Page caching is an approach to caching where the entire action output of is stored as a HTML file that the web server can serve without going through Action Pack. This is the fastest way to cache your content as opposed to going dynamically through the process of generating the content. Unfortunately, this incredible speed-up is only available to stateless pages where all visitors are treated the same. Content management systems — including weblogs and wikis — have many pages that are a great fit for this approach, but account-based systems where people log in and manipulate their own data are often less likely candidates.

Specifying which actions to cache is done through the caches_page class method:

  class WeblogController < ActionController::Base
    caches_page :show, :new

This will generate cache files such as weblog/show/5.html and weblog/new.html, which match the URLs used to trigger the dynamic generation. This is how the web server is able pick up a cache file when it exists and otherwise let the request pass on to Action Pack to generate it.

Expiration of the cache is handled by deleting the cached file, which results in a lazy regeneration approach where the cache is not restored before another hit is made against it. The API for doing so mimics the options from url_for and friends:

  class WeblogController < ActionController::Base
    def update
      List.update(params[:list][:id], params[:list])
      expire_page :action => "show", :id => params[:list][:id]
      redirect_to :action => "show", :id => params[:list][:id]

Additionally, you can expire caches using Sweepers that act on changes in the model to determine when a cache is supposed to be expired.

Setting the cache directory

The cache directory should be the document root for the web server and is set using Base.page_cache_directory = "/document/root". For <a href="/rails/Rails">Rails,</a> this directory has already been set to Rails.public_path (which is usually set to RAILS_ROOT + "/public"). Changing this setting can be useful to avoid naming conflicts with files in public/, but doing so will likely require configuring your web server to look in the new location for cached files.

Setting the cache extension

Most <a href="/rails/Rails">Rails</a> requests do not have an extension, such as /weblog/new. In these cases, the page caching mechanism will add one in order to make it easy for the cached files to be picked up properly by the web server. By default, this cache extension is .html. If you want something else, like .php or .shtml, just set Base.page_cache_extension. In cases where a request already has an extension, such as .xml or .rss, page caching will not add an extension. This allows it to work well with RESTful apps.

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December 21, 2009
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expire_page outside controller and sweepers

If you want to expire cached pages from scripts or console just use class-method expire_page. But don’t forget about difference between instance and class method. In class-method you pass page url, not a hash of action/controller:

ActionController::Base.expire_page your_page_url