is there a way in asp.net to make sure that a certain threaded sub is not run twice concurrently, no matter what?
the code i have now is
Public Class CheckClass
ReadOnly Property CheckSessionsLock As Object
Get
If HttpRuntime.Cache("CheckSessionsLock") Is Nothing Then HttpRuntime.Cache("CheckSessionsLock") = New Object
Return HttpRuntime.Cache("CheckSessionsLock")
End Get
End Property
Sub TryThreads()
Dim thread = New Thread(AddressOf TryLock)
thread.Priority = ThreadPriority.Lowest
thread.Start()
End Sub
Sub TryLock()
SyncLock CheckSessionsLock
DoTrace("entered locker")
For x = 0 To 10000
Next
DoTrace("exiting locker")
End SyncLock
DoTrace("exited locker")
End Sub
End Class
if i run this code on every page then several times the code overlaps. the DoTrace function in the code simply writes the message to a table.
the messages in the table should appear in order (entered,exiting,exited) again and again, but in reality, they don't. i get like entered, exiting,entered,exited,exiting...
this means that the synclock is not complete. is that true?
if so, how can we implement a complete synclock on a block of code, across requests and across sessions?
EDIT: i need this lock, as the real code will be sending emails, according to a list of mailing types in a db. after each mailing type is sent, its marked, then it continues with the next mailing. i cant have in middle of processing, another thread should see this mailing as unprocessed.
please advise
Rather than using the HttpRuntime Cache have you considered using a static variable?
Just as a note (it might be helpful to explain why you want this functionality) your website is not going to be very scalable if this can only be run once at a time.
In C# (sorry, don't know VB syntax) I use this:
private static readonly object Padlock = new object();
It's a field, not a property,
It's static (in VB, that's "shared" if I'm not mistaken) so it's the same throughout the entire application
It's initialised once as soon as you use this class, not when you explicitly use the field.
With your property/cache version, you could have two threads trying to get the lock-object and each creating a different one:
Thread 1 checks the cache and doesn't find the object
Thread 1 is parked
Thread 2 checks the cache, doesn't find the object
Thread 2 creates the object and caches it, retrieves it again and returns from the property
Thread 1 resumes
Thread 1 creates a new object and caches it, retrieves it again and returns a different lock object than thread 2 uses
Any further threads will use the lock object of thread 1
Related
I have a very long running process in an ASP.net application that we desperately need to dramatically shorten. The process in question is charging a large number of credit cards. Currently it performs at about 1 charge per second. We need this to be more like 10 per second.
So we decided that utilizing multiple simultaneous threads would be one way to go. So we basically take this large list of orders to process, divide the list into ten lists and then spawn a new thread to process each of the ten lists simultaneously.
An additional complication of this process is that we need to report progress on this process, and not only to the user session that initiated the process, but to any user, in any session in the application. So for example, if I log in and start this process, I will see a progress bar. If after I initiate the process, and it is still running, another user logs in elsewhere and goes to this same page, they will also see the progress bar.
I did some research and thought that I could use Application variables to store the relevant bits of information required to report progress. The client polls the server on a regular basis whenever on this page to see if there are any threads running, and if so, it returns various statistics on the progress of the process back to the client.
It would seem that this approach does not work. A simple counter of the number of currently running threads does not work as expected. It seems that the so-called thread safety of the Application object is safe in that no two threads will be able to access the same variable simultaneously, but not safe in that if two threads both attempt to increment a variable, one of them will be able to increment it, and the other will not, and rather than queue up and increment it in turn, the second thread just moves on. I'm sure this is my thread safety ignorance shining through.
Another issue is that using Debug.Print or Debug.WriteLine seem to be the same kind of "thread-safe" as the Application object. As each thread starts, we use Debug.WriteLine to output the name and start time of the thread, and as it completes, we do the same thing to write that it completed. We consistently see ten threads start and four threads end in the debug window.
I don't think we need to use Application.Lock() and Application.Unlock(), but I have tried it both with and without those calls before and after every write operation, but to no avail- the results are the same either way.
