Task scheduler for websites

Public Class OtherJob 
Implements ISchedulerJob 
 Public Sub Execute() Implements ISchedulerJob.Execute 
   MMCUtil.SendMail("tosomeone@domain.com", _
    "Subject:testing schedule from other job ", _
   "Body:Body of other job schedule") 
 End Sub 
End Class

As you can see it does exactly same as previous job but the subject and body are different so it technically is different job to execute.

You can add as many job classes as you like. The job classes does not have to be executed. To include the job in the the execution schedule each job class has to instantiated. I will discuss in more detail later on how to achive this.

The next step is to build configuration class. The configuration has two properties. One Integer that keeps the time (in milliseconds) that you wish to wait before the scheduled jobs execute, and one ArrayList that keeps Jobs to be executed.

Public Class SchedulerConfiguration

       Private m_sleepInterval As Integer
       Private m_jobs As New ArrayList()

        Public ReadOnly Property SleepInterval() As Integer

          Get
               Return m_sleepInterval
           End Get
         End Property
        Public ReadOnly Property Jobs() As ArrayList
            Get
                Return m_jobs
            End Get
        End Property
       Public Sub New(ByVal newSleepInterval As Integer)
           m_sleepInterval = newSleepInterval
        End Sub
End Class

The scheduler engine is the one that makes the jobs run. This is where you will wan to pay some attention (design your code carefully), since the jobs are recurring and the execution is placed in indefinite loop.

Public Class Scheduler 
  Private configuration As SchedulerConfiguration = Nothing 
    Public Sub New(ByVal config As SchedulerConfiguration) 
        configuration = config 
    End Sub 
   Public Sub Start() 
    While True 
         Try 
           For Each job As ISchedulerJob In configuration.Jobs 
             job.Execute() 
           Next 
        Catch
       Finally 
         Thread.Sleep(configuration.SleepInterval) 
       End Try 
     End While
End Sub 
End Class

If you wish to execute the jobs only once at specific time, then simply do not use the while/wend loop and make sure you place a check if the current time (date.Now) is greater then scheduled time. In this case you may also wish to set scedule interval to seconds or minutes. That is all it takes to make the scheduler. To add a Jobs and execute them you need to do the following.

1. Craeate a new instance of The Scheduler Configuration (in my case 1 hr)
2. Create the jobs by crating a new instance of each job in the scheduler configuration
3. Create new instance of the scheduler using the above config
4. Create a new thread that will start with the sub named Start in the scheduler class
5. Start the newly created thread

Private schedulerThread As System.Threading.Thread = Nothing 

Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
 Dim config As New MailScheduler.SchedulerConfiguration(60 * 60 * 1000)
 config.Jobs.Add(New MailScheduler.MailJob())
 config.Jobs.Add(New MailScheduler.OtherJob())
 Dim scheduler As New MailScheduler.Scheduler(config)
 Dim mailThreadStart As _
   New System.Threading.ThreadStart(AddressOf scheduler.Start)
  schedulerThread = New System.Threading.Thread(mailThreadStart)
  schedulerThread.Start()
End Sub




Sub Application_End(ByVal sender As Object, ByVal e As EventArgs)
  If Not IsNothing(schedulerThread) Then
   schedulerThread.Abort()
  End If
End Sub

I've placed the above code in the global.asax on application start with schedulerThread declared as private variable within the scope of global.asax. The reason is because I will need to reference it on application end to kill the same tread . That would be a perfect place or any scheduled task to run and then just continue to the site. Now as the site is working our jobs will execute in a separate thread that sleeps between the jobs executions for a specified period of time.

If you wish the code to run as long as the asp.net worker process is alive leave the scheduler thread (do not abort it). IT IS DANGEROUS to leave any thread open, especially on hosting provider this may cause undesired results. So be WARNED. The safer method is to create public reference to the scheduler thread so you can reference it from within a page and abort it by simply loading the page (on page load event). The approach on how to start and kill the thread is up to you. This article intention was to give a simple code on implementing the scheduler functionality for recurring tasks. You may wish to search this site for more samples on creating safe threading models for your web site.

Happy coding...