AWS CloudFormation

Template Structure

A CloudFormation template is composed of several major sections. While only Resources is required, a typical template is organized as follows:

1. AWSTemplateFormatVersion (Optional)

   • Specifies the template format version. The only valid value is 2010-09-09.

2. Description (Optional)

   • A text string that describes the template.

3. Metadata (Optional)

   • JSON or YAML objects that provide additional information about the template.

4. Parameters (Optional)

   • Allows you to input custom values to your template each time you create or update a stack. This makes templates reusable.

5. Mappings (Optional)

   • A mapping of keys and associated values that you can use to specify conditional parameter values, similar to a lookup table (e.g., mapping an AWS Region to a specific AMI ID).

6. Conditions (Optional)

   • Define conditions that control whether certain resources are created or whether certain resource properties are assigned a value during stack creation or update.

7. Transform (Optional)

   • Specifies one or more macros that AWS CloudFormation uses to process your template. The most common is AWS::Serverless for SAM templates.

8. Resources (Required)

   • The core of the template. This section specifies the stack resources and their properties, such as an EC2 instance or an S3 bucket.

9. Outputs (Optional)

   • Describes the output values that you can import into other stacks, view on the AWS CloudFormation console, or see in response to describe-stacks API calls.

Important Features

Drift Detection

• Drift detection enables you to detect whether a stack's actual configuration has "drifted" from its expected configuration as defined in the template.

• A stack is considered to have drifted if one or more of its resources have been changed manually outside of CloudFormation.

Cross-Stack References

• You can export output values from one stack to be used in another stack.

• The other stacks can then import these values using the Fn::ImportValue intrinsic function. This is useful for creating loosely coupled, modular architectures (e.g., a networking stack that exports a VPC ID for application stacks to use).

Resource Import

• Allows you to bring existing, manually-created AWS resources under CloudFormation management without having to delete and recreate them.

Deletion Policies

• You can add a DeletionPolicy attribute to any resource.

  • Delete (Default): CloudFormation deletes the resource when the stack is deleted.

  • Retain: CloudFormation leaves the resource intact when the stack is deleted.

  • Snapshot: For resources that support it (like EBS Volumes and RDS Databases), CloudFormation creates a snapshot before deleting the resource.

Custom Resources

• Enable you to write custom provisioning logic in your template. CloudFormation invokes a Lambda function or SNS topic to handle the creation, update, or deletion of these resources.

CloudFormation Designer

• A graphic tool for creating, viewing, and modifying CloudFormation templates with a drag-and-drop interface.