VBScript

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. VBScript

Dim fso, outFile
Set fso = CreateObject("Scripting.FileSystemObject")
'Create a Unicode (utf-16) output text file.
Set outFile = fso.CreateTextFile("output.txt", True, True)

success = 0

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

set http = CreateObject("Chilkat.Http")

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

set resp = CreateObject("Chilkat.HttpResponse")
success = http.HttpNoBody("HEAD",url,resp)
If (success = 0) Then
    outFile.WriteLine(http.LastErrorText)
    WScript.Quit
End If

remoteFileSize = resp.ContentLength

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

' Let's download in 4 chunks.
' (the last chunk will be whatever remains after the 1st 3 equal sized chunks)
chunkSize = 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..
set http1 = CreateObject("Chilkat.Http")
set http2 = CreateObject("Chilkat.Http")
set http3 = CreateObject("Chilkat.Http")
set http4 = CreateObject("Chilkat.Http")

set sbRange = CreateObject("Chilkat.StringBuilder")
success = sbRange.SetString("bytes=<range-start>-<range-end>")
numReplaced = sbRange.ReplaceI("<range-start>",0)
numReplaced = sbRange.ReplaceI("<range-end>",chunkSize - 1)
outFile.WriteLine(sbRange.GetAsString())
http1.SetRequestHeader "Range",sbRange.GetAsString()

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

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

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

' Start each range download
' task1 is a Chilkat.Task
Set task1 = http1.DownloadAsync(url,"qa_output/chunk1.dat")
success = task1.Run()

' task2 is a Chilkat.Task
Set task2 = http2.DownloadAsync(url,"qa_output/chunk2.dat")
success = task2.Run()

' task3 is a Chilkat.Task
Set task3 = http3.DownloadAsync(url,"qa_output/chunk3.dat")
success = task3.Run()

' task4 is a Chilkat.Task
Set task4 = http4.DownloadAsync(url,"qa_output/chunk4.dat")
success = task4.Run()

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

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

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

    If (task4.Live = 1) 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

Loop

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

If (task2.GetResultBool() = 0) Then
    outFile.WriteLine(task2.ResultErrorText)
    numErrors = numErrors + 1
End If

If (task3.GetResultBool() = 0) Then
    outFile.WriteLine(task3.ResultErrorText)
    numErrors = numErrors + 1
End If

If (task4.GetResultBool() = 0) Then
    outFile.WriteLine(task4.ResultErrorText)
    numErrors = numErrors + 1
End If

If (numErrors > 0) Then

    WScript.Quit
End If

' All downloads were successful.
' Compose the file from the parts.
set fac = CreateObject("Chilkat.FileAccess")
success = fac.ReassembleFile("qa_output","chunk","dat","qa_output/hamlet.xml")
If (success = 0) Then
    outFile.WriteLine(fac.LastErrorText)
Else
    outFile.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.
bSame = fac.FileContentsEqual("qa_output/hamlet.xml","qa_output/hamletRegular.xml")
outFile.WriteLine("bSame = " & bSame)

outFile.Close