conditionalget.rb

def defer(&block)
  def block.each ; yield call ; end
  block
end

app =
  lambda do |env|
    status = 200
    headers = { 'Last-Modified' => exec_sql('SELECT MAX(updated_at) FROM foos').httpdate }

    body = defer {
      rows = exec_sql('SELECT * FROM foos')
      # more heavy processing
      render_template :foos => rows
    }

    [ status, headers, body ]
  end

use Rack::ConditionalGet
run app

# Right? When the conditional GET hits, the defer block is never executed. If
# you can generate your validators quickly enough, you can get fairly decent
# throughput without having to resort to huge max-age w/ cache busting or manual
# purge.
#
# It feels like a lot of folks are under the impression that conditional GET is
# only useful for saving bandwidth when it's actually more interesting in these
# resource reducing cases. Put a validating cache in front of an app like this
# and your backends never have to generate the same response twice!