Question

Before I begin, I have to apologize for two things. One is that it is very difficult for me to explain things in a concise manner. Two is that I need to be somewhat vague due to the nature of the company I work for.

I am working on enhancing the functionality of an application that I’ve inherited. It is a very intensive application that runs a good portion of my company’s day to day business. Because of this I am limited to the scope of what I can change–otherwise I’d probably rewrite it from scratch. Anyways, here is what I need to do:

I have several threads that all perform the same task but on different data input streams. Each thread interacts through an API from another software system we pay licensing on to write out to what is called channels. Unfortunately we have only licensed a certain number of concurrently running channels, so this application is supposed to turn them on an off as needed.

Each thread should wait until there is an available channel, lock the channel for itself and perform its processing and then release the channel. Unfortunately, I don’t know how to do this, especially across multiple threads. I also don’t really know what to search Google or this site for, or I’d probably have my answer. This was my thought:

A class that handles the distribution of channel numbers. Each thread makes a call to a member of this class. When it does this it would enter a queue and block until the channel handling class recognizes that we have a channel, signals the waiting thread that a channel is available and passing it the channel id. I have no idea where to begin even looking this up. Below I have some horribly written PsuedoCode of how in my mind I would think it would work.

Public Class ChannelHandler

    Private Shared WaitQueue as New Queue(of Thread)
   '// calling thread adds itself to the queue
    Public Shared Sub WaitForChannel(byref t as thread) 
            WaitQueue.enqueue(t)
    End Sub

    Public Shared Sub ReleaseChannel(chanNum as integer)
        '// my own processing to make the chan num available again
    End Sub

    '// this would be running on a separate thread, polling my database
    '// for an available channel, when it finds one, somehow signal
    '// the first thread in the queue that its got a channel and here's the id
     Public Shared Sub ChannelLoop()
           while true
               if WaitQueue.length > 0 then 
                   if thereIsAChannelAvailable then '//i can figure this out my own
                        dim t as thread = ctype(WaitQueue.dequeue(), Thread)
                         lockTheChannel(TheAvailableChannelNumber) 'performed by me
                       '// signal the thread, passing it the channel number
                        t => SignalReady(theAvailableChannelNumber) '// how to signal?
                    end if
                end if
           end while


    End Sub

End Class

and then

'// this inside the function that is doing the processing:
ChannelHandler.requestChannel(CurrentThread)
while (waitingForSignal) '// how?
    block                '// how?

dim channelNumber as int => getChannelNumberThatWasSignaledBack

'// perform processing with channelNumber

ChannelHandler.ReleaseChannel(channelNumber)

I am working with the .NET Framework 3.5 in VB.NET. I am sure there has got to be some sort of mechanism already built for this, but as I said I have no idea exactly what keywords I should be searching for. Any input pointing me in the right direction (ie specific .NET framework classes to use or code samples) would be greatly appreciated. If I need to elaborate on anything, please let me know and I will to the best of my ability.

Edit: The other problem that I have is that these channels can be turned on/off from outside of this application, manually by the user (or as a result of a user initiated event). I am not concerned with a channel be shut down while a thread is using it (it would throw an exception and then pick back up next time it came through. But the issue is that there are not a constant number of threads fighting over a constant number of channels (if a user turns one on manually, the count is reduced, etc). Both items are variable, so I cant rely on the fact that there are no external forces (ie, something outside this set of threads, which is why I do some processing via my DB to determine an available channel number)

Answer 1

What I would do:

• Switch the System.Threading.Thread by the System.Threading.Tasks.Task class.
• If a new Task needs to be created, but the List(Of Task) (or, in your example, Queue(Of Task) ) count greater than the maximum permitted, use the Task.WaitAny method.

EDIT:

As I answered the previous block on my phone (which is pretty challenging for writing code), let now me write an example about how I would do it:

Imports System.Threading.Tasks
Imports System.Collections.Generic

Public Class Sample

    Private Const MAXIMUM_PERMITTED As Integer = 3
    Private _waitQueue As New Queue(Of Task)

    Public Sub AssignChannel()
        Static Dim queueManagerCreated As Boolean
        If Not queueManagerCreated Then
            Task.Factory.StartNew(Sub() ManageQueue())
            queueManagerCreated = True
        End If

        Dim newTask As New Task(Sub()
                                    ' Connect to 3rd Party software
                                End Sub)
        SyncLock (_waitQueue)
            _waitQueue.Enqueue(newTask)
        End SyncLock
    End Sub

    Private Sub ManageQueue()
        Dim tasksRunning As New List(Of Task)

        While True
            If _waitQueue.Count <= 0 Then
                Threading.Thread.Sleep(10)
                Continue While
            End If

            If tasksRunning.Count > MAXIMUM_PERMITTED Then
                Dim endedTaskPos As Integer = Task.WaitAny(tasksRunning.ToArray)

                If endedTaskPos > -1 AndAlso
                    endedTaskPos <= tasksRunning.Count Then
                    tasksRunning.RemoveAt(endedTaskPos)
                Else
                    Continue While
                End If
            End If

            Dim taskToStart As Task
            SyncLock (_waitQueue)
                taskToStart = _waitQueue.Dequeue()
            End SyncLock

            tasksRunning.Add(taskToStart)
            taskToStart.Start()
        End While
    End Sub

End Class