Fixing i18n backend cache key when using memcached and Ruby 1.8.6

Paul Engel vr 11 jun 10

Implementing internationalization for Rails sites is a common thing. Most of you are likely using the i18n library developed by Sven Fuchs.

Backends and chaining them together

The default backend (I18n::Backend:: Simple) reads its translations from YAML files stored in the /config/locales directory and uses an in-memory hash to ensure efficiency. In most cases, this backend is sufficient enough when the application doesn’t require translation management by end-users. But if so, storing translations within YAML files isn’t preferred. You want to use a database for doing this.

Fortunately, i18n also provides a backend for database storage (I18n::Backend::ActiveRecord). Chaining both backends enables you to define translations within the YAML files and to override them with translations stored in the database.

I18n.backend = I18n::Backend::Chain.new(I18n::Backend::ActiveRecord.new, I18n.backend)

The usage of memcached ensures efficiency.

require "memcache"

yaml   = YAML.load(File.read(File.join(RAILS_ROOT, "config", "memcached.yml")))
config = yaml["defaults"].merge yaml[RAILS_ENV || "development"]

mem_cache               = MemCache.new config
mem_cache.servers = config["servers"]

I18n.cache_store = ActiveSupport::Cache.lookup_store :mem_cache_store, mem_cache

The problem

Using this setup in Ruby 1.8.6 can raise an error caused by an invalid memcached cache key. This key is generated by the protected method I18n::Backend::Cache.cache_key which doesn’t provide a clean key in Ruby 1.8.6 as it uses the Hash.hash method to create the key. The pitfall is that isn’t reliable in Ruby < 1.8.7:

{}.hash != {}.hash.

The fix

A solution is to upgrade Ruby, but doing this can bring a lot of side effects to Ruby dependent libraries on your system. We are clearly searching for some 1.8.7 goodies to use in 1.8.6. Fortunately, Marc-André Lafortune has created the gem Backports which provides us just that.

But as backports is not endorsed by ruby-core and you’re noted to use this with caution!

To fix the I18n cache key problem, just extract the hash method backport and use it in your project:

class Hash
  
  def hash
    h = 0
    each do |key, value|
      h ^= key.hash ^ value.hash
    end
    h
  end unless {}.hash == {}.hash
  
end

Enjoy!

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