ANN extended_fragment_cache plugin

From:
http://blog.zvents.com/2006/11/3/rails-plugin-extended-fragment-cache

The extended_fragment_cache plugin provides content interpolation and
an in-process memory cache for fragment caching. It also integrates
the features of Yan Pritzker's memcache_fragments plugin since they
both operate on the same methods.

== Installation

1. This plugin requires that the memcache-client gem is installed.
   # gem install memcache-client

2. Install the plugin OR the gem
   $ script/plugin install
svn://rubyforge.org/var/svn/zventstools/projects/extended_fragment_cache
   - OR -
   # gem install extended_fragment_cache

== In-Process Memory Cache for Fragment Caching

Fragment caching has a slight inefficiency that requires two lookups
within the fragment cache store to render a single cached fragment. The
two cache lookups are:

1. The read_fragment method invoked in a controller to determine if a
   fragment has already been cached. e.g.,
     unless read_fragment("/x/y/z")
      ...
     end
2. The cache helper method invoked in a view that renders the fragment.
e.g.,
     <% cache("/x/y/z") do %>
       ...
     <% end %>

This plugin adds an in-process cache that saves the value retrieved
from the fragment cache store. The in-process cache has two benefits:

1. It cuts in half the number of read requests sent to the fragment
cache store. This can result in a considerable saving for sites that
make heavy use of memcached.
2. Retrieving the fragment from the in-process cache is faster than
going to fragment cache store. On a typical dev box, the savings are
relatively small but would be noticeable in standard production
environment using memcached (where the fragment cache could be remote)

Peter Zaitsev has a great post comparing the latencies of different
cache types on the MySQL Performance blog:
http://www.mysqlperformanceblog.com/2006/08/09/cache-performance-comparison/

The plugin automatically installs a before_filter on the
ApplicationController that flushes the in-process memory cache at the
start of every request.

== Content Interpolation for Fragment Caching

Many modern websites mix a lot of static and dynamic content. The more
dynamic content you have in your site, the harder it becomes to
implement caching. In an effort to scale, you've implemented fragment
caching all over the place. Fragment caching can be difficult if your
static content is interleaved with your dynamic content. Your views
become littered
with cache calls which not only hurts performance (multiple calls to
the cache backend), it also makes them harder to read. Content
interpolation allows you substitude dynamic content into cached
fragment.

Take this example view:
<% cache("/first_part") do %>
  This content is very expensive to generate, so let's fragment cache
it.<br/>
<% end %>
<%= Time.now %><br/>
<% cache("/second_part") do %>
  This content is also very expensive to generate.<br/>
<% end %>

We can replace it with:
<% cache("/only_part", {}, {"__TIME_GOES_HERE__" => Time.now}) do %>
  This content is very expensive to generate, so let's fragment cache
it.<br/>
  __TIME_GOES_HERE__<br/>
  This content is also very expensive to generate.<br/>
<% end %>

The latter is easier to read and induces less load on the cache
backend.

We use content interpolation at Zvents to speed up our JSON methods.
Converting objects to JSON representation is notoriously slow.
Unfortunately, in our application, each JSON request must return some
unique data. This makes caching tedious because 99% of the content
returned is static for a given object, but there's a little bit of
dynamic data that
must be sent back in each response. Using content interpolation, we
cache the object in JSON format and substitue the dynamic values in the
view.

This plugin integrates Yan Pritzker's extension that allows content to
be cached with an expiry time (from the memcache_fragments plugin)
since they both operate on the same method. This allows you to do
things like:

<% cache("/only_part", {:expire => 15.minutes}) do %>
  This content is very expensive to generate, so let's fragment cache
it.
<% end %>

== Bugs, Code and Contributing

There.s a RubyForge project set up at:

http://rubyforge.org/projects/zventstools/

Anonymous SVN access:

$ svn checkout svn://rubyforge.org/var/svn/zventstools