How to share a resouce (a serial port) with several threads - vb.net

Question

I currently have a program with 4 threads.
4 Threads are "Worker Threads" that each have a dedicated serial port that monitors a dedicated device. So Worker Thread 1 monitors Com port 1, Thread 2 monitors Com Port 2 etc.
All this all working fine. No conflicts.
However, the 4 worker threads all have to send commands to a 5th Comm Port as well, which is a communication link to a device that can powercycle the other devices.
I.E. they all have to share a specific resource, the 5th com port.
When they do send a command to this 5th shared each thread has to wait until the command has finished before continuing.
I've followed the coding example from Dan (thanks!) and tried to form a prototype test code.
This SEEMS to work.
I would appreciate a critical review of the code to see if I'm going in the right direction.
Apologies if I'm not explaining this very well as whilst I've used threads before. Handling a shared resource is new to me. Also I'm just getting to grips with how Stackoverflow works!!
Many thanks

Answer 1

A simplified solution using a shared instance of a resource and a lock.
Public Class Resource
Public Function Read() As String
Return "result"
End Function
End Class
Public Class ResourceUser
Private Shared resourceLock As New Object
Private Shared r As New Resource()
Public Function Read()
Dim res As String
SyncLock resourceLock
res = r.Read()
End SyncLock
Return res
End Function
End Class
Example usage:
Sub Main()
Dim t1 As New Threading.Thread(AddressOf DoSomethingWithResourceUser)
Dim t2 As New Threading.Thread(AddressOf DoSomethingWithResourceUser)
t1.Start()
t2.Start()
End Sub
Private Sub DoSomethingWithResourceUser()
Dim ru As New ResourceUser()
ru.Read()
End Sub

Answer 2

Here's another example more specific to serial comms. It uses a dictionary to keep track of physical comm resources and their relevant locks, so that you can do asynchronous access to different comm ports, but synchronize access to each single comm port.
Sub Main()
Dim c1 As New CommPortThreadSafe("COM1")
Dim c2 As New CommPortThreadSafe("COM2")
Dim c3 As New CommPortThreadSafe("COM1")
Dim t1 As New Threading.Thread(Sub() c1.Read())
Dim t2 As New Threading.Thread(Sub() c2.Read())
Dim t3 As New Threading.Thread(Sub() c3.Read())
' t1 and t3 can't be in critical region at same time
' t2 will be able to run through critical region
t1.Start()
t2.Start()
t3.Start()
End Sub
Public Class CommPort
Public Property Name As String
Public Function Read() As String
Return "result"
End Function
End Class
Public Class CommPortThreadSafe
Private Shared resourceLocks As New Dictionary(Of String, Object)()
Private Shared comms As New Dictionary(Of String, CommPort)()
Private Shared collectionLock As New Object()
Private commPortName As String
' constructor takes the comm port name
' so the appropriate dictionaries can be set up
Public Sub New(commPortName As String)
SyncLock collectionLock
Me.commPortName = commPortName
If Not comms.ContainsKey(commPortName) Then
Dim c As New CommPort()
Dim o As New Object()
c.Name = commPortName
' configure comm port further etc.
comms.Add(commPortName, c)
resourceLocks.Add(commPortName, o)
End If
End SyncLock
End Sub
Public Function Read()
Dim res As String
SyncLock resourceLocks(Me.commPortName)
res = comms(Me.commPortName).Read()
End SyncLock
Return res
End Function
End Class
To address your recent edits:
Threads A would all declare the comm port in the same way. Actually this is a benefit of this pattern (similar to multiton pattern) which works like a singleton when only one comm port is used. This code could be used in all threads:
Dim myCommPort As New CommPortThreadSafe("COM1")
The lock inside the read is going to synchronize access to COM1 because "COM1" (the name of the comm port) is actually the key to the Dictionary<string, object> used for locking. So when any thread reaches this code, keying with the same key, that region will be accessible only to a single thread because they all use the same key.
SyncLock resourceLocks(Me.commPortName)
res = comms(Me.commPortName).Read()
End SyncLock
As you saw, that string is set up in the constructor so as long as all the threads create their object passing the same string to the constructor, they will all have underlying indirect references to the same CommPort. The constructor can only create the instance if the name doesn't already exist in its dictionary:
If Not comms.ContainsKey(commPortName) Then
Dim c As New CommPort()
Here's another example usage with just one comm port:
Sub Main()
Dim ts As New ThreadStart(
Sub()
Dim c As New CommPortThreadSafe("COM1")
For i As Integer = 0 To 99
c.Read()
Next
End Sub)
Dim t1 As New Threading.Thread(ts)
Dim t2 As New Threading.Thread(ts)
Dim t3 As New Threading.Thread(ts)
Dim t4 As New Threading.Thread(ts)
t1.Start()
t2.Start()
t3.Start()
t4.Start()
End Sub
In this example we start 4 threads which each perform the code in the threadstart. There is a loop reading the comm port. If you test this out, you will see that it is thread-safe as long as the entire read takes place inside Read(), which you will need to develop of course. You may have another layer in which you are sending custom commands and waiting for a response. These two actions should both be inside a single SyncLock in each custom function. Thread B should use the same class if it's doing a similar thing.