I have a ton of code, so I'm not sure exactly which parts to share, but here are some of the relevant parts:
This is how we create and start the threads:
For Each oBatch As List(Of Guid) In oOrderBatches
Dim t As New Threading.Thread(Sub() ProcessPaymentBatch(oBatch, clubrunid, oToken.UserID))
t.IsBackground = True
t.Start()
Next
Here is the sub that is started by each thread:
Private Sub ProcessPaymentBatch(oBatch As List(Of Guid), clubrunid As String, UserID As Guid)
ThreadsRunning(clubrunid) += 1
Try
Debug.Print("Thread Start")
For Each oID As Guid In oBatch
‘Do a bunch of processing stuff…
Next
Finally
ThreadsRunning(clubrunid) -= 1
Debug.Print("Thread End")
End Try
End Sub
Finally, this is an example of one of the application variables that the threads attempt to access, but seems to be failing.
Private Const _THREADSRUNNING As String = "ThreadsRunningThisRun_"
Public Property ThreadsRunning(clubid As String) As Integer
Get
Dim sToken As String = _THREADSRUNNING & clubid
If Application(sToken) Is Nothing Then
ThreadsRunning(clubid) = 0
End If
Return Application(sToken)
End Get
Set(ByVal value As Integer)
Debug.Print(value)
Dim sToken As String = _THREADSRUNNING & clubid
Application.Lock()
Application(sToken) = value
Application.UnLock()
End Set
End Property
The Debug output from this property looks something like this:
Thread Start
1
Thread Start
Thread Start
1
1
4
Thread End
5
3
Thread Start
6
3
1
-1
Thread End
-2
-3
I can't understand why there would be a different number of "Thread Start" and "Thread End" debug statements, and I don't understand how the thread count could get to negative numbers. This is why I am confused by the thread safety of the Application and Debug objects.
Your help in this matter would be greatly appreciated!
Nevermind, I was just being an idiot. The problem had nothing to do with the Application or Debug objects not being thread safe, the problem was in my methodology (as was expected really).
To clarify, the issue was that we were locking the global variables in the application object when writing, but not when reading. We then tried also locking when reading, but still had the same problem. What we failed to realize was that when incrementing a value, you are getting the current value, adding onto that, then setting the new value. The lock needed to bridge all three of those operations, so it goes like this:
Lock
Get
Add
Set
Unlock
What we were doing previously was:
Lock
Get
Unlock
Add
Lock
Set
Unlock
Which allowed for multiple threads to Get and then Set the same values as one another, which explains all of the oddities we were seeing in the debug window.
I have a function that parses an input file.
Private Function getSvSpelOdds(ByVal BombNo As Integer) As Boolean
Dim InputFileBase As String = HttpContext.Current.Application("InputFileBase")
strInputFile = InputFileBase & "PC_P7_D.TXT"
OddsReader = New StreamReader(strInputFile)
'some other code
End Function
If the file is not there (getSvSpelOdds returns False), I would like to retry after 30 seconds.
To achieve this I use a timer.
If Not getSvSpelOdds(y) Then
Timer1.Interval = 30000
End If
Private Sub Timer1_Elapsed(sender As Object, e As System.Timers.ElapsedEventArgs) Handles Timer1.Elapsed
getSvSpelOdds(y)
End Sub
Problem is that when timer fires the HttpContext.Current (used to get the value of gloal variable) is null.
Should I use some other approach to get this to work?
As already described HttpContext should be null as Timer_Elapsed is called in different thread. But you may use System.Web.HttpRuntime.Cache to pass filename, cache should be accessible from all threads.
HttpContext.Current only gives you the context you want when you call it on the thread that handles the incoming thread.
When calling it outside of such threads, you get null. That matches your case, as Timer1_Elapsed is executed on a new thread.
Should I use some other approach to get this to work?
Almost certainly, yes. 30 seconds is a long time to wait without giving any feedback to users.
It would probably be better to return a "no results are available yet, but we're still looking" page to the user. That page can be set to refresh automatically after 30 seconds, by adding a suitable meta-tag:
<META HTTP-EQUIV="refresh" CONTENT="30">
And you then get a fresh request/response cycle on the server. And haven't tied up server resources in the meantime.
Other answers seems to address the other part of your question (about why it doesn't work in the timer callback)
The Elapsed event on the Timer will run on a separate thread therefore its expected behaviour for the current context to be null.
You can only access it from the same thread.
Should I use some other approach to get this to work?
Yes, it's not generally a good idea to mix ASP.NET and threads given the complexity of how ASP.NET works. Like already mentioned its not a great UX to have no feedback for 30 seconds, its better to let the user know what's actually going on.
