Params

CloudFormation parameters allow you to create different environments with the same template code. Lono params simplify this process by providing an env-like file syntax.

Structure

The config files are located at config/blueprints/BLUEPRINT/params. Example:

config/blueprints
└── demo
    └── params
        ├── dev.env
        └── prod.env

Examples

Lono allows you to set parameters in a simple KEY=VALUE format commonly found in env files. This format is less prone to human error than the AWS more verbose parameter file format. When lono build runs it processes the files in config/blueprints/BLUEPRINT/params folders and outputs the AWS JSON format file in output/BLUEPRINT/params folder. Here’s an example:

config/blueprints/demo/params/dev.env

KeyName=default
InstanceType=t2.micro

This results in:

[
  {
    "ParameterKey": "InstanceType",
    "ParameterValue": "t2.micro"
  },
  {
    "ParameterKey": "KeyName",
    "ParameterValue": "default"
  }
]

These files can directly be used to launch the CloudFormation stack with the aws cloudformation CLI tool manually. Though, the lono up command automates this for you. For example, running lono up BLUEPRINT will automatically build the param file and use it when deploying the stack.

ERB Support and SSM Helper

Parameters files support ERB evaluation. This allows you to use the ssm helper method to substitute secrets from SSM parameter store. This avoids committing secret information to your repo. Example:

config/blueprints/demo/params/dev.env

KeyName=<%= ssm("/#{Lono.env}/key_name") %>
InstanceType=t2.micro

Here’s an example of storing an SSM parameter:

aws ssm put-parameter --name /dev/key_name --value demo-key-pair --type SecureString

To confirm that parameter is stored:

 aws ssm get-parameter --name /dev/key_name --with-decryption

Variables Substitution

Variables are also available in the params file. Here’s an example:

config/blueprints/demo/vars/base.rb

@ami = "ami-12345"

config/blueprints/demo/params/base.rb

Ami=<%= @ami %>

Will produce:

output/demo/params/mystack.json

[
  {
    "ParameterKey": "Ami",
    "ParameterValue": "ami-12345"
  }
]

Layering Support

Lono params support layering which is covered in Layering Support. This allows you to create different environments with the same CloudFormation template.

Getting Started