Memcache Memoizing in Ruby

Posted 26 Jan 2011 to ruby, memcache and has Comments

I realized recently with some amount of astonishment that the ruby memcache gem does not have the ability to memoize values. In the general case of memoization, it can be suffienct to simply “remember” values by setting some sort of static class variable. In the case, however, where you have multiple machines which may all need to calculate the same value, it is better to have a single location for the collective memory so that if one machine has recently calcuated it all of the other machines can use the result of that calcuation. Enter memcached.

I’ve written some very simple code to provide memoization within the MemCache client class. There are two additional methods - one which allows simple memoization and the other allows you to wrap the key and age for the value into a Proc that can then be called. Here’s the code:

require 'memcache'

class MemCache
  def memoize(key, age=0)
    value = get(key)
    if value.nil? and block_given?
      value = yield
      add(key, value, age)

  def memoize_proc(key, age=0, &block) { memoize key, age, &block }

Then, if there’s some expensive code that is run quite a bit within a method, you can easily memoize the result:

m = 'localhost'

class Record
  def self.lookup(id)
    m.memoize(id, 10) { 
      # perform intensive lookup here - as example, return some dummy data
      { :name => 'value', :age => '20' }

If you need to fetch the result frequently and don’t want to keep calling the memoize method with the key name and age, you can have your code wrapped in a Proc:

m = 'localhost'
f = m.memoize_proc('key', 10) {
  # this is your computationally intensive part
  "something hard to compute"

puts  # calls codeblock, stores result in memcache
puts  # gets result from memcache

There are probably other examples of this out there - but since I was doing it anyway I thought I’d put it up on the tubes.