Fix

Gemfile.lock

@@ -100,6 +100,8 @@ GEM
       rack (>= 3.0.0)
     rack-test (2.1.0)
       rack (>= 1.3)
+    rackup (2.2.1)
+      rack (>= 3)
     rails-dom-testing (2.2.0)
       activesupport (>= 5.0.0)
       minitest
@@ -161,6 +163,7 @@ DEPENDENCIES
   ostruct
   pry-byebug
   rack
+  rackup
   rake
   rspec
   rubocop (~> 0.75.0)

apollo-federation.gemspec

@@ -35,6 +35,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'ostruct'
   spec.add_development_dependency 'pry-byebug'
   spec.add_development_dependency 'rack'
+  spec.add_development_dependency 'rackup'
   spec.add_development_dependency 'rake'
   spec.add_development_dependency 'rspec'
   spec.add_development_dependency 'rubocop', '~> 0.75.0'

example/graphql_server.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'rack'
+require 'rackup/handler/webrick'
 require 'json'
 require 'graphql'
 require 'pry-byebug'
@@ -34,7 +35,7 @@ def self.run(schema, options = {})
       end
     end.parse!
 
-    Rack::Handler::WEBrick.run(GraphQLServer.new(schema), **handler_options) do
+    Rackup::Handler::WEBrick.run(GraphQLServer.new(schema), **handler_options) do
       if test_mode
         $stdout.puts '_READY_'
         $stdout.flush