non blocking webrequests vb.net - vb.net

Question

I am making a program that must process about 5000 strings as quickly as possible. about 2000 of these strings must be translated via a webrequest to mymemory.translated.net. (see code below, JSON part removed since not needed here)
Try
url = "http://api.mymemory.translated.net/get?q=" & Firstpart & "!&langpair=de|it&de=somemail#christmas.com"
request = DirectCast(WebRequest.Create(url), HttpWebRequest)
response = DirectCast(request.GetResponse(), HttpWebResponse)
myreader = New StreamReader(response.GetResponseStream())
Dim rawresp As String
rawresp = myreader.ReadToEnd()
Debug.WriteLine("Raw:" & rawresp)
Catch ex As Exception
MessageBox.Show(ex.ToString)
End Try
the code itself is working fine, problem is it is a blocking code and needs about 1 second per string. Thats more then half an hour for all my strings. i would need to convert this code to a non blocking one and make multiple calls on the same time. Could somebody please tell me how i could do that? I was thinking of a background worker but that wouldnt speed things up.. it would just execute the code on a different thread...
thanks!

Answer 1

The problem is you aren't just being held back by the maximum number of concurrent operations. HttpWebRequests are throttled by nature (I believe the default policy allows only 2 at any given time), so you have to override that behaviour too. Please refer to the code below.
Imports System.Diagnostics
Imports System.IO
Imports System.Net
Imports System.Threading
Imports System.Threading.Tasks
Public Class Form1
''' <summary>
''' Test entry point.
''' </summary>
Private Sub Form1_Load() Handles MyBase.Load
' Generate enough words for us to test thoroughput.
Dim words = Enumerable.Range(1, 100) _
.Select(Function(i) "Word" + i.ToString()) _
.ToArray()
' Maximum theoretical number of concurrent requests.
Dim maxDegreeOfParallelism = 24
Dim sw = Stopwatch.StartNew()
' Capture information regarding current SynchronizationContext
' so that we can perform thread marshalling later on.
Dim uiScheduler = TaskScheduler.FromCurrentSynchronizationContext()
Dim uiFactory = New TaskFactory(uiScheduler)
Dim transformTask = Task.Factory.StartNew(
Sub()
' Apply the transformation in parallel.
' Parallel.ForEach implements clever load
' balancing, so, since each request won't
' be doing much CPU work, it will spawn
' many parallel streams - likely more than
' the number of CPUs available.
Parallel.ForEach(words, New ParallelOptions With {.MaxDegreeOfParallelism = maxDegreeOfParallelism},
Sub(word)
' We are running on a thread pool thread now.
' Be careful not to access any UI until we hit
' uiFactory.StartNew(...)
' Perform transformation.
Dim url = "http://api.mymemory.translated.net/get?q=" & word & "!&langpair=de|it&de=somemail#christmas.com"
Dim request = DirectCast(WebRequest.Create(url), HttpWebRequest)
' Note that unless you specify this explicitly,
' the framework will use the default and you
' will be limited to 2 parallel requests
' regardless of how many threads you spawn.
request.ServicePoint.ConnectionLimit = maxDegreeOfParallelism
Using response = DirectCast(request.GetResponse(), HttpWebResponse)
Using myreader As New StreamReader(response.GetResponseStream())
Dim rawresp = myreader.ReadToEnd()
Debug.WriteLine("Raw:" & rawresp)
' Transform the raw response here.
Dim processed = rawresp
uiFactory.StartNew(
Sub()
' This is running on the UI thread,
' so we can access the controls,
' i.e. add the processed result
' to the data grid.
Me.Text = processed
End Sub, TaskCreationOptions.PreferFairness)
End Using
End Using
End Sub)
End Sub)
transformTask.ContinueWith(
Sub(t As Task)
' Always stop the stopwatch.
sw.Stop()
' Again, we are back on the UI thread, so we
' could access UI controls if we needed to.
If t.Status = TaskStatus.Faulted Then
Debug.Print("The transformation errored: {0}", t.Exception)
Else
Debug.Print("Operation completed in {0} s.", sw.ElapsedMilliseconds / 1000)
End If
End Sub,
uiScheduler)
End Sub
End Class

Answer 2

If you want to send 10 parallel requests, you must create 10 BackgroundWorkers. Or manually create 10 threads. Then iterate, and whenever a worker/thread is done, give it a new task.
I do not recommend firing 5000 parallel threads/workers, you must be careful:
A load like that could be interpreted as spamming or an attack by the server. Don't overdo it, maybe talk to translated.net and ask them about the workload they accept.
Also think about what your machine and your internet upstream can handle.

Answer 3

I would create a Task for every request, so you can have a Callback for every call using ContinueWith:
For Each InputString As String In myCollectionString
Tasks.Task(Of String).Factory.StartNew(Function(inputString)
Dim request As HttpWebRequest
Dim myreader As StreamReader
Dim response As HttpWebResponse
Dim rawResp As String = String.Empty
Try
Dim url As String = "http://api.mymemory.translated.net/get?q=" & inputString & "!&langpair=de|it&de=somemail#christmas.com"
request = DirectCast(WebRequest.Create(url), HttpWebRequest)
response = DirectCast(request.GetResponse(), HttpWebResponse)
myreader = New StreamReader(response.GetResponseStream())
rawResp = myreader.ReadToEnd()
Debug.WriteLine("Raw:" & rawResp)
Catch ex As Exception
MessageBox.Show(ex.ToString)
End Try
Return rawResp
End Function, CancellationToken.None, _
Tasks.TaskCreationOptions.None).ContinueWith _
(Sub(task As Tasks.Task(Of String))
'Dom something with result
Console.WriteLine(task.Result)
End Sub)
Next