At the Forge - Memcached Integration in Rails

Last month, we talked about memcached, a distributed caching system that is in widespread use among Web sites. The reason for memcached's popularity is its simplicity. With a minimum of overhead and setup, it's possible to set and retrieve nearly any value. Caching values that otherwise would come from the database makes it possible to avoid the database altogether on many occasions, speeding the throughput of a Web application and reducing the load on the database server.

Memcached is a wonderful tool, and it is something nearly every Web developer should have in his or her arsenal to improve site performance. But with the release of Ruby on Rails 2.1, it got even better. Rails now has integrated support for memcached, allowing you to use it almost for free from within your application. There are some caveats and tricks to its use, but once you have those under your belt, you quickly will discover that memcached has improved your site performance dramatically.

This month, we take a look at how to make memcached work inside your Rails applications. We further explore some issues you might encounter when using memcached, some of which are easier to work around than others.

Ruby on Rails has, since its inception, tried to make Web developers' lives easier by coming out with many tools such developers might need. It comes with an excellent object-relational mapper (ORM), ActiveRecord. It comes with a way to test your code at a variety of different levels (called, in Rails-speak, unit, functional and integration). It comes with a first-class JavaScript library and associated effects, in Prototype and Scriptaculous. As numerous demonstrations and tutorials have shown, Rails allows you to jump right in to Web development, writing and testing your code with a minimum of dependencies. If you need to include some functionality that was left out by the Rails authors, it's not very difficult to include a Ruby gem (downloadable library) or even a “plugin” that sits inside your Rails application.

Rails has long come with a multilayered caching system that programmers can tap to speed up applications. You can cache individual pages, controller actions or even page fragments. And indeed, judicious use of the Rails caching commands can result in serious improvements to performance.

But, it was only in version 2.1 that Rails integrated support for caching individual objects. The support for object caching not only has the potential to improve your application's performance dramatically, but it also allows you to work with a variety of different storage facilities, so you can choose the one that's most appropriate for you. Although this article concentrates on the use of memcached, you should know that it's possible to work with not only memcached, but also with caches on the local filesystem, in local memory or even on another Rails-aware server using DRb (distributed Ruby, available as a Ruby gem).

To demonstrate how to use memcached, I'm going to create a simple Rails application, using PostgreSQL as the database:

createdb atf
rails --database=postgresql atf

Next, I create a simple object, person, for my application, with the Rails built-in scaffolding that includes a RESTful interface:

./script/generate scaffold person firstname:string 
 ↪lastname:string email_address:string

To import this definition into the database, I run the migration that it created:

rake db:migrate

Sure enough, if I connect to the database, I can see that the table has been created (Listing 1).

atf_development=# \d people
                      Table "public.people"
 Column       |            Type             | Modifiers
--------------+-----------------------------+-----------------------------------
id            | integer                     | not null default nextval
                                              ↪('people_id_seq'::regclass)
firstname     | character varying(255)      |
lastname      | character varying(255)      |
email_address | character varying(255)      |
created_at    | timestamp without time zone |
updated_at    | timestamp without time zone |
  Indexes:
      "people_pkey" PRIMARY KEY, btree (id)

And, if I run the application, I have access (via the RESTful interface) to the various CRUD functions associated with a Person object: Create, Retrieve, Update and Delete. I simply type:

./script/server