object sharing at application scope

[jay desi]

Now, what I intend to do is as soon as user requests for this page, queries are executed, beans are loaded and placed in arraylist and arraylist is
cached at application and not session scope.Next time user requests the page, certain queries are performed to determine validity of existing cache. If
cache is valid, just use the same cache instead of generating a new report.


I have following configuration in dispatcher-servlet.xml:-

 
    <bean id="TestController1" class="Controllers.TestController1">   
        <property name="sessionForm">   
            <value>true</value>   
        </property>   
  
        <property name="commandName">   
            <value>tco</value>   
        </property>   
        <property name="commandClass">   
            <value>entities.TestCommandObject</value>   
  
        </property>   
        <property name="formView">   
            <value>TestInput1</value>   
        </property>   
        <property name="successView">   
            <value>TestOutput1</value>   
        </property>   
  
    </bean>

where TestController1 extends SimpleFormController..



I intend to use below approach in my TestController class:-

public class TestController1 extends SimpleFormController {

     //only 1 output obj to be shared by multiple threads
     private TestControllerOutput tco= new TestControllerOutput ();

...
...

public TestController1 () {
...
}

 protected Object formBackingObject(HttpServletRequest request) throws ServletException{
....
...
}

        protected ModelAndView onSubmit(HttpServletRequest request,HttpServletResponse response,Object command,BindException errors) throws Exception {
.....
.....
ModelAndView mv=null;
....
            synchronized(this){
             ....
             ....
             //this method will check validity of cache and refresh cache if invalid
             tco.generateReport()
             mv = new ModelAndView(this.getSuccessView(),"tco",tco);
             return mv;


            }


}
}

I have a TestControllerOutput Class

public TestControllerOutput {

private CacheBean cb = new CacheBean
public void generateReport(){
....
....
//operates on CacheBean cb...
....
}
}

and then there is CacheBean class which actually contains data....

Now my approach is to have 1 instance of CacheBean used by 1 instance of TestControllerOutput and
I believe from above approach, TestController1 will give same instance of TestControllerOutput to every threads.

Is this approach valid?

[Gerardo Tasistro]

I wouldn't put the synchronize in the model view part

    
 synchronized(this){  
              ....  
              ....  
              //this method will check validity of cache and refresh cache if invalid  
              tco.generateReport()  
              mv = new ModelAndView(this.getSuccessView(),"tco",tco);  
              return mv;  
   
   
  }  

That will lock that bit of code regardless if the report is being generated

I would do something like:

 

tcoReport=tco.getReport();
mv = new ModelAndView(this.getSuccessView(),"tco",tcoReport);  

Then the getReport metod would be more like

    

	public TcoReport getReport() {
		synchronized(doUpdate) {  // if we're not swapping the temp report to the report then let threads read
			return tcoReport;  // returns the report cache data
		}
	}

	public void updateReport() {
		TcoReport tempReport=null;
		// do all the code to popuplate the report
		// this code will take a lot of time and can be running while the other threads keep reading the soon to be stale data
		synchronized(doUpdate) {
			// we're finished, now we quickly update the info
			tcoReport=tempReport;  // gain lock on doUpdate so no thread can read while we update the report
		}

	}

You could have an updateReport method which could be called every time you need an update. That could run in the background every so many minutes and update the tcoReport array. This minimizes the amount of time threads would wait for a lock release.

[jay desi]

thanks Gerardo..This is a very efficient approach..However, I feel wrt my app, I might face some implementation difficulties...



1) I will give a brief explanation on this. Whether to update cache or not is decided by query and not by time.

The first query gives me a series of numbers like resulset row1-> 56, resultset row2-> 34 and so on. Everytime I take the count of rows that has value greater then cache's max value..eg 56 here. If count is more then zero, that means report needs to be updated.

So, I was thinking that with every request, this query should be executed. So essentially I want to execute this query before "getReport" for every request.something like

 
    public TcoReport getReport() {   
        synchronized(doUpdate) {  // if we're not swapping the temp report to the report then let threads read   
            //execute query to determine if any new records added in database
            //if added, call updateReport
            //else return report
            return tcoReport;  // returns the report cache data   
        }   
    }

    public void updateReport() {   
        TcoReport tempReport=null;   
        // do all the code to popuplate the report   
        // this code will take a lot of time and can be running while the other threads keep reading the soon to be stale data   
        synchronized(doUpdate) {   
            // we're finished, now we quickly update the info   
            tcoReport=tempReport;  // gain lock on doUpdate so no thread can read while we update the report   
        }   
  
    }

But then, it will again increase amont of time threads have to wait. The database operations are such that there are fewer inserts/updates but more reads.
So, in this case is this approach more feasible or running "updateReport" in background? Also, how do I run "updateReport" in background in spring? I guess I would have to implement "updateReport" in class that extends Runnable or something like that.

2) You also mentioned in TestControllerServlet:-

tcoReport=tco.getReport();   
mv = new ModelAndView(this.getSuccessView(),"tco",tcoReport);

I guess, here "tcoReport" and "tco" are class level variables and not local variables inside "onSubmit" method. Essentially, I will be using
same instance of "TestControllerOutput" ie tco for every requests. Am I right?

Sorry for a very long post.

[Gerardo Tasistro]

Quick note tcoReport is method scope and tco is class scope. You don't want multiple treads reading and writing to tcoReport.

Regarding your question. I'm under the impression that the INSERTs and UPDATEs are the ones that eventually lead to cache update. What you can do is check after an insert/update is done and verify if an update needs to be done. That would lead to some bottleneck in the inserts/updates, but you say it is less common. So it would be less impacting.

The other option is to run updates from a separate thread. This thread wakes up on every insert and does the job while the insert process is freed to continue accepting inserts.

[jay desi]

Thanks Gerardo...

My webapp does not allow inserts or updates...The db inserts/updates take place at a different facility which is completely independent
of this webapp and I cannot modify anything inside db schema....So when you said:-

What you can do is check after an insert/update is done and verify if an update needs to be done
...
This thread wakes up on every insert and does the job while the insert process is freed to continue accepting inserts.

you meant writing some kind of database triggers, which I am not allowed to modify. I dont see any other way in which insert or update in db can trigger a thread.

Your approach has proved a good learning experience for me.

[Gerardo Tasistro]

In that case I'd set up a thread that periodically checks to see if the cache needs to be updated. Since there is no way to know if the cache is needed and for a period of time no read requests might arrive, you run the possibility of having your data not refresh for a while. And the first read request that comes in will take a while to respond. Based on your experience you can set the system to check every minute, 5 minutes or whatever you see fit. If an update is needed then you just fire the method and go to sleep again. That will make your application seem responsive for all requests.