Deferred Dispatcher

README.markdown

The DeferredDispatcher is a way to get around the 30-second limit for App Engine requests. We don't want spin-up to be a "show stopper", and we want every request to have a full 30 seconds to do "real work". We added this to appengine-rack 0.0.7, so folks won't need an additional require. The DeferredDispatcher works by using redirects to break up a loading request into 3 parts.

1. initialize the jruby runtime and jruby-rack
2. require the framework, and create a new dispatcher
3. process the real request, with a full 30 seconds

We are inside jruby-rack, so obviously the runtime and Jruby-rack have initialized, so first we set @runtime to true (from nil) and redirect. Next pass, we notice that the @rack_app is nil, so we require the framework (which can take some time) and create a new dispatcher (these have been passed in as a hash) and redirect. In the final pass, we hand the environment to the dispatcher, to do the original "work" of the request.

appengine-rack lib appengine-rack.rb

module AppEngine
  module Rack

    # ...

    class DeferredDispatcher
      def initialize args
        @args = args
      end
 
      def call env
        if @runtime.nil?
          @runtime = true
          # 1: redirect with runtime and jruby-rack loaded
          redirect_or_error(env)
        elsif @rack_app.nil?
          require @args[:require]
          @rack_app = Object.module_eval(@args[:dispatch]).new
          # 2: redirect with framework required & dispatched
          redirect_or_error(env)
        else
          # 3: process all other requests
          @rack_app.call(env)
        end
      end
 
      def redirect_or_error(env)
        if env['REQUEST_METHOD'].eql?('GET')
          redir_url = env['REQUEST_URI'] +
              (env['QUERY_STRING'].eql?('') ? '?' : '&') +
              "_spinup_=#{Time.now.to_i}"
          res = ::Rack::Response.new('*', 302)
          res['Location'] = redir_url
          res.finish
        else
          ::Rack::Response.new('Service Unavailable', 503).finish
        end
      end
    end

    # ...

  end
end

complex config.ru

require 'appengine-rack'
AppEngine::Rack.configure_app(
    :application => 'complex-app',
    :precompilation_enabled => true,
    :sessions_enabled => true,
    :version => "1")

AppEngine::Rack.app.resource_files.exclude :rails_excludes
ENV['RAILS_ENV'] = AppEngine::Rack.environment

deferred_dispatcher = AppEngine::Rack::DeferredDispatcher.new(
     :require => File.expand_path('../config/environment', __FILE__),
     :dispatch => 'ActionController::Dispatcher')
 
map '/admin' do
  use AppEngine::Rack::AdminRequired
  run deferred_dispatcher
end
 
map '/' do
  use AppEngine::Rack::LoginRequired
  run deferred_dispatcher
end

simple config.ru

require 'appengine-rack'
AppEngine::Rack.configure_app(
    :application => 'simple-app',
    :precompilation_enabled => true,
    :sessions_enabled => true,
    :version => "1")

AppEngine::Rack.app.resource_files.exclude :rails_excludes
ENV['RAILS_ENV'] = AppEngine::Rack.environment

run AppEngine::Rack::DeferredDispatcher.new(
    :require => File.expand_path('../config/environment', __FILE__),
    :dispatch => 'ActionController::Dispatcher')