[3/25] Building a RSS Reader with Quartz Plugin – Grails Tutorial 4
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Groovy Version: 1.6
Grails Version: 1.1
Plugin Version: 0.4.1-SNAPSHOT
Plugin Docs: http://grails.org/plugin/quartz
Download Resources: source code screencast 1Â screencast 2
Hello,
In this tutorial, we’ll talk about the Quartz plugin used to schedule jobs executions in your application. The plugin is build on top of the Quartz Job Scheduler Library from OpenSymphony. OpenSympony is the company that built the WebWork framework, that is now called Struts2 after Apache “aquisition”.
“Scheduling jobs” is very useful in your application to cover background needs. Some tasks you’ll need to execute undercover your application some times (invalidating old users that have not logged for more than 1 month) or even async processes that you’ll have to do if you do not have a JMS infrastructure, for example, sending e-mails to a lot of people.
In our example, we’ll build a simple RSS Reader that will use the quartz plugin to schedule fetchs it will be done in the feeds and insert in the database. Our application will mainly have one domain class called Post (seen in the last tutorial), a Feed domain class to store our feeds and a similar RSS Parser from technorati. Â (Yes, I love the RSS format).
Initially, we’ll create the app, install the quartz plugin, create the domain classes and the Feed scaffold structure
grails create-app feedreader cd feedreader grails install-plugin quartz grails create-domain-class Post
We’ll insert the Post domain class code
class Post {
   String title
   String link
   String body
}
We have to create the Feed domain class and its scaffold structure.
grails create-domain-class Feed
class Feed {
   String word
   String url
}
Scaffolding…
grails generate-all Feed
screencast-1
After this, we’ll create our Technorati Feed Parser from this code above.
class TechnoratiService {
   boolean transactional = false
   def parseAndSave(rss) {
      def rssObj = new XmlSlurper().parse(rss)
      rssObj.channel.item.each {
         def post = new Post(title: it.title.toString(),
               link: it.link.toString(),
               body: it.description.toString())
         post.save()
         println "Post [${post}] saved."
      }
   }
}
We’ll run our application using the grails run-app command and insert some feeds. Note that we’ve configured our datasource to use hsqldb storing in the filesystem instead of regular memory setup.Â
Note that we have one JobController that Quartz install for us, forget about it, ok? We’ll create our own job after the second screencast.
screencast-2
Now, we have to understant some quartz properties and commands.Â
When we install the quatz plugin, it installs another command for us the grails create-job MyJob, with it we’ll create our FeedParserJob. Note that we use convention over configuration with all jobs having *Job names.Â
grails create-job FeedParser
Job classes have to implement the execute() method. This method is the one that Quartz will trigger when it’s time to execute the job. To define when the job it will be executed and what’s the interval between executions, I suggest you read the plugin documentation witch shows N ways to do this. In our example we’ll use a cron expression similar to *N*X OS systems setting our job to execute once in five minutes.
Our cron expression will be like this: Â “0 0/5 * * * ?”
Depending on your jobs requisites, it may run concurrently with another instance of it or not. In our case, we’ll not start other job execution if the last on is still running. To prevent this behavior, we can set the concurrent property to false
def concurrent = false
Our job will essentially look for the feeds we’ve inserted on the database, and for each one it will call the Technorati service asking for new Posts. The final source for our job is the one below:
class FeedParserJob {
   def concurrent = false
   def cronExpression = "0 0/5 * * * ?"
  Â
   def technoratiService
   def execute() {
     def feedList = Feed.findAll()
      for (Feed feed : feedList) {
         println "Reading feed ${feed.word} @ ${feed.url}"
         technoratiService.parseAndSave(feed.url)
      }
   }
}
As you can see in the example above, you can inject any spring bean in your job, just declare it as I did with my TechnoratiService! 
(this is really great!)
That’s it, if you run you application you’ll see that every 5 minutes (minutes 0,5,10,15…) the job will be called and every posts technorati returns will be inserted on your database. Note that in this simple example we did not check if the post had been already inserted in the database before inserting it, this will just grow our database with a lot of instances representing the same post. This can be avoided checking if the post already exists before inserting it  (just check if you have any Post with the same link), but I’ll left this for you!
Before finish this, let’s just improve a little bit our post list view.
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Now, try to enrich its interface, adding some ajax to get only the new posts since the last fech! Maybe you can start from this your new Google Reader killer! 
Now, let me know, are you using this plugin in your production environment? What for? What kind of jobs you do with it?Â
Thanks!
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