Serverless Sinatra

June 10, 2017

"AWS Lambda allows you to host serverless functions written in almost any language. Let me show you how to use Ruby."

At the moment the AWS Lambda service does not support ruby. It is however possible to spawn another process from within one of the supported runtimes e.g. nodejs. This post shows that it possible to run a rack based web application in AWS Lambda.

Show me the code!

The code that shows how to do this is at serverless-ruby-sinatra-example

How does it work?

serverless ruby

The project uses the serverless framework to deploy a lambda function that responds to a AWS Api Gateway events. The lambda function then spawns a process that execute some ruby code that invokes the sinatra application via the rack webserver interface.

Packaging the ruby code

AWS Lambda functions run within a container on an AWS EC2 instances. At the time of writing this post the ruby runtime is unavailable. In order to make the ruby runtime available I am using travelling ruby to package the ruby runtime and all the gem dependencies as a zip file. These are included in the zip file the serverless framework uploads to s3.

The code for packaging the ruby runtime and dependencies can be found in the Rakefile


The shim.js script is a lambda function that handles API Gateways events by spawning the script and then passes the lambda event and context objects to rackadapter.rb as json text.

var proc = spawn('./ruby-bin/', 
    [JSON.stringify(event, null, 2), JSON.stringify(context, null, 2)]


The rack_adapter.rb simply converts the event and context structures into a rack environment hash and then calls the sinatra application. The response is written to stdout.

What are the pain points of this approach?

The main issues with this approach is that every time the lambda function is invoked a new ruby vm is launched. This take about 700ms for this really simple hello world style app. This is about 7 times slower then the equivalent nodejs lambda.

Another limitation is that travelling ruby only supports up to ruby 2.2 and there are limited pre built native extension gems available that are compatible with Amazon linux. You could always build your application and native dependencies on an Amazon Linux build agent to work around this.