local_connection_pool.rb

# config/server.rb
require "em-synchrony"
require "em-synchrony/mysql2"

config['db'] = EM::Synchrony::ConnectionPool.new(:size => 25) do
  ::Mysql2::EM::Client.new({
            :adapter => 'em_mysql2',
            :database => 'shard001',
            :username => 'xxx',
            :password => 'xxx',
            :host => 'localhost',
            :reconnect => true})
end



# server.rb
def response(env)
  ...
  multi = EventMachine::Synchrony::Multi.new

  ids.each do |id|
    multi.add id, db.aquery("select * from users where id = #{id}")
  end

   users = []
   res = multi.perform
   res.responses[:callback].each do |res|
     res.last.callback{|r| users << r.first}
   end
  ...
end

shared_connection_pool.rb

# server.rb
def response(env)
  ...
  db = EventMachine::Synchrony::ConnectionPool.new(size: 25) do
        Mysql2::EM::Client.new({
            :adapter => 'em_mysql2',
            :database => 'shard001',
            :username => 'xxx',
            :password => 'xxx',
            :host => 'localhost',
            :reconnect => true})
    end

  multi = EventMachine::Synchrony::Multi.new

  ids.each do |id|
    multi.add id, db.aquery("select * from users where id = #{id}")
  end

   users = []
   res = multi.perform
   res.responses[:callback].each do |res|
     res.last.callback{|r| users << r.first}
   end
  ...
end