How to throttle concurrent Async webrequests - vb.net

Question

I often need to make a large number of webrequests, without overloading the network
I currently do this by running synchronous requests in parallel, utilizing ThreadPool.SetMinThreads and MaxDegreeOfParallelism to exactly specify how many requests run concurrently
Now this works just fine, but it feels wrong.
I would really like to utilize async methods, but i cant work out how to limit the number of concurrent requests.
A simplified example of my parallel way of doing this( using a webclient and no error handling for brevity):
Private Function SearchSitesForKeywordInParallel(ByVal keyword As String, ByVal sites As String(), ByVal maxConcurrency As Integer) As String()
Dim po As New ParallelOptions
po.MaxDegreeOfParallelism = maxConcurrency
Threading.ThreadPool.SetMinThreads(maxConcurrency, 2)
Dim sitesContainingKeyword As New Concurrent.ConcurrentBag(Of String)
Parallel.For(0, sites.Count, po, Sub(i)
Dim wc As New Net.WebClient
wc.Proxy = Nothing
Dim pageSource As String = wc.DownloadString(sites(i))
If pageSource.Contains(keyword) Then
sitesContainingKeyword.Add(sites(i))
End If
End Sub)
Return sitesContainingKeyword.ToArray
End Function
This is a blocking function, which is what i require.
Now i have tested the webclient.downloadStringAsync method in a regular for loop, and it will fire all the requests pretty much at once, overloading the network.
What i would like to do is initially make X requests, then make new ones as each response comes back.
I am fairly sure tasks is the way to go, and im positive a have read some very nice implementations in c#, but my c# experience is limited, and i have a hard time translating c# lambadas to vb.net.
I am also limited to vs2010 and .net4, so the niceties of .net4.5 async await are not an option for me.
Any help very much appreciated

Answer 1

Not sure, if I understand completey, what exactly you want to achieve, but if you want to use aync methods, you can do it like this:
Dim google As String = "http://www.google.com/#&q="
Dim qsites As New Concurrent.ConcurrentQueue(Of String)
For Each k In {"foo", "bar", "john", "jack", "stackoverflow", "basic", "ship", "car", "42"}
qsites.Enqueue(google & k)
Next
Dim cde As New System.Threading.CountdownEvent(qsites.Count)
Dim strings As New Concurrent.ConcurrentBag(Of String)
Dim completedhandler = Sub(wco As Object, ev As Net.DownloadStringCompletedEventArgs)
Dim wc = DirectCast(wco, Net.WebClient)
Debug.Print("got one!")
strings.Add(ev.Result)
cde.Signal()
Dim s As String = String.Empty
If qsites.TryDequeue(s) Then
Debug.Print("downloading from {0}", s)
wc.DownloadStringAsync(New Uri(s))
End If
End Sub
Dim numthreads As Integer = 4
System.Threading.Tasks.Task.Factory.StartNew(Sub()
For i = 1 To numthreads
Dim s As String = String.Empty
If qsites.TryDequeue(s) Then
Dim wc As New Net.WebClient
wc.Proxy = Nothing
AddHandler wc.DownloadStringCompleted, completedhandler
Debug.Print("downloading from {0}", s)
wc.DownloadStringAsync(New Uri(s))
End If
Next
End Sub)
cde.Wait()
You only need to "start" the async downloads in a different thread/task because (afaik) the WC's downloadcompleted events fire in the UI thread (or currentsync..context) and the cde.wait would then not allow the events to be handled.

Answer 2

I just want to add another answer to this as I have recently solved a similar problem (note the code snippet is in C#, but should give the idea).
I used to have number of parallel http synchronous requests sent to http server on different thread and used to limit the number of requests I sent using semaphore.
Now, I have adapted to new TPL (c# 5.0 - aysn/await - quite handy (basically continuation introduced in TPL sound natural to me - and with async/await it has become much easier to use)), to invoke network I/O asynchronously.
i.e. ideally now I will be using only one thread in caller (unless I really need to get results before continuing), and let .net, os and I/o completion port threads work together to invoke my continuation code in thread pool to complete operation (basically 'callback' in APM, on completed event in event based pattern, 'continuation' in TPL, code after await in C# 5.0 (4.5 .net))
The principle I followed when I have embraced async i/o is simple - don't let thread wait and waste CPU and resources, unless it is really necessary!

