How to Use Parameters in CloudFormation Templates

In AWS CloudFormation, you use Parameters to pass values into your template at stack creation time. Define parameters in the Parameters section, then reference them in resources using Ref or Fn::GetAtt. This lets you customize your infrastructure without changing the template code.

Syntax

The Parameters section defines inputs your template accepts. Each parameter has a name and properties like Type, Description, and optional Default value. You reference parameters inside resources using Ref to get their value.

• Parameters: The section where you declare inputs.
• Type: Defines the kind of input (e.g., String, Number, AWS-specific types).
• Default: Optional default value if none is provided.
• Ref: Used to access the parameter value in the template.

yaml

Parameters:
  InstanceTypeParameter:
    Type: String
    Description: EC2 instance type
    Default: t2.micro

Resources:
  MyEC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: !Ref InstanceTypeParameter
      ImageId: ami-0abcdef1234567890

Example

This example shows a CloudFormation template that uses a parameter to set the EC2 instance type. When you create the stack, you can choose the instance type or use the default t2.micro. The template references the parameter with !Ref to assign the instance type.

yaml

AWSTemplateFormatVersion: '2010-09-09'
Description: Simple EC2 instance with parameterized instance type
Parameters:
  InstanceTypeParameter:
    Type: String
    Description: EC2 instance type
    Default: t2.micro
Resources:
  MyEC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: !Ref InstanceTypeParameter
      ImageId: ami-0abcdef1234567890

Output

When deployed, the EC2 instance will launch with the instance type specified by the parameter or the default t2.micro if none is provided.

Common Pitfalls

Common mistakes when using parameters include:

• Not specifying a Type for the parameter, causing deployment errors.
• Using Ref incorrectly on resources instead of parameters.
• Forgetting to provide a value when no default is set, which causes stack creation to fail.
• Using parameters for values that should be hardcoded or managed by other means.

Always validate your template and test parameter inputs before deployment.

yaml

Parameters:
  InstanceTypeParameter:
    Description: EC2 instance type

Resources:
  MyEC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: !Ref MyEC2Instance  # Incorrect: Ref should point to parameter, not resource
      ImageId: ami-0abcdef1234567890

# Correct usage:
# InstanceType: !Ref InstanceTypeParameter

Quick Reference

Here is a quick summary of key points when using parameters in CloudFormation:

Concept	Description
Parameters Section	Defines inputs your template accepts.
Type	Specifies the kind of input (String, Number, AWS-specific).
Default	Optional default value if no input is given.
Ref	Used to get the parameter value inside the template.
No Default	Requires user to provide a value at stack creation.
Validation	Use AllowedValues or AllowedPattern to restrict inputs.

Key Takeaways

Define parameters in the Parameters section with a clear Type and optional Default.

Use !Ref to access parameter values inside your resources.

Always provide a value for parameters without defaults when creating a stack.

Validate parameter inputs to avoid deployment errors.

Parameters let you customize templates without changing the code.