Also, you need to determine whether the timeout length is appropriate or whether a timeout is needed at all. I don't know the nature of your application but I assume there is some external means for the file to be generated and picked up by your site.
I am an absolute beginner on ASP.net (VB.) Please pardon me if the question is too obvious for the experienced members.
I tried to make a simple WebRequest in the async mode in case the target URL takes long to provide the data. In my code below, I just want to see if the callback block (RespCallback) is called correctly every time. If all goes well, lblResult should have the string '123' appended to it every time I click the button which calls the 'GetData' sub.
However, the lblResult only shows 123 after the first click. After the subsequent click, the lblResult only gets appended with '12', as if RespCallback is never called. When I tried to debug this in Visual Studio, the execution actually stepped right into the RespCallback part and the lblResult.Text watch actually shows '123123' but the resulting Web page always shows only '12312'
I am sure I am missing something basic here, but I just don't know what. I was even guessing that it has to do with browser cache (hence the result changes for the second time) but I don't know how to fix that either.
Can someone please help? Thanks in advance.
Jim
Dim myWebRequest As WebRequest
Public Shared allDone As New ManualResetEvent(False)
Private Shared BUFFER_SIZE As Integer = 1024
Public Class RequestState
' This class stores the state of the request
Private Shared BUFFER_SIZE As Integer = 1024
Public requestData As StringBuilder
Public bufferRead() As Byte
Public request As WebRequest
Public response As WebResponse
Public responseStream As Stream
Public Sub New()
bufferRead = New Byte(BUFFER_SIZE) {}
requestData = New StringBuilder("")
request = Nothing
responseStream = Nothing
End Sub ' New
End Class ' RequestState
Public Sub GetData(Sender As Object, Args As System.EventArgs)
lblResult.Text += "1"
myWebRequest = WebRequest.Create(dataURL)
Dim myRequestState As New RequestState()
myRequestState.request = myWebRequest
' Start the asynchronous request.
Dim asyncResult As IAsyncResult = CType(myWebRequest.BeginGetResponse(AddressOf RespCallback, myRequestState), IAsyncResult)
lblResult.Text += "2"
allDone.WaitOne()
End Sub
Private Sub RespCallback(asynchronousResult As IAsyncResult)
lblResult.Text += "3"
allDone.Set()
End Sub
I don't know VB so it's hard to read for me but I'm suspecting GetData is your onClick handler.
First thing that is not right is that you have Shared members. Why your reset event is Shared? It makes all requests use the same object.
Basically Your code with ManualResetEvent won't work because after first allDone.Set(), your object remains set (as long as web application lives). To get "123" every time you should add allDone.Reset() after allDone.WaitOne().
In Your situation web request returns to client before RespCallback is called every time except first call (when your reset event is in non-signaled state).
AutoResetEvent resets automatically. That's why it worked.
But! You can't do this this way. Making your ResetEvent Shared you make all request use the same object. When more than one request will be processed by your application at the same time you will get undetermined behavior.
Remove Shared from your code. Than your code will work (but not asynchronously) without allDone.Reset() and without AutoResetEvent. But it will provide known results (not depending on amount of requests).
About asynchronous call (now that we have code "working"). Well. There is no async request to your web page. allDone.WaitOne() waits until your async webRequest finish. So basically you could just as well do synchronous request.
You need a special pattern for asynchronous web pages. You can read how to do this here.
But i'm not sure it's what you wanted. Do you want your request to be called asynchronously so that it will not use server resources or do you want to display some message to the user (like "Data is being downloaded...") while your web page will remain fully responsible?
If it's the second one you should use AJAX functionality (Like UpdatePanel or using JavaScript directly). You can read about it here.
Couple things to check:
If your label is a fixed width, then it's possible the text is being clipped
If you are using an UpdatePanel, you will need to set its mode to 'Conditional' and call Update() on it in the RespCallback callback method so that the UI gets refreshed with the latest label text value.
I have a long running task that I need to implement on a webpage. What I would like to do is run the task on a separate thread, but have a progress bar on the webpage.
I am struggling to find an easy way of doing this. Here is a very simplified example that I want to do this on. Basically, I want the ResetAll() in a thread, and pool the variable y to update the webpage UI.
Can someone help me?
