How do I cache a method with Ruby/Rails?

Question

I have an expensive (time-consuming) external request to another web service I need to make, and I'd like to cache it. So I attempted to use this idiom, by putting the following in the application controller:

def get_listings
  cache(:get_listings!)
end

def get_listings!
  return Hpricot.XML(open(xml_feed))
end

When I call get_listings! in my controller everything is cool, but when I call get_listings Rails complains that no block was given. And when I look up that method I see that it does indeed expect a block, and additionally it looks like that method is only for use in views? So I'm guessing that although it wasn't stated, that the example is just pseudocode.

So my question is, how do I cache something like this? I tried various other ways but couldn't figure it out. Thanks!

Answer 1

an in-code approach could look something like this:

def get_listings
  @listings ||= get_listings!
end

def get_listings!
  Hpricot.XML(open(xml_feed))
end

which will cache the result on a per-request basis (new controller instance per request), though you may like to look at the 'memoize' helpers as an api option.

If you want to share across requests don't save data on the class objects, as your app will not be threadsafe, unless you're good at concurrent programming & make sure the threads don't interfere with each other's data access to the shared variable.

The "rails way" to cache across requests is the Rails.cache store. Memcached gets used a lot, but you might find the file or memory stores fit your needs. It really depends on how you're deploying and whether you want to prioritise cache hits, response time, storage (RAM), or use a hosted solution e.g. a heroku addon.