Serverless Rack

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A Serverless v1.x plugin to build your deploy Ruby Rack applications using Serverless. Compatible Rack application frameworks include Sinatra, Cuba and Padrino.

Features

Install

sls plugin install -n serverless-rack

This will automatically add the plugin to package.json and the plugins section of serverless.yml.

Sinatra configuration example

project
├── api.rb
├── config.ru
├── Gemfile
└── serverless.yml

api.rb

A regular Sinatra application.

require 'sinatra'

get '/cats' do
  'Cats'
end

get '/dogs/:id' do
  'Dog'
end

config.ru

require './api'
run Sinatra::Application

serverless.yml

All functions that will use Rack need to have rack_adapter.handler set as the Lambda handler and use the default lambda-proxy integration for API Gateway. This configuration example treats API Gateway as a transparent proxy, passing all requests directly to your Sinatra application, and letting the application handle errors, 404s etc.

service: example

provider:
  name: aws
  runtime: ruby2.5

plugins:
  - serverless-rack

functions:
  api:
    handler: rack_adapter.handler
    events:
      - http: ANY /
      - http: ANY {proxy+}

Gemfile

Add Sinatra to the application bundle.

source 'https://rubygems.org'

gem 'sinatra'

Deployment

Simply run the serverless deploy command as usual:

$ bundle install --path vendor/bundle
$ sls deploy
Serverless: Packaging Ruby Rack handler...
Serverless: Packaging gem dependencies using docker...
Serverless: Packaging service...
Serverless: Excluding development dependencies...
Serverless: Uploading CloudFormation file to S3...
Serverless: Uploading artifacts...
Serverless: Uploading service .zip file to S3 (1.64 MB)...
Serverless: Validating template...
Serverless: Updating Stack...
Serverless: Checking Stack update progress...
..............
Serverless: Stack update finished...

Usage

Automatic bundling of gems

You’ll need to include any gems that your application uses in the bundle that’s deployed to AWS Lambda. This plugin helps you out by doing this automatically, as long as you specify your required gems in a Gemfile:

source 'https://rubygems.org'

gem 'rake'
gem 'sinatra'

For more information, see https://bundler.io/docs.html.

Dockerized bundling

If your application depends on any gems that include compiled binaries, these must be compiled for the lambda execution environment. Enabling the dockerizeBundler configuration option will fetch and build the gems using a docker image that emulates the lambda environment:

custom:
  rack:
    dockerizeBundler: false

Bundler configuration

You can use the automatic bundling functionality of serverless-rack without the Rack request handler itself by including the plugin in your serverless.yml configuration, without specifying rack_adapter.handler as the handler for any of your lambda functions. This will omit the Rack handler from the package, but include any gems specified in the Gemfile.

If you don’t want to use automatic gem bundling you can set custom.rack.enableBundler to false:

custom:
  rack:
    enableBundler: false

In order to pass additional arguments to bundler when installing requirements, the bundlerArgs configuration option is available:

custom:
  rack:
    bundlerArgs: --no-cache

If your bundler executable is not in $PATH, set the path explicitly using the bundlerBin configuration option:

custom:
  rack:
    bundlerBin: /path/to/bundler

Local server

For convenience, a sls rack serve command is provided to run your Rack application locally. This command requires the rack gem to be installed, and acts as a simple wrapper for rackup.

By default, the server will start on port 5000.

$ sls rack serve
[2019-01-03 18:13:21] INFO  WEBrick 1.4.2
[2019-01-03 18:13:21] INFO  ruby 2.5.1 (2018-03-29) [x86_64-linux-gnu]
[2019-01-03 18:13:21] INFO  WEBrick::HTTPServer#start: pid=25678 port=5000

Configure the port using the -p parameter:

$ sls rack serve -p 8000
[2019-01-03 18:13:21] INFO  WEBrick 1.4.2
[2019-01-03 18:13:21] INFO  ruby 2.5.1 (2018-03-29) [x86_64-linux-gnu]
[2019-01-03 18:13:21] INFO  WEBrick::HTTPServer#start: pid=25678 port=8000

When running locally, an environment variable named IS_OFFLINE will be set to True. So, if you want to know when the application is running locally, check ENV["IS_OFFLINE"].

