Sweepers are the terminators of the caching world and responsible for expiring caches when model objects change. They do this by being half-observers, half-filters and implementing callbacks for both roles. A Sweeper example:

class ListSweeper < ActionController::Caching::Sweeper
  observe List, Item

  def after_save(record)
    list = record.is_a?(List) ? record : record.list
    expire_page(:controller => "lists", :action => %w( show public feed ), :id => list.id)
    expire_action(:controller => "lists", :action => "all")
    list.shares.each { |share| expire_page(:controller => "lists", :action => "show", :id => share.url_key) }
  end
end

The sweeper is assigned in the controllers that wish to have its job performed using the cache_sweeper class method:

class ListsController < ApplicationController
  caches_action :index, :show, :public, :feed
  cache_sweeper :list_sweeper, :only => [ :edit, :destroy, :share ]
end

In the example above, four actions are cached and three actions are responsible for expiring those caches.