cfn 0.0.11

Stacks

A stack defines the static values and dynamic parameters that are put into the template to generate a usable CloudFormation template.

You can define multiple stacks that use the same template by using a different region or environment.

In the examples below I use a single template to define a CloudFormation stack for the regions eu-west-1 and eu-central-1. The example creates one subnet per availability zone, something that differs between the two regions. So, instead of limiting ourself to just using two availability zones, we use a bit of Jinja2 templating and simply generate the subnets from a list.

Templates

A template is a YAML file that is pre-processed by Jinja2 before being turned into JSON. It gets values from the stack and is then generated to provide a finished stack that can be used by CloudFormation.

Create your first stack and template

First, create two directories, stacks and templates. These folders will contain your stacks and your templates.

Then, create two stacks, one for region eu-west-1 and one for eu-central-1. You can copy paste the code below.

# stacks/example.eu-west-1.yml
---
name: Example
values:
  azs:
    - eu-west-1a
    - eu-west-1b
    - eu-west-1c

# stacks/example.eu-central-1.yml
---
name: Example
values:
  azs:
    - eu-central-1a
    - eu-central-1b

Next, create a template that looks like this:

# templates/example.yml
---
AWSTemplateFormatVersion: "2010-09-09"

Description: Example stack template

Resources:
{% for az in azs %}
  {{ "Subnet%s:" | format(az[-1].upper()) }}
    Type: AWS::EC2::Subnet
    Properties:
      AvailabilityZone: {{ az }}
      # Other properties are not present, because the example
      # would become too long...
{% endfor %}

That’s it! Now it’s time to generate the CloudFormation files. Run cfn --region eu-west-1 generate example and cfn --region eu-central-1 generate example. You should have a new folder output which contains two files, example.eu-west-1.json and example.eu-central-1.json. These are the ready to use CloudFormation templates.

The generated CloudFormation stack template for stacks/example.eu-west-1.yml would look something like this:

{
    "AWSTemplateFormatVersion": "2010-09-09",
    "Description": "Example stack template",
    "Resources": {
        "SubnetA": {
            "Type": "AWS::EC2::Subnet",
            "Properties": {
                "AvailabilityZone": "eu-west-1a"
            }
        },
        "SubnetB": {
            "Type": "AWS::EC2::Subnet",
            "Properties": {
                "AvailabilityZone": "eu-west-1b"
            }
        },
        "SubnetC": {
            "Type": "AWS::EC2::Subnet",
            "Properties": {
                "AvailabilityZone": "eu-west-1c"
            }
        }
    }
}

You can validate these files with the aws-cli tool by running aws cloudformation validate-template --template-body file://output/example.eu-central-1.json.

To create the stack using the cfn tool, run cfn --region eu-west-1 create example.

If you’ve made changes to the stack and wish to update, run cfn --region eu-west-1 update example.

Run cfn --region eu-west-1 delete example to delete an existing stack created by the cfn tool.