Thread and Timer

[Chris Hani]

I have been a new project for a month.

I have seen a little server program that is designed to multi thread but using timer to repeat jobs. The server basically looking for some records that a flag has been set as 1 and creat one thread per record. Inside the thread, it will modify the flag setting as 0, fetch another record from another table, create a brand new record and so on.


Rough structure of this program is

 
//Inside class A
public void run(){
  .....
 
  //set up timer
  Timer timer = new Timer(true);
  timer.schedule(this,60000,5000)
)
 
public class B extends A{
  public void run(){
    super.run();
    // does its stuff
    .....
    // Check the DB and collect those record that a specific field = 1
    // creating a thread per a record
  }

In my understanding class A is scheduled to be executed every 5000 milisecond. First and foremost, am I right?

I think it is NOT a good design.
I have seen that the main program rescheduled and start to execute before a thread finished. The problem is the most recent rescheduled task interfere the work from thread. The rescheduled task try to fetch same record that is already under work for a thread.

So I modified like this

At first remove class A
Design class B as following

 
publc class B implements Runnable{
 
   public void run(){
        while (1){
             // Check the DB and collect those record that a specific field = 1
              // Set the flag first and
              // creating a thread per a record
        }
}

My brand new server didn't accept from the tech leader since the previous one is already running for 3 years.

I think the issue here is I use infinity loop for main program and current server is using Timer for it.

What you guys think? Do you have any valuable link for this?

[ December 12, 2006: Message edited by: Chris Hani ]



[HENRY: Added Code Tags]
[ December 12, 2006: Message edited by: Henry Wong ]

[Stan James]

I think your extends relationship is just confusing things. Let's work on the Timer approach a bit more.

First write a TimerTask and schedule it for execution.

TimerTask task = new MyTimerTask();
Timer timer = new Timer(true);
timer.schedule(task,60000,5000);

That will execute your task every 5 seconds as you want. It will run on the Timer's thread so you don't have to start one yourself.

So what should your task do? If your database update is very fast or if there are no other tasks scheduled you can just do all the work right there in the run() of your TimerTask. If the Timer is managing a bunch of other tasks you want to get the database update off the Timer thread and into its own thread. So one of these two things should happen:

// in MyTimerTask
private void run()
{
   doDatabaseUpdates();
}
 
// OR
 
private void run()
{
   Runnable r = new DatabaseUpdater();
   Thread t = new Thread(r);
   t.start();
}

Does that make sense?

BTW: If you start a new thread and your database update runs longer than your repeat time (5 seconds) you can run into database concurrency problems. Maybe set the repeat time higher or get rid of the new thread idea.
[ December 13, 2006: Message edited by: Stan James ]

[Henry Wong]

In my understanding class A is scheduled to be executed every 5000 milisecond. First and foremost, am I right?

I think it is NOT a good design.
I have seen that the main program rescheduled and start to execute before a thread finished. The problem is the most recent rescheduled task interfere the work from thread. The rescheduled task try to fetch same record that is already under work for a thread.

The period specified by the schedule() method is the period between executions. So whether the task takes a second, or a minute, or an hour, the next run of the task will, at earliest, take place 5 seconds after the previous runs finishes. No overlap. No interference between runs.

Henry

[Chris Hani]

James,

Your second code is exactly same as the current server does.
It is working fine because the timer is set longer that the database update thread's execution time. I think I understand that design.

What I did was I completely change the design.
I didn't use the Timer at all. Instead I use infinite loop.

My question is
1. Above two what is the popular( or more likely correct in the point of design) approach to desing multi thread server program in the real world?