为何 Connection 设计为 class 内的 module，这样有何作用呢？

class Redis
  module Connection
    class Memory
      include Redis::Connection::CommandHelper
      include FakeRedis
      include SortMethod
      include TransactionCommands
      include CommandExecutor

      attr_accessor :options

      # Tracks all databases for all instances across the current process.
      # We have to be able to handle two clients with the same host/port accessing
      # different databases at once without overwriting each other. So we store our
      # "data" outside the client instances, in this class level instance method.
      # Client instances access it with a key made up of their host/port, and then select
      # which DB out of the array of them they want. Allows the access we need.
      def self.databases
        @databases ||= Hash.new {|h,k| h[k] = [] }
      end

      # Used for resetting everything in specs
      def self.reset_all_databases
        @databases = nil
      end

      def self.connect(options = {})
        new(options)
      end

      def initialize(options = {})
        self.options = options
      end