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
Related
In short, remaining in the HTTP context, I would like the user, after clicking on an order completion button, not to wait for the mails to be sent before being sent back to a "thak you page".
I saw that HostingEnvironment.QueueBackgroundWorkItem could help me with this but there is always the risk of it being killed by IIS recycling.
What is the best solution to do this?
I know the best would be to develop a separate console solution but it wouldn't be worth it for 3/4 emails, alternatively I could consider speeding it up by making them asynchronous?
Protected Sub btnConcludiOrdine_Click(sender As Object, e As System.EventArgs) Handles btnConcludiOrdine.Click
If IsValidOrder(Me._cart, msg) Then
If Me._cart.SaveOrder(Me._user, Me._orderCode, Me._lang) then
'Update quantity in db
Dim mail As New EmailBLL
mail.SendOrderNotice(Me._cart, Me._lang) '2 Mails
mail.SendProductNotice() '2 Mails
End If
Else
Response.Redirect("*Error URL*")
End If
End Sub
The way you approach this is as suggested – start a task, or so called new processor thread.
So, what you would do is break out the code – this even works if code behind is for a web form.
So, the first step is to move out the “slow” parts, or the parts we want to run separate.
The main issue is that to start/launch/want/desire/achieve a brand new processor thread?
The sub call CAN ONLY PASS ONE “parameter” and the sub can only accept one parameter!!!!!
I note in your case that routine needs two values.
However, that “one parameter” can be a array of “many” values, or even a collection or whatever. In our case we pass the two values.
So just keep in mind that what you call can NOT update or “use” the values of controls on the form – the instance of that form will go out of scope.
But we can of course PASS the values you need. This will allow that routine to run 100% independent of the web form.
I also VERY strong suggest that if you DO place the sub in the same web page code behind? You should/can mark that sub as shared. Doing so will allow the compiler to get mad at you and spit out errors if that routine say tries to use or update a control value on the form.
However, it is MUCH better is to place this sub in a separate standard code module out side of the web forms code behind.
Regardless of above, we can now re-write the code we have as this:
If Me._cart.SaveOrder(Me._user, Me._orderCode, Me._lang) then
Dim myInfo(1) as object
myInfo(0) = me.cart
myInfo(1) = me_._lng
Call MyUpdateQ(myInfo)
End If
' bla bla lba
Shared Sub MyUPdateQ(p() as object)
'Update quantity in db
Dim mail As New EmailBLL
mail.SendOrderNotice(p(0),p(1)
mail.SendProductNotice() '2 Mails
End Sub
Ok, so far, we not achieved much, but we re-writing to accept the ONE array is KEY here.
So, now now make sure the above runs/works and is all happy.
Now, because we moved out the "work load" to that one routine, it is now a simple matter to start a thread.
Now, Our above code becomes this:
Protected Sub btnConcludiOrdine_Click(sender As Object, e As System.EventArgs) Handles btnConcludiOrdine.Click
If IsValidOrder(Me._cart, msg) Then
If Me._cart.SaveOrder(Me._user, Me._orderCode, Me._lang) then
Dim myInfo(1) as object
myInfo(0) = me.cart
myInfo(1) = me_._lng
Dim MyThread As New Thread(New ParameterizedThreadStart(AddressOf MyUpdateQ))
MyThread.Start(myInfo)
End If
Else
Response.Redirect("*Error URL*")
End If
End Sub
Shared Sub MyUPdateQ(p() as object)
'Update quantity in db
Dim mail As New EmailBLL
mail.SendOrderNotice(p(0),p(1)
mail.SendProductNotice() '2 Mails
End Sub
That is it. Now when you click your button it will wait ZERO time, since the long running routine is now going to run 100% as a separate thread. And this will also mean that when the user clicks the button - the page will respond instant and post back to user will be done. So if that thread takes 6 seconds, or even 25 seconds, the user will not notice this delay.
Just push your sending mail logic in Task and if you are not interested in result don't await it. c# syntax
Task.Run(() => SendEmail());
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 am using Flex and Actionscript 3, along with Webservices, rpc and a callResponder. I want to be able to, for example, say:
loadData1(); // Loads webservice data 1
loadData2(); // Loads webservice data 2
loadData3(); // Loads webservice data 3
However, Actionscript 3 works with async events, so for every call you need to wait for the ResultEvent to trigger when it is done. So, I might want to do the next request every time an event is done. However, I am afraid that threading issues might arise, and some events might not happen at all. I don't think I'm doing a good job of explaining, so I will try to show some code:
private var service:Service1;
var cp:CallResponder = new CallResponder();
public function Webservice()
{
cp.addEventListener(ResultEvent.RESULT, webcalldone);
service = new Service1();
}
public function doWebserviceCall()
{
// Check if already doing call, otherwise do this:
cp.token = service.WebserviceTest_1("test");
}
protected function webcalldone(event:ResultEvent):void
{
// Get the result
var result:String = cp.lastResult as String;
// Check if other calls need to be done, do those
}
Now, I could ofcourse save the actions in an arraylist, but whose to say that the addToArrayList and the check if other calls are available do not mess eachother up, or just miss each other, thereby halting execution? Is there something like a volatile Arraylist? Or is there a completely different, but better solution for this problem?
Use an AsyncToken to keep track of which call the returned data was for http://flexdiary.blogspot.com/2008/11/more-thoughts-on-remoting.html
When I want to store data in an async manor I put it in an array and make a function that will "pop" the element as I send it off.
This function will be called on complete and on error events.
Yes I know there could be an issue with the server and data lost but oh well. That can also be handled
Events will always fire however, it may not be a complete event that gets fired but could be an error event.
Once the array is empty the function is done.
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