Answer 3

You can do this asynchronously in VB.NET using the Wintellect Powerthreading library's AsyncEnumerator class, which you can get from NuGet.
This gives you some of the functionality of Await but works in VS2010 with .Net 2.0 to 4.0 while giving you an upgrade path to the 4.5 async features.
The downside is that the WebClient async methods require an EAP-to-APM shim based on Task<> to be used with AsyncEnumerator, so the code is quite a lot more complicated.
The simplest way to control the number of concurrent requests is to initiate X async operations, then just initiate another every time one completes.
Example code:
Imports System.Collections.Generic
Imports System.Runtime.CompilerServices
Imports System.Threading.Tasks
Imports System.Net
Imports Wintellect.Threading.AsyncProgModel
Module TaskExtension
REM http://msdn.microsoft.com/en-us/library/hh873178.aspx
<Extension()>
Public Function AsApm(Of T1)(ByVal task As Task(Of T1), callback As AsyncCallback, state As Object) As IAsyncResult
If (task Is Nothing) Then
Throw New ArgumentNullException("task")
End If
Dim tcs = New TaskCompletionSource(Of T1)(state)
task.ContinueWith(Sub(t As Task(Of T1))
If (t.IsFaulted) Then
tcs.TrySetException(t.Exception.InnerExceptions)
ElseIf t.IsCanceled Then
tcs.TrySetCanceled()
Else : tcs.TrySetResult(t.Result)
End If
If (Not callback Is Nothing) Then
callback(tcs.Task)
End If
End Sub, TaskScheduler.Default)
Return tcs.Task
End Function
End Module
Module ApmAsyncDownload
Public Function DownloadStringAsync(url As Uri) As Task(Of String)
Dim tcs As New TaskCompletionSource(Of String)
Dim wc As New WebClient()
AddHandler wc.DownloadStringCompleted, Sub(s As Object, e As System.Net.DownloadStringCompletedEventArgs)
If (Not (e.Error Is Nothing)) Then
tcs.TrySetException(e.Error)
ElseIf e.Cancelled Then
tcs.TrySetCanceled()
Else : tcs.TrySetResult(e.Result)
End If
End Sub
wc.DownloadStringAsync(url)
Return tcs.Task
End Function
Public Function BeginDownloadString(url As Uri, callback As AsyncCallback, state As Object) As IAsyncResult
Return DownloadStringAsync(url).AsApm(callback, state)
End Function
Public Function EndDownloadString(asyncResult As IAsyncResult) As String
Dim castToTask As Task(Of String) = asyncResult
Return castToTask.Result
End Function
End Module
Public Class AsyncIterators
Private Shared Iterator Function SearchUrl(ae As AsyncEnumerator(Of Boolean), keyword As String, uri As Uri) As IEnumerator(Of Int32)
ae.Result = False
ApmAsyncDownload.BeginDownloadString(uri, ae.End(0, AddressOf ApmAsyncDownload.EndDownloadString), Nothing)
Yield 1
If (ae.IsCanceled()) Then
Return
End If
Try
Dim page As String = ApmAsyncDownload.EndDownloadString(ae.DequeueAsyncResult)
ae.Result = page.Contains(keyword)
Catch ex As AggregateException
End Try
End Function
Public Shared Iterator Function SearchIterator(ae As AsyncEnumerator(Of List(Of String)), keyword As String, urls As List(Of Uri)) As IEnumerator(Of Int32)
ae.Result = New List(Of String)
'Control how many searches are started asynchonously
Dim startSearches = Math.Min(3, urls.Count)
Dim enumerator = urls.GetEnumerator
Dim toBeCompleted = urls.Count
Do Until (toBeCompleted <= 0)
While (startSearches > 0)
If enumerator.MoveNext Then
Dim subAe = New AsyncEnumerator(Of Boolean)()
subAe.SyncContext = Nothing
subAe.BeginExecute(SearchUrl(subAe, keyword, enumerator.Current), ae.End(0, Function(ar As IAsyncResult) As AsyncEnumerator.EndObjectXxx
subAe.EndExecute(ar)
End Function), enumerator.Current)
End If
startSearches = startSearches - 1
End While
'Wait for first async search to complete
Yield 1
toBeCompleted = toBeCompleted - 1
If (ae.IsCanceled()) Then
Exit Do
End If
'Get result of the search and add to results
Dim result = ae.DequeueAsyncResult()
Dim completedAe = AsyncEnumerator(Of Boolean).FromAsyncResult(result)
If (completedAe.EndExecute(result)) Then
Dim uri As Uri = result.AsyncState
ae.Result.Add(uri.OriginalString)
End If
'Start 1 more search
startSearches = startSearches + 1
Loop
End Function
End Class
Module Module1
Sub Main()
Dim searchAe = New AsyncEnumerator(Of List(Of String))()
searchAe.SyncContext = Nothing
Dim urlStrings = New List(Of String) From {"http://www.google.com", "http://www.yahoo.com", "http://www.dogpile.com"}
Dim uris = urlStrings.Select(Function(urlString As String) As Uri
Return New Uri(urlString)
End Function).ToList()
For Each Str As String In searchAe.EndExecute(searchAe.BeginExecute(AsyncIterators.SearchIterator(searchAe, "search", uris), Nothing, Nothing))
Console.WriteLine(Str)
Next
Console.ReadKey()
End Sub
End Module
And I now see what you mean about translating c# lambdas!