Sinatra is a wonderful small Ruby web framework built on top of the Rack interface. Sinatra is very fast out of the box making it perfect for light weight tasks. Sinatra does not come with support for memcached out of the box but adding support for memcached is fairly straightforward.

Requirements

1. Download the file cache.rb.

2. You will need to have installed memcached and also the memcache-client gem:

sudo gem install memcache-client
 

Installation

At the top of your Sinatra application should be

require 'sinatra'
require 'cache'    

CONFIG['memcached'] = 'localhost:11211'

# add caching to Sinatra
class Sinatra::Event
  include CacheableEvent
end

Usage

A standard Sinatra controller method looks like this:

get '/:name' do
    ...
    erb 'name'
end

To add caching change to:

get '/:name', :cache_key => Name.cache_key do
    ...
    erb 'name'
end

You will want Name.cache_key to return a string and to change whenever the content returned by name.html.erb view will change. You do not have to invalidate any caches directly. Furthermore the computed cache_key will automatically include details of the parameters of the request.

If you want to cache anything else inside your application use:

result = Sinatra::Cache.cache(cache_key) do
    this_is_an_expensive_method
end

How it Works

There are essentially two modification that must be made. Override the way Sinatra stores blocks that make up the controller methods and also to override the way Sinatra calls those stored blocks.

Overriding the block storage

When you declare a controller method in your application file like

get '/:name' do
    ...
    erb 'name'
end

Sinatra stores the block provided and saves it to be executed when responding to a request. The caching mechanism replaces this block with another block. This replacement block uses the caching mechanism. The cache_key and original block are saved by means of Proc.new which creates a closure and allows the block to access these values at the time of execution.

def _invoke_with_caching(*args)
  if options[:cache_key]
    # replace the block with another block that can be cached
    def wrap_block(key,block)
      Proc.new do
        Sinatra::Cache.cache(key + "/" + params.to_a.join("/")) { instance_eval(&block) }
      end
    end
    @block = wrap_block(options[:cache_key], block)
  end
  _invoke_without_caching(*args)
end

How fast is it?

This depends on how long it takes do generate your cache_key. There is also additional overhead in accessing memcached. In my case it usually takes about 15 milliseconds to respond from the cache.

It doesn't matter

However it doesn't matter that much. You web application speed usually doesn't depend on how fast your application responds, especially when the response time is less than 150 milliseconds. To understand why see this explanation of where your application is really slow.