async.rb

require 'sinatra'
set :port, 3000

TEMPLATE = DATA.read

class Async
  # execute a block identified by `id` synchronously if block is done returns
  # the block return value if not finished it returns the `default` value the
  # if the `id` id not provided it uses the block source location as an id
  # Example:
  # having a long running script calling
  # `Asybc.command { `ssh user@ip-address ps`}`
  # will return nil immediatly until the server responds with the `ps` output,
  # only then that line will return the output.
  # this pattern is useful if you have a long running server that needs to do
  # alot of async operations and aggregate the response of whatever returned,
  # and refreshes the result periodically, allowing you to render the result and
  # update it on periods
  def self.command(id: nil, default: nil, &block)
    id ||= block.source_location
    result(id, default, &block) || default
  end

  def self.result(id, default, &block)
    create(id, default, &block) || get(id)
  end

  def self.create(id, default, &block)
    @commands ||= {}
    return if @commands.key?(id)

    @commands[id] = Thread.new do
      Thread.current[:output] = block.call
    end
    default
  end

  def self.get(id)
    thread = @commands[id]
    return if thread.status

    thread[:output]
  end
end

get '/' do
  Async.command do
    `ssh vps ps`
  end
end

__END__
Hello world!