Move Over Rails. Here Comes Merb. : Page 3

Using DataMapper and a Database for Rendered Feed Data
To read all of the seed RSS URLs and fetch the RSS feed XML for each seed RSS URL, I created a model class FeedDataManager that reads the feed_sources database table using the DataMapper-generated class FeedSource. You can look at the source file app/models/feed_data_manager.rb for the full implementation, but the following few code snippets show how you fetch and process RSS data:

   feed_sources = FeedSource.all.collect {|f| f.url}
   feed_sources.each {|source|
     puts "\n* * processing #{source}"
     begin
       rss = SimpleRSS.parse(open(source))
       rss.items.each {|item|
         puts "  link: #{item.link}  title: #{item.title}  feed title: #{rss.feed.title}"
         # item.context # all content text
         # save data to database
         feed = Feed.new
         feed.url = item.link
         feed.title = item.title
         feed.content = item.content
         feed.feed_name = rss.feed.title
         feed.posted_date = Time.now
         feed.save!
         puts "* saved #{feed}"
       }
     rescue
       print "Error: #{$!}"
     end
  }

Running this periodically in the background, the ideal implementation, is relatively easy. Merb has built-in support for running background tasks from controllers, but I wanted to do something different: start a work thread as soon as all application code has loaded. To do this, I edited the config/init.rb file by adding a statement to the run after loading block:

Merb::BootLoader.after_app_loads do
  # This will get executed after your app's classes have been loaded.
  DataMapper.setup(:default, "sqlite3://#{Dir.pwd}/dev.db")
  # Start up the background work thread:
  Thread.new { FeedDataManager.new }
end

The initialize method for class FeedDataManager never returns. Rather, it initializes instance data and enters a long loop that fetches feeds, and then the thread sleeps for five hours before re-fetching the feeds.

Now you need a controller to get the feed data for a view (note that I have removed the unnecessary controller methods that merb-gen resource feeds ... created):

class Feeds < Application
  # provides :xml, :yaml, :js ## do not need to handle web service clients

  def index
    @feeds = Feed.all
    display @feeds  ## make available to the view
  end
end

My initial edits for the app/views/feeds/index.html.erb were:

<h1>Latest Ruby Blog Entries</h1>

<% @feeds.each {|feed| %>
    <h4> <%= feed.title %> </h4>
    <a href="<%=feed.url%>" target="new">link</a> <%= feed.content[0..70]%>
    <br/><br/>
<% } %>

Finally, before running the first implementation, I wanted to set the method index in the controller class Feeds to be the default route for the web application in config/router.rb:

    match('/').to(:controller => 'feeds', :action =>'index')

That is pretty much bare bones, but you now have a complete web application (that you will improve) for fetching feeds and displaying them.

One thing that surprised me when I first ran this prototype was its speed. It was fast without caching and despite database fetches for the data it would display. I ran the Apache Benchmark utility and discovered that this prototype provided 44 page renders per second on my MacBook:

ab -n 5 http://localhost:4000/

The next section explains how you can change the design of the demo application to incorporate in-memory storage.

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