Answer 3

FORM CODE FOR TESTING
Imports System.Threading
Public Class Form1
Private Worker(4) As jWorker '4 Worker Object
Public myWorkerThread(4) As Threading.Thread '4 runtime threads
Private Checker As jChecker '1 Checking Object
Public myCheckerThread As Threading.Thread ' Thread to check status of Resource
Dim MainThreadResouce As jResourceUser
'Assume the actual serial port is opened here
'so its available for access by the jResource Object.
'Pressing button 1 will start up all the threads
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'setup a another ResourceUser object here so can see one of the shared values from the form.
MainThreadResouce = New jResourceUser
'Setup and start worker threads - basically these do work and regularly
'send commands to a shared resource
For I = 1 To 4
Worker(I) = New jWorker
Worker(I).id = I
myWorkerThread(I) = New Threading.Thread(AddressOf Worker(I).doWork)
myWorkerThread(I).Start()
Next I
'Start Checking thread - regularly checks something in the resource
'please ignore this for now!
Checker = New jChecker
myCheckerThread = New Threading.Thread(AddressOf Checker.dochecking)
myCheckerThread.Start()
End Sub
End Class
========================
WORKER THREAD, MODELLING THE MONITORING THREADS
Imports System.Threading
Public Class jWorker
Private _id As Integer = 0
Private _workCount As Integer = 0
Private resourceUser As jResourceUser
Public workerStatus As String = ""
Public Property id() As Integer
Get
Return _id
End Get
Set(ByVal Value As Integer)
Me._id = Value
End Set
End Property
Public ReadOnly Property count() As Integer
Get
Return _workCount
End Get
End Property
Public Sub New()
resourceUser = New jResourceUser
End Sub
Sub doWork()
workerStatus = "Started"
Do Until False
Thread.Sleep(1000)
_workCount += 1
If _workCount Mod 5 = 0 Then
'THe line below would cause a bottleneck
'As it prevents the system trying to powercycle any other
'modems whilst its doing just one.
'resourceUser.PowerCycle(_id)
Debug.Print("W" & _id & ", Worker - Requesting Power OFF +++++++++++++")
resourceUser.Poweroff(_id)
Debug.Print("W" & _id & ", Worker - Waiting 10 secs for modem to settle")
Thread.Sleep(10000)
Debug.Print("W" & _id & ", Worker - Requesting Power ON")
resourceUser.PowerOn(_id)
Debug.Print("W" & _id & ", Worker - Finished Power cycle ------------")
End If
Loop
End Sub
End Class
===================
RESOURCE USER - TO CONTROL ACCESS TO SHARED RESOURCE
Public Class jResourceUser
'Variables for handling resouce locking
Private Shared resourceLock As New Object
Private Shared _resource As New jResource("Com1", "ON")
'keeps a status of which workers have signalled an OFF or ON via the Resource
'in the form of a string 11213141 (device number (1-4) - 1 for on, 0 for off
Private Shared _powerStatus As String
Public ReadOnly Property PowerStatus As String
Get
Return _powerStatus
End Get
End Property
Public Sub New()
_powerStatus = _resource.PowerState
End Sub
Sub PowerOn(ByVal WorkerID As Integer)
Debug.Print("W" & WorkerID & ", ResouceUser - requesting Lock for Power ON [" & _powerStatus & "]")
SyncLock resourceLock
_resource.TurnOn(WorkerID)
_powerStatus = _resource.PowerState
Debug.Print("W" & WorkerID & ", ResouceUser - Turned On, Device statuses " & _powerStatus & "]")
End SyncLock
End Sub
Sub Poweroff(ByVal WorkerID As Integer)
Debug.Print("W" & WorkerID & ", ResouceUser requesting Lock for Power OFF [" & _powerStatus & "]")
SyncLock resourceLock
_resource.TurnOff(WorkerID)
_powerStatus = _resource.PowerState
