We generate each templates CloudFormation .json file and check them in to spot subtle diffs when refactoring. We also validate all configs on every PR. Doing this serially takes a lot of time, but it can be parallelized easily, while also avoiding ruby boot overhead.

This took us from ~5 minutes runtime to 30s, enjoy!

desc "Validate templates via cloudformation api"
task :validate do
  each_template do |template|
    execute_sfn_command :validate, template
  end
end

desc "Generates cloudformation json files"
task :generate, [:pattern] do |_t, args|
  pattern = /#{args[:pattern]}/ if args[:pattern]
  previous = Dir["generated/*.json"]

  used = each_template do |template|
    next if pattern && template !~ pattern
    output = execute_sfn_command :print, template

    generated = "generated/#{File.basename(template.sub('.rb', '.json'))}"
    File.write generated, output
    generated
  end

  (previous - used).each { |f| File.unlink(f) } unless pattern
end

...

require 'parallel'
require 'timeout'

def each_template(&block)
  # preload slow requires
  require 'bogo-cli'
  require 'sfn'

  # run in parallel, but isolated to avoid caches from being reused
  options = {in_processes: 10, progress: "Progress", isolation: true}
  Parallel.map(Dir["templates/**/*.rb"], options, &block)
end

private

def execute_sfn_command(command, template, *args)
  Timeout.timeout(20) do
    capture_stdout do
      Sfn::Command.const_get(command.capitalize).new({
        defaults: true, # do not ask questions about parameters
        file: template,
        retry: {type: :flat, interval: 1} # we will run into rate limits, ignore them quickly
      }, args).execute!
    end
  end
rescue StandardError
  # give users context when something failed
  warn "bundle exec sfn #{command} #{args.join(" ")} --defaults -f #{template}"
  raise
end

def capture_stdout
  old = $stdout
  $stdout = StringIO.new
  yield
  $stdout.string
ensure
  $stdout = old
end