Deploying Sinatra: small, sharp, shippable.: what should I watch out for? (#1)

The classic-style Sinatra app is a singleton — don't stash per-request state in constants or class ivars.

Deploying Sinatra: small, sharp, shippable.: what should I watch out for? (#2)

Rack 3 changed the response body contract to require `each` returning strings, not arrays. Some old middleware will break.

Deploying Sinatra: small, sharp, shippable.: what should I watch out for? (#3)

If you use `Sinatra::Reloader`, gate it behind `configure(:development)` — it's slow and unsafe in production.

// framework · sinatra

Sinatra: small, sharp, shippable.

The 40-line Dockerfile framework. Perfect for internal APIs, webhooks, and 'we just need one endpoint' services.

Sinatra doesn't need a deployment guide as much as it needs a discipline guide — the framework gets out of your way, which means every deployment decision is yours. Here's the boring, correct default.

128 MB

typical image size

220 ms

cold boot

~4k

req/s on 1 vCPU

01 / 03

config.ru is the deploy target

Every cloud that speaks Rack — which is all of them — will boot a Sinatra app given a config.ru and a Gemfile. Keep it that simple.

ruby
# config.ru
require "./app"
run Sinatra::Application

# Gemfile
source "https://rubygems.org"
ruby "3.3.5"
gem "sinatra", "~> 4.1"
gem "puma", "~> 6.4"
gem "json"

02 / 03

Puma config that scales

Sinatra apps are usually thread-safe and stateless — perfect for a threaded Puma. Set workers to your CPU count on VMs, and to 1 on Fargate / Cloud Run.

ruby
# config/puma.rb
threads Integer(ENV.fetch("RAILS_MIN_THREADS", 5)),
        Integer(ENV.fetch("RAILS_MAX_THREADS", 5))
workers Integer(ENV.fetch("WEB_CONCURRENCY", 1))
preload_app!
port    ENV.fetch("PORT", 3000)
environment ENV.fetch("RACK_ENV", "production")

03 / 03

Health checks

Add a `/healthz` route that returns 200 without touching the database, and a `/readyz` that does. Every load balancer expects both.

ruby
get "/healthz" do
  content_type :json
  { status: "ok", pid: Process.pid }.to_json
end

Gotchas we learned the hard way

!The classic-style Sinatra app is a singleton — don't stash per-request state in constants or class ivars.
!Rack 3 changed the response body contract to require `each` returning strings, not arrays. Some old middleware will break.
!If you use `Sinatra::Reloader`, gate it behind `configure(:development)` — it's slow and unsafe in production.