A dead simple API web server contains three endpoints: fibonacci numbers, Google's home page sha1, a simple store.

api_server.rb

require 'socket'
require 'net/https'
require 'uri'
require 'digest/sha1'

def fibonacci(n)
  n < 2 ? n : fibonacci(n-1) + fibonacci(n-2)
end

def google_body
  uri = URI.parse 'https://www.google.com.ua/'
  http = Net::HTTP.new uri.host, uri.port
  http.use_ssl = true
  http.verify_mode = OpenSSL::SSL::VERIFY_NONE
  response = http.request Net::HTTP::Get.new(uri.request_uri)
  Digest::SHA1.hexdigest response.body
end

def set_value(value)
  `echo "#{value}" > store`
end

def get_value
  `cat store`.chop
end

def value_from_request(socket)
  while (line = socket.gets)
    if line =~ /^Content-Length: (\d+)/
      content_length = $1.to_i
    elsif line == "\r\n"
      break
    end
  end
  socket.read(content_length).match(/^value=(.+)/)[1]
end

server = TCPServer.new 2000

loop do
  socket = server.accept

  case socket.gets
    when /^GET \/fib\/(\d+)/  then socket.puts "{\"response\": #{fibonacci($1.to_i)}}"
    when /^GET \/google-body/ then socket.puts "{\"response\": #{google_body}}"
    when /^GET \/google-body/ then socket.puts "{\"response\": #{google_body}}"
    when /^GET \/store/       then socket.puts "{\"response\": \"#{get_value}\"}"
    when /^POST \/store/      then set_value(value_from_request(socket)) and socket.puts("{\"response\": \"ok\"}")
    else socket.puts "{\"error\": \"Unknown request\"}"
  end

  socket.close
end

here I loose testing gets for nil. Not sure if it's important.

Assuming it is a POST request with "value=blah" payload, you can safely skip the check.