Multi-Threading in a Windows Service

Read record information in to some time of storage (array, queue, etc.)  called myStorage.  Then create a subroutine called Process() like the pseudo code below.

Public thr1, thr2, thr3 As New System.Threading.Thread(AddressOf Process)
Public myRecords 'Declare whatever type of storage you choosse to use.

Sub Main()
   thr1.Start
   thr2.Start
   thr3.Start
End Sub

Sub Process()
beginThread:    
   'Synclock allows you to block access to an object from other threads until you release it
   Synclock myRecords
      'See if there is still a records left in myRecords to process
      'If a record is available
      'Get record information and remove the record from myRecords so that it can not be accessed by other threads once myRecords is released
   End Synclock

   If not (record is nothing) then
      'Process Record
   End if
   System.Threading.Thread.Sleep(2000)    'Will make current thread sleep for 2 seconds, adjust as necessary
   GoTo beginThread      'Will start process all over again
End Sub

Thanks for the replies guys.  I have given up on low-level threading for now.  Instead I'm trying to use the ThreadPool class without much luck.  This is what I have so far:

MailProcessor Class - Does the bulk of the work
  Mailbox Property - Must be set with a Mailbox object
  UsedCachedRules Property - Used for various things
  BeginProcessing Method - The function I want to call asynchronously from each thread... does a lot of work ranging from less than a second to a few minutes depending on the mailbox.

So first I declare and start a Timers.Timer object when the service starts to fire off every 30 seconds.  Everytime the Elapsed event is raised, I grab all of the mailboxes from the DB that are NOT currently being processed, translate them into  Mailbox objects, and mark them all as "currently being processed" in the DB.

Then, I have a loop that loops through each Mailbox in the MailboxesCollection and performs the following tasks:

1. Instantiate a new MailProcessor object (as 'proc')
2. Set the Mailbox property to the next Mailbox in the collection
3. Set the UsedCachedRules property to false
4. Executes the following line:

ThreadPool.QueueUserWorkItem(AddressOf proc.BeginProcessing)

The problem is, it looks like the BeginProcessing method is never actually executed.  I have logging statements (using my own logging function) before the thread is spawned and also inside of the MailProcessor's BeginProcessing method.  The only debugging text being written to the log is coming from the main thread (before the thread is spawned and afterward).

Also, no execeptions are being thrown.  Not sure what to make of it.  I can post some actual code samples if necessary.

Thanks,

Brian

Make sure to put a limit on the number of threads your program spawns. There will be a point at which making new threads no longer increases overall performance. Start with 2.

I thought that you couldn't set a maximum number of concurrent threads with the ThreadPool class.

Also, more importantly, do you have any advice on the issue at hand?

No, not without calling your program design into question first.

By all means please do.  My number one priority is to get this thing to work.

I wrapped my "mail processing" functionality into a separate MailProcessor class because I read that it was the only way to pass parameters to a thread delegate.  So I created my two necessary parameters as properties of my class, and the delegate as a method.

I understand now that with the ThreadPool class, you can pass parameters via the "state object".  Maybe the fact that the main functionality is wrapped into a separate class, which is being instanciated once for each thread is the issue?  I would use just one instance for all threads, but it does so much, the synclocks would be too numerous I think.

However, if that's what's causing the execution to never enter the delegate function, I'll gladly spend some time adding the synclocks.  Any ideas?

Thanks again,

Brian

Sorry for the delay.

Unfortunately, I can't answer the question as to why your delegate isn't being called. I'll need to see more code to help you diagnose that (if it is still a problem).

My criticism of your stated design is simply the lack of any limitation on the number of concurrent threads that may be spawned. You are correct that the ThreadPool class doesn't provide a facility to dynamically set the maximum number of threads. Regardless, if you are planning to have multiple threads performing database access, there will be a point at which the threads will spend more time contending with each other than doing what you want which could cause all sorts of problems, not least with users potentially being locked out of mailboxes!

I'll definitely do that.  I think I'm going to end up ditching the ThreadPool class.  Doesn't seem like it's built to do what I'm attempting to accomplish.  I haven't replied here in a few days because the priority of this particular project has dropped significantly, however, it still needs to be done sooner than later.  Thanks for your replies.

Brian

You're most welcome. I'll stay subscribed.

Thanks Working Journeyman.  Unfortunately, other projects took priority over this one and it doesn't look like I'll be able to resume anytime soon.  You did, however, help me to understand more some of the concepts of threading and I believe I know now how to accomplish the requirements.  You've been a great help.

Brian

Again, you're most welcome, except for keeping me waiting FOUR MONTHS for payment. I don't care what your priorities are - I want to be paid promptly. Please don't delay payment without good reason as it pisses me off and makes other people think they can also do the same, which pisses me off so much more that it is the reason I no longer participate on Experts Exchange. I don't get paid quickly enough.

In the real world I'd sue your ass off. You don't want to be sued by professionals. We always, always win and it always, always hurts.

Consider yourself spanked. Please come again. :)

Paul

Oh, I should point out that I make part of my living as a professional programming tutor. That may explain why not getting paid pisses me off so much...

I sincerely hope you're only kidding.  For one, and most importantly, I'm not sure if they told you, but "expert points" don't have any monetary value.  How can they have any value at all when there exists an unlimited abundance?  Secondly, I'm not sure of your country of residence, but here in the great USA, professionals very rarely win in petty suits like the imagined one you are referring to.  Trust me.  Even if the suit is "won", the litigation process leeches away any margin you may have garnered.  That is why there are so many collections agencies.  Much higher propensity for a positive ROI.

Anyway, it's not my place to school you in economics or a/r management.  I think once the time is again available, I'll have one recursive wrapper method being called by each thread so the job is executed until I tell it that it's checked all of the mailboxes.  I'll spawn a user-defined number of threads at the very beginning to guard against the unproductive "thread collision" you warned me of.

You've been helpful and are finally awarded your commission.  Don't spend it all in one place! :)

Brian