface:
    ParameterGroups:
      - Label:
          default: "Configuration Réseau"
        Parameters:
          - VPCCidr
      - Label:
          default: "Configuration Instance"
        Parameters:
          - InstanceType

Parameters:
  VPCCidr:
    Type: String
    Default: '10.0.0.0/16'
    Description: CIDR block pour le VPC
    AllowedPattern: '(\d{1,3}\.){3}\d{1,3}/\d{1,2}'
  
  InstanceType:
    Type: String
    Default: t3.micro
    AllowedValues:
      - t3.micro
      - t3.small
      - t3.medium
    Description: Type d'instance EC2

Mappings:
  RegionAMI:
    eu-west-1:
      AMI: ami-0c55b159cbfafe1f0
    eu-west-2:
      AMI: ami-0a669382ea0feb73a
    us-east-1:
      AMI: ami-0ff8a91507f77f867

Conditions:
  IsProduction: !Equals 
    - !Ref AWS::StackName
    - production

Resources:
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: !Ref VPCCidr
      EnableDnsHostnames: true
      EnableDnsSupport: true
      Tags:
        - Key: Name
          Value: !Sub '${AWS::StackName}-vpc'

  PublicSubnet:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: !Select 
        - 0
        - !Cidr 
          - !Ref VPCCidr
          - 4
          - 8
      MapPublicIpOnLaunch: true
      AvailabilityZone: !Select 
        - 0
        - !GetAZs ''
      Tags:
        - Key: Name
          Value: !Sub '${AWS::StackName}-public-subnet'

  WebServer:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: !Ref InstanceType
      ImageId: !FindInMap 
        - RegionAMI
        - !Ref AWS::Region
        - AMI
      SubnetId: !Ref PublicSubnet
      Tags:
        - Key: Name
          Value: !Sub '${AWS::StackName}-webserver'

Outputs:
  VPCId:
    Description: ID du VPC
    Value: !Ref VPC
    Export:
      Name: !Sub '${AWS::StackName}-VPCId'
  
  InstanceId:
    Description: ID de l'instance EC2
    Value: !Ref WebServer
  
  PublicIP:
    Description: IP publique de l'instance
    Value: !GetAtt WebServer.PublicIp
    Condition: IsProduction

Syntax best practices

1. Use descriptive names

# ❌ Mauvais
Resources:
  R1:
    Type: AWS::EC2::VPC
  
# ✅ Bon
Resources:
  ProductionVPC:
    Type: AWS::EC2::VPC

2. Comment the code

Resources:
  # VPC principal pour l'environnement de production
  # CIDR: 10.0.0.0/16 (65536 IPs)
  MainVPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: '10.0.0.0/16'

3. Organize resources logically

Resources:
  # ========================================
  # NETWORK
  # ========================================
  VPC:
    Type: AWS::EC2::VPC
    # ...
  
  PublicSubnet:
    Type: AWS::EC2::Subnet
    # ...
  
  # ========================================
  # SECURITY
  # ========================================
  SecurityGroup:
    Type: AWS::EC2::SecurityGroup
    # ...
  
  # ========================================
  # COMPUTE
  # ========================================
  WebServer:
    Type: AWS::EC2::Instance
    # ...

Summary

Templates are written in YAML or JSON
YAML is recommended for its readability
The Resources section is mandatory
Pseudo-parameters provide dynamic values
Always validate your templates before deployment

← Introduction | Resources →

Structure of a template​

Sections in detail​

AWSTemplateFormatVersion​

Description​

Metadata​

YAML vs JSON​

YAML​

Equivalent JSON​

Comparison​

YAML syntax for CloudFormation​

Data types​

Anchors and aliases​

Pseudo-parameters​

Template validation​

Via CLI​

Via cfn-lint​

Common errors​

Complete template example​

Syntax best practices​

1. Use descriptive names​

2. Comment the code​

3. Organize resources logically​

Summary​