everycache.rb

#
# Demo for how to
# - use memoization with any cache
# - set up invalidation rules
#

class Cache
  def initialize
    @data = {}
  end

  def read(key)
    if value = @data[key]
      value.dup
    end
  end

  def write(key)
    value = yield
    value = value.dup unless Symbol === value
    @data[key] = value
  end

  def delete(key)
    @data.delete(key)
  end
end

$cache = Cache.new

module Everycache
  def cache_class_method(name)
    overwrite(name) do |name, old_method, *args|
      cache.read(key(name, args)) ||
        old_method.call(*args).tap do |value|
          cache.write(key(name, args)) { value }
        end
    end
  end

  def invalidate(name, trigger, &relevant_args)
    overwrite(trigger) do |trigger, old_method, *args|
      old_method.call(*args).tap do |result|
        cache.delete(key(name, relevant_args.call(*args)))
      end
    end
  end

  def key(name, args)
    [ self.to_s.to_sym, name, Array(args) ]
  end

  def cache
    raise "cache is not initialized"
  end

private
  def overwrite(name, &block)
    old_method = method(name)
    metaclass.instance_eval do
      define_method(name) do |*args|
        block.call(name, old_method, *args)
      end
    end
  end

  def metaclass
    class << self; self; end
  end
end

class Recursion
  def self.fib(n)
    puts "# fib(#{n})"
    case n
    when 1..3
      [ 0, 1, 1 ][0...n]
    else
      result = fib(n - 1)
      result << (result[-2] + result[-1])
    end
  end

private
  extend Everycache
  cache_class_method(:fib)

  def self.cache
    $cache
  end
end

class Invalidation
  @objects = {
    1 => "a",
    2 => :b,
  }

  def self.find(id)
    puts "# find(#{id})"
    @objects[id]
  end

  def self.save(id, value)
    puts "# save(#{id}, #{value.inspect})"
    @objects[id] = value
  end

private
  extend Everycache
  cache_class_method(:find)
  invalidate(:find, :save) { |id, value| id }

  def self.cache
    $cache
  end
end

puts "# ------ recusion -------------- "
puts Recursion.fib(7).inspect
puts Recursion.fib(5).inspect
puts Recursion.fib(8).inspect
puts Recursion.fib(7).inspect

puts "# ------ invalidation ---------- "
puts Invalidation.find(1).inspect
puts Invalidation.find(1).inspect
Invalidation.save(1, "new value for 1")
puts Invalidation.find(1).inspect
Invalidation.save(2, "new value for 2")
puts Invalidation.find(2).inspect