EJB3 Timer Scheduler

[iterrell]

I'm using jBPM in an EJB3 application, and I didn't feel that the web tier was the proper place for the scheduler. I wrote a stateless session bean that uses EJB3 timers for scheduling instead. It's basically the same algorithm (oh, and same code :) that's in the servlet, just now instead of using threads directly it lets the application server handle the details of the timing. I took out the listeners for now.

Obviously since it needs to be started on startup I have a servlet calling down to the start() method, but this fits in nicely since my application has other timers that need to be started on application startup.

To use transactions I had to add these parameters to jbpm.sar/jbpm.sar.cfg.jar/jbpm.hibernate.properties:

hibernate.transaction.factory_class=org.hibernate.transaction.JTATransactionFactory
hibernate.transaction.manager_lookup_class=org.hibernate.transaction.JBossTransactionManagerLookup

package com.something.workflow;

public interface Scheduler {

 void start();
 void stop();
}

package com.something.workflow.impl;

import static java.lang.Math.min;

import java.util.Collection;
import java.util.Date;
import java.util.Iterator;

import javax.annotation.Resource;
import javax.ejb.SessionContext;
import javax.ejb.Stateless;
import javax.ejb.Timeout;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.jbpm.calendar.BusinessCalendar;
import org.jbpm.calendar.Duration;
import org.jbpm.db.JbpmSession;
import org.jbpm.db.JbpmSessionFactory;
import org.jbpm.db.SchedulerSession;
import org.jbpm.scheduler.exe.Timer;

import com.something.workflow.Scheduler;

@Stateless
public class SchedulerBean implements Scheduler {
 private static final Log log = LogFactory.getLog(SchedulerBean.class);

 static JbpmSessionFactory jbpmSessionFactory = JbpmSessionFactory.getInstance();
 static BusinessCalendar businessCalendar = new BusinessCalendar();

 @Resource
 private SessionContext ctx;

 private static final long DEFAULT_TIMEOUT = 5000L;

 public void start()
 {
 log.debug("starting Scheduler");
 ctx.getTimerService().createTimer(DEFAULT_TIMEOUT,null);
 }

 @Timeout
 public void timeoutHandler(javax.ejb.Timer timer)
 {
 long time = min(executeTimers(),DEFAULT_TIMEOUT);
 timer.cancel();
 ctx.getTimerService().createTimer(time,null);
 }

 public void stop() {
 log.debug("stopping Scheduler");
 Collection timers = ctx.getTimerService().getTimers();
 for (Object o : timers) {
 javax.ejb.Timer t = (javax.ejb.Timer)o;
 t.cancel();
 }
 }

 private long executeTimers() {
 long millisTillNextTimerIsDue = DEFAULT_TIMEOUT;
 boolean isDueDateInPast = true;

 JbpmSession jbpmSession = jbpmSessionFactory.openJbpmSessionAndBeginTransaction();
 try {
 SchedulerSession schedulerSession = new SchedulerSession(jbpmSession);

 log.debug("checking for jBPM timers");
 Iterator iter = schedulerSession.findTimersByDueDate();
 while ((iter.hasNext()) && (isDueDateInPast)) {
 Timer timer = (Timer) iter.next();
 log.debug("found timer "+timer);

 // if this timer is due
 if (timer.isDue()) {
 log.debug("executing timer '"+timer+"'");

 // execute
 timer.execute();

 // if there was an exception, just save the timer
 if (timer.getException()!=null) {
 schedulerSession.saveTimer(timer);

 // if repeat is specified
 }
 else if (timer.getRepeat()!=null) {
 // update timer by adding the repeat duration
 Date dueDate = timer.getDueDate();

 // suppose that it took the timer runner thread a
 // very long time to execute the timers.
 // then the repeat action dueDate could already have passed.
 while (dueDate.getTime()<=System.currentTimeMillis()) {
 dueDate = businessCalendar.add(dueDate,new Duration(timer.getRepeat()));
 }
 timer.setDueDate( dueDate );
 // save the updated timer in the database
 log.debug("saving updated timer for repetition '"+timer+"' in '"+(dueDate.getTime()-System.currentTimeMillis())+"' millis");
 schedulerSession.saveTimer(timer);
 }
 else {
 // delete this timer
 log.debug("deleting timer '"+timer+"'");
 schedulerSession.deleteTimer(timer);
 }
 }
 else { // this is the first timer that is not yet due
 isDueDateInPast = false;
 millisTillNextTimerIsDue = timer.getDueDate().getTime() - System.currentTimeMillis();
 }
 }
 }
 finally {
 jbpmSession.commitTransactionAndClose();
 }

 return millisTillNextTimerIsDue;
 }

}

Did I duplicate anything already in jBPM? I looked around for this first, but didn't find it. Anyone see any reasons to not use this approach?

I appreciate all advice,
Ian

[ralfoeldi]

Hi Ian,

looks good - on principle - I haven't checked the code in detail.

You still have the problem that all Timers get executed in the same transaction. There's a Jira issue for that, but I guess it will take some time till that gets changes.

I've basically done the same - using Quartz and a SLSB.

Greetings

Rainer

[supernovasoftware.com]

Could you post a code sample for looking up an EJB3 session bean from a quartz timer?