Console app timer to call web methods every x minutes

Question

I am coding in VB.Net, VS 2008. I wrote a console app that consumes 2 web methods from a web site application. I need to enhance this console app so that it launches the web methods continuously, perhaps every x minutes (during business hours), but never before the last invocation has terminated, whose duration may vary, depending on how many accounts there are to process.

Originally, I scheduled the application using Task Scheduler, but I think this doesn't prevent two invocations at the same time.

Although I have seen many posts on using timers, I haven't found exactly what I need.

So far I have:

Dim aTimer As New System.Timers.Timer()
    AddHandler aTimer.Elapsed, AddressOf TriggerWebMethods

' Set the Interval to 10 minutes:
aTimer.Interval = 1000 * 60 * 10            '(1 second * 60 = 1 minute * 10 = 10 minutes)
aTimer.Enabled = True
aTimer.AutoReset = True

1. When should Timer.Elapsed be used vs. Timer.Tick?
2. What is the difference between Timer.Enabled vs Timer.Start, and should I be selecting just one?
3. I would like the 2nd web method to kick off when the first one is done.

I'd like to keep this as simple as possible. Thank you for all help.

Answer 1

If you are dealing with a System.Timers.Timer, then you'd only have the Elapsed event available. If a System.Windows.Forms.Timer, then you'd use the Tick event. You're not writing a WinForms app so you would be using the System.Timers.Timer.

Personally, I would only use the Enabled property to see if the timer has been started. I wouldn't use it to start or stop it. Using the Start() or Stop() method makes it very clear what's happening to the timer.

If your web methods execute synchronously, you could just call them one after the other in your TriggerWebMethods() method. The second will not be called until the first completes.

Sub TriggerWebMethods(source As Object, e As ElapsedEventArgs)
    FirstWebMethod()
    SecondWebMethod()
End Sub

If asynchronously, you'd have to register a callback on the first web method to execute the second when it completes. In VB, I believe you can use the second directly as the callback, depending on how you make the asynchronous call. (Sorry, my VB is very rusty now so might not be 100% correct syntax)

Sub FirstWebMethod()
    ' ...
End Sub

Sub SecondWebMethod()
    ' ...
End Sub

Sub TriggerWebMethods(source As Object, e As ElapsedEventArgs)
    Dim first As Action = AddressOf FirstWebMethod
    first.BeginInvoke(AddressOf SecondWebMethod, first)
End Sub

Answer 2

Just to add a little to Jeff M's answer. The Timer.Elapsed Event has the following note.

If the SynchronizingObject property is null, the Elapsed event is raised on a ThreadPool thread. If the processing of the Elapsed event lasts longer than Interval, the event might be raised again on another ThreadPool thread. In this situation, the event handler should be reentrant.

Since you're in a Console app you can either hand roll you own SynchronizingObject or you can set the AutoReset to False, and change your TriggerWebMethods to have a start at the end. You may even want to offset the interval to take into consideration the amount of processing time.

e.g.

    Dim start As Date = Now


    'do stuff


    Dim ts As TimeSpan = Now - start

    Dim i As Integer = (1000 * 60 * 10) - ts.TotalMilliseconds

    Dim aTimer As Timers
    aTimer.Interval = i