Parallel I/O with GraphQL::Dataloader

dataloader_example.rb

require "bundler/inline"

gemfile do
  gem "graphql"
  gem "libev_scheduler"
end

class MySchema < GraphQL::Schema
  class FetchThings < GraphQL::Dataloader::Source
    def initialize(starting_at:)
      @starting_at = starting_at
    end

    def fetch(_ids)
      # Pretend it takes a long time:
      sleep_for = [0.5, 1].sample
      sleep sleep_for
      # Return the things:
      things = 4.times.map do |i|
        { value: @starting_at + i }
      end
      # There's only ever one ID here, but return an Array to make it match:
      [ things ]
    end
  end

  class Thing < GraphQL::Schema::Object
    field :value, Int
  end

  class Query < GraphQL::Schema::Object
    field :things, [Thing] do
      argument :starting_at, Int
    end

    def things(starting_at:)
      dataloader.with(FetchThings, starting_at: starting_at).load(nil)
    end
  end

  query(Query)
  use GraphQL::Dataloader, nonblocking: true
end

Fiber.set_scheduler(Libev::Scheduler.new)

def run_query
  start = Time.now

  pp MySchema.execute("{
    t1: things(startingAt: 0) { value }
    t2: things(startingAt: 4) { value }
  }").to_h
  # {"data"=>
  #   {"t1"=>[{"value"=>0}, {"value"=>1}, {"value"=>2}, {"value"=>3}],
  #    "t2"=>[{"value"=>4}, {"value"=>5}, {"value"=>6}, {"value"=>7}]}
  # }

  pp [:elapsed, Time.now - start, :seconds]
  # [:elapsed, 1.006262, :seconds]
  # It only took one second -- the two sleeps happened simultaneously
end


10.times do
  run_query
end