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
end
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]
end
end
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 Rails, 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 Rails 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.