At the Forge - Memcached Integration in Rails
Now, it's all well and good that we have cached information about each person in memcached. Our database certainly will thank us for that. But, what happens when data about the person changes? The way we've written this application, we're out of luck. Updated information will make its way to the database, but the cache will continue to give us the data it stored long ago. Even if this weren't the case, we still would want to empty the cache on occasion, allowing data to expire if we haven't used it in a while.
To solve the second problem, we can invoke our cache function in a slightly different way, indicating how long we want it to stick around in a second (and optional) argument:
@person = cache(['Person', params[:id]], :expires_in => 30.minutes) do Person.find(params[:id]) end
The :expires_in parameter accepts a number of seconds, which we either can enter by hand or via one of the super-convenient Rails extensions to the Fixnum class.
The second problem, one of expiring data manually, requires that we use a less beautiful, but also convenient, way of accessing the cache storage system:
Rails.cache.delete(['controller', 'Person', ↪params[:id]].join('/'))
Basically, we access the cache system using the Rails.cache object and invoke the delete method on it. That method accepts a memcached key. As you might remember, we previously saw that the elements of our key array (as used by the helpful cache method) were joined by slashes and prefixed with controller. Thus, the above works, even though it's not quite as nice as I might have liked. We can see that this is the case in the memcached logs:
<7 delete controller/Person/1 0 >7 DELETED
And, sure enough, we then find that our next invocation of show for person 1 retrieves the information from the database and caches it in memcached.
Caching has long been an excellent way to improve performance in the computer industry, from the hardware level all the way up to operating systems and applications. Rails programmers have incorporated memcached into their applications over the last few years, but I believe that its complete integration in version 2.1 will make it even easier, and more widespread, to find memcached-enabled Rails applications. As you can see, adding just a few lines of configuration and application code can speed up an application by many times, without having to sacrifice accuracy.
If you are looking for information on memcached, you should begin at www.danga.com/memcached, the home page for the open-source project and the source of a great deal of good documentation, code and general information.
For information on Ruby on Rails, start by going to www.rubyonrails.com, which has pointers to documentation, mailing lists and (of course) software you can download.
For information on the integration of memcached into Rails, try www.thewebfellas.com/blog/2008/6/9/rails-2-1-now-with-better-integrated-caching.
There are some Rails plugins that might make it even easier to cache objects. For example, take a look at www.inwebwetrust.net/post/2008/09/08/query-memcached and lucaguidi.com/pages/cached_models, both of which have gained some attention since Rails 2.1 caching was released.
Finally, a tutorial on the use of memcached with Rails is included in a chapter of Advanced Rails Recipes, published by the Pragmatic Programmers. I have greatly enjoyed this book and recommend it to anyone planning to use Rails for more than a simple application. The chapter on memcached is one that has been released as a free sample, and it is available in PDF as media.pragprog.com/titles/fr_arr/cache_data_easily.pdf.
Reuven M. Lerner, a longtime Web/database developer and consultant, is a PhD candidate in learning sciences at Northwestern University, studying on-line learning communities. He recently returned (with his wife and three children) to their home in Modi'in, Israel, after four years in the Chicago area.
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