Remote command execution

The rack exec command lets you execute ruby code remotely:

$ sls rack exec -c "puts (1 + Math.sqrt(5)) / 2"
1.618033988749895

$ cat count.rb
3.times do |i|
  puts i
end

$ sls rack exec -f count.rb
0
1
2

The rack command command lets you execute shell commands remotely:

$ sls rack command -c "pwd"
/var/task

$ cat script.sh
#!/bin/bash
echo "dlrow olleh" | rev

$ sls rack command -f script.sh
hello world

The rack rake command lets you execute Rake tasks remotely:

$ sls rack rake -t "db:rollback STEP=3"

Explicit routes

If you’d like to be explicit about which routes and HTTP methods should pass through to your application, see the following example:

service: example

provider:
  name: aws
  runtime: ruby2.5

plugins:
  - serverless-rack

functions:
  api:
    handler: rack_adapter.handler
    events:
      - http:
          path: cats
          method: get
          integration: lambda-proxy
      - http:
          path: dogs/{id}
          method: get
          integration: lambda-proxy

Custom domain names

If you use custom domain names with API Gateway, you might have a base path that is at the beginning of your path, such as the stage (/dev, /stage, /prod). In this case, set the API_GATEWAY_BASE_PATH environment variable to let serverless-rack know.

The example below uses the serverless-domain-manager plugin to handle custom domains in API Gateway:

service: example

provider:
  name: aws
  runtime: ruby2.5
  environment:
    API_GATEWAY_BASE_PATH: ${self:custom.customDomain.basePath}

plugins:
  - serverless-rack
  - serverless-domain-manager

functions:
  api:
    handler: rack_adapter.handler
    events:
      - http: ANY /
      - http: ANY {proxy+}

custom:
  customDomain:
    basePath: ${opt:stage}
    domainName: mydomain.name.com
    stage: ${opt:stage}
    createRoute53Record: true

File uploads

In order to accept file uploads from HTML forms, make sure to add multipart/form-data to the list of content types with Binary Support in your API Gateway API. The serverless-apigw-binary Serverless plugin can be used to automate this process.

Keep in mind that, when building Serverless applications, uploading directly to S3 from the browser is usually the preferred approach.

Raw context and event

The raw context and event from AWS Lambda are both accessible through the Rack request. The following example shows how to access them when using Flask:

require 'sinatra'

get '/' do
  puts request.env['serverless.event']
  puts request.env['serverless.context']
end

Text MIME types

By default, all MIME types starting with text/ and the following whitelist are sent through API Gateway in plain text. All other MIME types will have their response body base64 encoded (and the isBase64Encoded API Gateway flag set) in order to be delivered by API Gateway as binary data (remember to add any binary MIME types that you’re using to the Binary Support list in API Gateway).

This is the default whitelist of plain text MIME types:

In order to add additional plain text MIME types to this whitelist, use the textMimeTypes configuration option:

custom:
  rack:
    textMimeTypes:
      - application/custom+json
      - application/vnd.company+json

Usage without Serverless

The AWS API Gateway to Rack mapping module is available as a gem.

Use this gem if you need to deploy Ruby functions to handle API Gateway events directly, without using the Serverless framework.

gem install --install-dir vendor/bundle serverless-rack

Initialize your Rack application and in your Lambda event handler, call the request mapper:

require 'serverless_rack'

$app ||= Proc.new do |env|
  ['200', {'Content-Type' => 'text/html'}, ['A barebones rack app.']]
end

def handler(event:, context:)
  handle_request(app: $app, event: event, context: context)
end