VB.NET

HTTP Download in Parallel with Simultaneous Range Requests

See more HTTP Examples

Demonstrates how to download a large file with parallel simultaneous requests, where each request downloads a segment (range) of the remote file. VB.NET

Dim success As Boolean = False

' This requires the Chilkat API to have been previously unlocked.
' See Global Unlock Sample for sample code.

Dim http As New Chilkat.Http

' First get the size of the file to be downloaded.
Dim url As String = "https://www.chilkatsoft.com/hamlet.xml"

Dim resp As New Chilkat.HttpResponse
success = http.HttpNoBody("HEAD",url,resp)
If (success = False) Then
    Debug.WriteLine(http.LastErrorText)
    Exit Sub
End If


Dim remoteFileSize As Integer = resp.ContentLength

Debug.WriteLine("Downloading " & remoteFileSize & " bytes...")

' Let's download in 4 chunks.
' (the last chunk will be whatever remains after the 1st 3 equal sized chunks)
Dim chunkSize As Integer = remoteFileSize / 4

' The Range header is used to download a range from a resource
' Range: bytes=<range-start>-<range-end>
' or
' Range: bytes=<range-start>-

' We're writing code this way for clarity..
Dim http1 As New Chilkat.Http
Dim http2 As New Chilkat.Http
Dim http3 As New Chilkat.Http
Dim http4 As New Chilkat.Http

Dim sbRange As New Chilkat.StringBuilder
sbRange.SetString("bytes=<range-start>-<range-end>")
Dim numReplaced As Integer = sbRange.ReplaceI("<range-start>",0)
numReplaced = sbRange.ReplaceI("<range-end>",chunkSize - 1)
Debug.WriteLine(sbRange.GetAsString())
http1.SetRequestHeader("Range",sbRange.GetAsString())

sbRange.SetString("bytes=<range-start>-<range-end>")
numReplaced = sbRange.ReplaceI("<range-start>",chunkSize)
numReplaced = sbRange.ReplaceI("<range-end>",2 * chunkSize - 1)
Debug.WriteLine(sbRange.GetAsString())
http2.SetRequestHeader("Range",sbRange.GetAsString())

sbRange.SetString("bytes=<range-start>-<range-end>")
numReplaced = sbRange.ReplaceI("<range-start>",2 * chunkSize)
numReplaced = sbRange.ReplaceI("<range-end>",3 * chunkSize - 1)
Debug.WriteLine(sbRange.GetAsString())
http3.SetRequestHeader("Range",sbRange.GetAsString())

sbRange.SetString("bytes=<range-start>-")
numReplaced = sbRange.ReplaceI("<range-start>",3 * chunkSize)
Debug.WriteLine(sbRange.GetAsString())
http4.SetRequestHeader("Range",sbRange.GetAsString())

' Start each range download
Dim task1 As Chilkat.Task = http1.DownloadAsync(url,"qa_output/chunk1.dat")
task1.Run()

Dim task2 As Chilkat.Task = http2.DownloadAsync(url,"qa_output/chunk2.dat")
task2.Run()

Dim task3 As Chilkat.Task = http3.DownloadAsync(url,"qa_output/chunk3.dat")
task3.Run()

Dim task4 As Chilkat.Task = http4.DownloadAsync(url,"qa_output/chunk4.dat")
task4.Run()

' Wait for the downloads to complete.
Dim numLive As Integer = 4
While numLive > 0
    numLive = 0
    If (task1.Live = True) Then
        numLive = numLive + 1
    End If

    If (task2.Live = True) Then
        numLive = numLive + 1
    End If

    If (task3.Live = True) Then
        numLive = numLive + 1
    End If

    If (task4.Live = True) Then
        numLive = numLive + 1
    End If

    If (numLive > 0) Then
        ' SleepMs is a convenience method to cause the caller to sleep for N millisec.
        ' It does not cause the given task to sleep..
        task1.SleepMs(10)
    End If

End While

' All should be downloaded now..
' Examine the result of each Download.
Dim numErrors As Integer = 0
If (task1.GetResultBool() = False) Then
    Debug.WriteLine(task1.ResultErrorText)
    numErrors = numErrors + 1
End If

If (task2.GetResultBool() = False) Then
    Debug.WriteLine(task2.ResultErrorText)
    numErrors = numErrors + 1
End If

If (task3.GetResultBool() = False) Then
    Debug.WriteLine(task3.ResultErrorText)
    numErrors = numErrors + 1
End If

If (task4.GetResultBool() = False) Then
    Debug.WriteLine(task4.ResultErrorText)
    numErrors = numErrors + 1
End If

If (numErrors > 0) Then




    Exit Sub
End If


' All downloads were successful.
' Compose the file from the parts.
Dim fac As New Chilkat.FileAccess
success = fac.ReassembleFile("qa_output","chunk","dat","qa_output/hamlet.xml")
If (success = False) Then
    Debug.WriteLine(fac.LastErrorText)
Else
    Debug.WriteLine("Success.")
End If







' Let's download in the regular way, and then compare files..
success = http.Download(url,"qa_output/hamletRegular.xml")

' Compare files.
Dim bSame As Boolean = fac.FileContentsEqual("qa_output/hamlet.xml","qa_output/hamletRegular.xml")
Debug.WriteLine("bSame = " & bSame)