lambda.rb

# this code was taken from the AWS serverless-sinatra-sample repo
# https://github.com/aws-samples/serverless-sinatra-sample/tree/master

require "json"
require "rack"
require "base64"

# Global object that responds to the call method. Stay outside of the handler
# to take advantage of container reuse
$app ||= Rack::Builder.parse_file("#{__dir__}/app/config.ru") # .first
ENV["RACK_ENV"] ||= "production"

def handler(event:, context:)
  # Check if the body is base64 encoded. If it is, try to decode it
  body = if event["isBase64Encoded"]
    Base64.decode64 event["body"]
  else
    event["body"]
  end

  # Rack expects the querystring in plain text, not a hash
  headers = event.fetch "headers", {}

  # Environment required by Rack (http://www.rubydoc.info/github/rack/rack/file/SPEC)
  env = {
    "REQUEST_METHOD" => event.fetch("httpMethod"),
    "SCRIPT_NAME" => "",
    "PATH_INFO" => event.fetch("path", ""),
    "QUERY_STRING" => (event["queryStringParameters"] || {}).map { |k, v| "#{k}=#{v}" }.join("&"),
    "SERVER_NAME" => headers.fetch("Host", "localhost"),
    "SERVER_PORT" => headers.fetch("X-Forwarded-Port", 443).to_s,

    "rack.version" => Rack::VERSION,
    "rack.url_scheme" => headers.fetch("CloudFront-Forwarded-Proto") { headers.fetch("X-Forwarded-Proto", "https") },
    "rack.input" => StringIO.new(body.to_s),
    "rack.errors" => $stderr
  }

  # Pass request headers to Rack if they are available
  headers.each_pair do |key, value|
    # 'CloudFront-Forwarded-Proto' => 'CLOUDFRONT_FORWARDED_PROTO'
    # Content-Type and Content-Length are handled specially per the Rack SPEC linked above.
    name = key.upcase.tr "-", "_"
    header = case name
    when "CONTENT_TYPE", "CONTENT_LENGTH"
      name
    else
      "HTTP_#{name}"
    end
    env[header] = value.to_s
  end

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

    # body is an array. We 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
    }
    if event["requestContext"].has_key?("elb")
      # Required if we use Application Load Balancer instead of API Gateway
      response["isBase64Encoded"] = false
    end
  rescue StandardException => exception
    # If there is _any_ exception, we return a 500 error with an error message
    response = {
      "statusCode" => 500,
      "body" => exception.message
    }
  end

  # By default, the response serializer will call #to_json for us
  response
end