Protected Sub btnReset_Click(sender As Object, e As System.EventArgs) Handles btnResetLowConductor.Click
ResetAll()
End Sub
Private Sub ResetAll()
Dim y As Integer = 0
While y < x
y += 1
Reset()
lblProgress.Text = y & "/" & x
End While
End Sub
Private Sub Reset()
Threading.Thread.Sleep(200)
End Sub
your lblProgress will not update as long as thread is alive. You will get only final value of y & x i.e when thread is dead. You can store the values of Y & X inside a session variable.
Private Sub ResetAll()
Dim y As Integer = 0
While y < x
y += 1
Reset()
Session("CurrentStatus") = y & "/" & x
End While
End Sub
From Your UI you will fire an asynchronous event using PageMethod i.e
function GetCurrentThreadStatus()
{
PageMethods.GetThreadStatus(function(status){
// success
$("span[id*='lblProgress']").text(status);
});
}
Code Behind : C#
[WebMethod]
public static string GetThreadStatus()
{
return (string)Session["CurrentStatus"];
}
As this is a website, you will probably need to use some sort of ajax method to call back to the server periodically. Here is the basic flow of your program as far as I can see it:
User makes initial call
Spin up new thread to run process and return with a 0% status
Periodically, make a call to the web server via ajax (A GetStatus call that will return the percentage of completion)
If the call is complete, update/refresh the page appropriately
If the call is not complete, then use the returned status
Now, I am not as sure about the details given that this is a web page, but you could try to store the status in the session variable (which may or may not be possible once you have already returned) periodically, and then the GetStatus will just read the most current status. If session does not work, then you will need to persist the status another way (db, file, etc).
Just be careful on how often you store the status. Too often and you slow your process down, and too little and you do not give an accurate representation of the status.
Last, if you can upgrade to .NET 4.0, then this becomes even more trivial using the TPL (Task Parallel Library)
You are confused about how web pages work.
Once the thread is fired and you write the response to the client, you won't be able to continue updating the page as the Thread progresses.
One way to do this sort of thing is using Ajax. Ulhas already showed you a way top do this; all you need to do is have that javascript function he wrote call itself until you get a 100% completion. You can do that using a timer.
Okay, with the help of various websites, I managed to do this.
Here is a sample project. Please let me know if there are any problems with this. I'm no expert, so I would like some feedback!
http://www.mediafire.com/?7vn16vp78rave6a
''' <summary>
''' Returns true if a submission by the same IP address has not been submitted in the past n minutes.
'' </summary>
Protected Function EnforceMinTimeBetweenSubmissions(ByVal minTimeBetweenRequestsMinutes as Integer) As Boolean
If minTimeBetweenRequestsMinutes = 0 Then
Return True
End If
If Cache("submitted-requests") Is Nothing Then
Cache("submitted-requests") = New Dictionary(Of String, Date)
End If
' Remove old requests. '
Dim submittedRequests As Dictionary(Of String, Date) = CType(Cache("submitted-requests"), Dictionary(Of String, Date))
Dim itemsToRemove = submittedRequests.Where(Function(s) s.Value < Now).Select(Function(s) s.Key).ToList
For Each key As String In itemsToRemove
submittedRequests.Remove(key)
Next
If submittedRequests.ContainsKey(Request.UserHostAddress) Then
' User has submitted a request in the past n minutes. '
Return False
Else
submittedRequests.Add(Request.UserHostAddress, Now.AddMinutes(minTimeBetweenRequestsMinutes))
End If
Return True
End Function
No. The ASP.NET Cache is not inherently thread-safe and it looks like you are creating objects in the Cache depending on whether they exist or not.
You need to lock the Cache when writing to it.
Let me word things a little differently. The code is, in fact, thread safe. The way you currently have it coded though could cause performance issues in multi-threaded situations.
In this case, multiple users would be running the same code simultaneously, theoretically accessing and modifying the same cache objects at the same time. As that scenario scales up, performance suffers.
Creating a lock will improve performance under heavy load (while imposing a slight overhead under light load) because you won't be fetching data neadlessly due to Caching issues.
The System.Web.Caching.Cache class is thread-safe according to the MSDN documenation. However, the documenation also shows an example where a read and a write are performed on the cache without locking. That cannot possibily be thread-safe since the write is dependent on the read. The code you posted basically looks like the example. I definitely recommend putting a lock around the entire method.