Sinatra

Sinatra’s domain-specific language maps HTTP verbs and paths to blocks of code — a complete API can fit in a single file. It influenced Flask, Express, and a generation of micro-frameworks. For Ruby services where Rails’ weight is unnecessary, Sinatra remains the clean, minimal choice.

Quick start

gem install sinatra puma

# app.rb
require 'sinatra'
require 'json'

posts = []

get '/posts' do
  content_type :json
  posts.to_json
end

get '/posts/:id' do
  content_type :json
  post = posts.find { |p| p[:id] == params[:id].to_i }
  halt 404, { error: 'Not found' }.to_json unless post
  post.to_json
end

post '/posts' do
  content_type :json
  data = JSON.parse(request.body.read, symbolize_names: true)
  post = { id: posts.length + 1, **data }
  posts << post
  status 201
  post.to_json
end

ruby app.rb
# Server running on http://localhost:4567

When to use

Sinatra is the right choice for small Ruby services, internal APIs, and scripts that need HTTP endpoints — anything where the full Rails stack would be overkill. It’s particularly popular for microservices, webhooks, and Rack middleware apps. For larger applications with database models, authentication, and complex routing, Rails is a better fit. Flask and Express are the Python and Node.js equivalents.