Using Memcached with Ruby on Engine Yard Cloud

Memcached is a high-performance, in-memory key-value store that can be used to speed up your Rails applications by caching the results of database calls, API calls or page rendering. Using memcached in a clustered environment

Rails

The memcached client that we recommend for use with newer version of Rails is  Dalli by  Mike Perham, the former maintainer of the memcache-client gem.

The first step is to add dalli to your Gemfile:

gem 'dalli'

By default, EY Cloud sets up and monitors memcached on solo, app master and app instances. We also write a memcached config file,  memcached.yml , to your application's shared config directory,  /data/yourapp/shared/config/ . This file will be symlinked to  /data/yourapp/current/config/memcached.yml  when you deploy your application. Finally, we need to parse the  memcached.yml  and tell Dalli where our memcached servers are located. We can do this by adding the following to  config/environments/production.rb :

# parse the memcached.yml
 memcached\_config = YAML.load\_file(Rails.root.join('config/memcached.yml'))
 memcached\_hosts = memcached\_config['defaults']['servers']
 # pass the servers to dalli setup
 config.cache\_store = :dalli\_store, \*memcached\_hosts

When app instances are added or removed, your  memcached.yml  will be updated automatically. So your application will always be using the correct hosts.

Rails 2.3

Unfortunately, Dalli offers no support for Rails 2.3 applications. However, Rails 2.3 comes with a prebundled version of the memcache-client. We simply need to make the following updates to  config/environments/production.rb :

# parse the memcached.yml
 memcached\_config = YAML.load\_file(Rails.root.join('config/memcached.yml'))
 memcached\_hosts = memcached\_config['defaults']['servers']
 # pass the servers to memcached setup
 config.action\_controller.cache\_store = :mem\_cache\_store, \*memcached\_hosts

Using a utility instance as your memcached server

To run memcached on a utility instance, first of all, follow the steps listed above to setup your cache store to use the hosts listed in  memcached.yml .

Next, you will have to use a  custom Chef recipe to set up memcached to run on your utility instance. The following is an example of a Chef recipe that will set up memcached on a utility instance named memcached. It will then write a new version of memcached.yml  that points to the utility instance.