lambda.rb

require 'json'
require 'rack'

$app ||= Rack::Builder.parse_file("#{File.dirname(__FILE__)}/config.ru").first

def handler(event:, context:)
  # Environment required by Rack (http://www.rubydoc.info/github/rack/rack/file/SPEC)
  env = {
    'REQUEST_METHOD' => event['httpMethod'],
    'SCRIPT_NAME' => '',
    'PATH_INFO' => event['path'] || '',
    'QUERY_STRING' => event['queryStringParameters'] || '',
    'SERVER_NAME' => 'localhost',
    'SERVER_PORT' => 443,

    'rack.version' => Rack::VERSION,
    'rack.url_scheme' => 'https',
    'rack.input' => StringIO.new(event['body'] || ''),
    'rack.errors' => $stderr,
  }
  # Pass request headers to Rack if they are available
  unless event['headers'].nil?
    event['headers'].each { |key, value| env["HTTP_#{key}"] = value }
  end

  begin
    # Response from Rack must have status, headers and body
    status, headers, body = $app.call(env)

    # body is an array. We simply combine all the items to a single string
    body_content = ''
    body.each do |item|
      body_content += item.to_s
    end

    # We return the structure required by AWS API Gateway since we integrate with it
    # https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html
    response = {
      'statusCode' => status,
      'headers' => headers,
      'body' => body_content
    }
  rescue Exception => msg
    # If there is any exception, we return a 500 error with an error message
    response = {
      'statusCode' => 500,
      'body' => msg
    }
  end
  # By default, the response serializer will call #to_json for us
  response
end