cloudformation-ruby-dsl

A Ruby DSL and helper utilities for building Cloudformation templates dynamically.

Written by Bazaarvoice: see the contributors page and the initial contributions for more details.

Motivation

Cloudformation templates often contain repeated stanzas, information which must be loaded from external sources, and other functionality that would be easier handled as code, instead of configuration.

Consider when a userdata script needs to be added to a Cloudformation template. Traditionally, you would re-write the script by hand in a valid JSON format. Using the DSL, you can specify the file containing the script and generate the correct information at runtime.

:UserData => base64(interpolate(file('userdata.sh')))

Additionally, Cloudformation templates are just massive JSON documents, making general readability and reusability an issue. The DSL allows not only a cleaner format (and comments!), but will also allow the same DSL template to be reused as needed.

Installation

Run gem install cloudformation-ruby-dsl to install system-wide.

To use in a specific project, add gem 'cloudformation-ruby-dsl' to your Gemfile, and then run bundle.

Usage

To convert existing JSON templates to use the DSL, run

cfntemplate-to-ruby [EXISTING_CFN] > [NEW_NAME.rb]

You may need to preface this with bundle exec if you installed via Bundler.

Make the resulting file executable (chmod +x [NEW_NAME.rb]). It can respond to the following subcommands (which are listed if you run without parameters):

• expand: output the JSON template to the command line (takes optional --nopretty to minimize the output)
• diff: compare output with existing JSON for a stack
• cfn-validate-template: run validation against the stack definition
• cfn-create-stack: create a new stack from the output
• cfn-update-stack: update an existing stack from the output

Below are the various functions currently available in the DSL. See the example script for more usage information.

DSL Statements

Add the named object to the appropriate collection.

• parameter(name, options) (may be marked :Immutable, which will raise error on a later change)
• mapping(name, options)
• condition(name, options)
• resource(name, options)
• output(name, options)

Cloudformation Function Calls

Invoke a native Cloudformation function.

• base64(value)
• find_in_map(map, key, name)
• get_att(resource, attribute)
• get_azs(region)
• join(delim, *list)
• select(index, list)
• ref(name)
• no_value()

Utility Functions

Additional capabilities for file inclusion, etc.

• tag(tag): add tags to the stack, which are inherited by all resources in that stack; can only be used at launch
• file(name): return the named file as a string, for further use
• load_from_file(filename): load the named file by a given type; currently handles YAML, JSON, and Ruby
• interpolate(string): embed CFN references into a string ({{ref('Service')}}) for later interpretation by the CFN engine
• Table.load(filename): load a table from the listed file, which can then be turned into mappings (via get_map)