Debug.Print("W" & WorkerID & ", ResouceUser - Turned Off, Device statuses [" & _powerStatus & "]")
End SyncLock
End Sub
'Not going to work as it blocks the whole system when it could be
'reseting other modems.
Sub PowerCycle(ByVal WorkerID As Integer)
SyncLock resourceLock
Debug.Print("W" & WorkerID & ", ResourceUser - Requesting Power CYcle LockPower")
_resource.PowerCycle(WorkerID)
_powerStatus = _resource.PowerState
Debug.Print("W" & WorkerID & ", ResourceUser - Power Cycled")
End SyncLock
End Sub
Function CheckState() As String
SyncLock resourceLock
Return _resource.CheckState
_powerStatus = _resource.PowerState
End SyncLock
End Function
End Class
============
RESOURCE - FOR ACTUAL WORK ON SHARED RESOURCE
'THis code would directly handle interactions
'with one specific com port that has already
'been configured and opened on the main thread.
Public Class jResource
Private _ComPort As String
Private _state As String
Private _PowerState As String
Private _CheckState As String
'Record the com port used for this resource
Public Property ComPort() As String
Get
Return _ComPort
End Get
Set(ByVal Value As String)
Me._ComPort = Value
End Set
End Property
'Returns the a particular status of the resouce
Public ReadOnly Property CheckState As String
Get
'here I'd send a few command to the comm port
'pick u the response and return it
Return _state
End Get
End Property
'The connected serial port is used to power cycle serveral devices
'this property returns the state of all those devices.
Public ReadOnly Property PowerState() As String
Get
Return _PowerState
End Get
End Property
Public Sub New(ByVal name, ByRef state)
Me._ComPort = name
Me._state = state
Me._PowerState = "11213141"
Me._CheckState = "ON"
End Sub
'Simulate a off command sent by a worker
'via its resourceUser object
Public Sub TurnOn(ByVal intWorker As Integer)
'simulate some work with the com port
Dim myTimeOut As DateTime
myTimeOut = Now.AddMilliseconds(500)
Do Until Now > myTimeOut
Loop
'Set the status to show that Device is on.
_PowerState = _PowerState.Replace(intWorker & "0", intWorker & "1")
Debug.Print("W" & intWorker & ", Resource - issued TurnON, Device Statuses [" & _PowerState & "]")
End Sub
Public Sub TurnOff(ByVal intWorker As Integer)
'simulate some work
Dim myTimeOut As DateTime
myTimeOut = Now.AddMilliseconds(500)
Do Until Now > myTimeOut
Loop
'Here would send command to Com port
'Set the status to show that Device is Off.
_PowerState = _PowerState.Replace(intWorker & "1", intWorker & "0")
Debug.Print("W" & intWorker & ", Resource - issued TurnOFF, Device Statuses [" & _PowerState & "]")
End Sub
Public Sub PowerCycle(ByVal intWorker As Integer)
Debug.Print("W" & intWorker & ", Resource - issued PowerCycle, Device Statuses [" & _PowerState & "]")
'Here would send command to Com port
'Set the status to show that Device is Off.
_PowerState = _PowerState.Replace(intWorker & "1", intWorker & "0")
Debug.Print("W" & intWorker & ", Resource - issued TurnOFF, Device Statuses [" & _PowerState & "]")
'simulate some work - takes a while for device to turn off
Dim myTimeOut As DateTime
myTimeOut = Now.AddMilliseconds(10000)
Do Until Now > myTimeOut
Loop
'Here would send command to Com port
'Set the status to show that Device is Off.
_PowerState = _PowerState.Replace(intWorker & "1", intWorker & "1")
Debug.Print("W" & intWorker & ", Resource - issued TurnON, Device Statuses [" & _PowerState & "]")
'simulate some work
myTimeOut = Now.AddMilliseconds(10000)
Do Until Now > myTimeOut
Loop
'Here would send command to Com port
End Sub
End Class