Visual Basic 6.0

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. Visual Basic 6.0

Dim success As Long
success = 0

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

Dim http As New ChilkatHttp

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

Dim resp As New ChilkatHttpResponse
success = http.HttpNoBody("HEAD",url,resp)
If (success = 0) Then
    Debug.Print http.LastErrorText
    Exit Sub
End If

Dim remoteFileSize As Long
remoteFileSize = resp.ContentLength

Debug.Print "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 Long
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..
Dim http1 As New ChilkatHttp
Dim http2 As New ChilkatHttp
Dim http3 As New ChilkatHttp
Dim http4 As New ChilkatHttp

Dim sbRange As New ChilkatStringBuilder
success = sbRange.SetString("bytes=<range-start>-<range-end>")
Dim numReplaced As Long
numReplaced = sbRange.ReplaceI("<range-start>",0)
numReplaced = sbRange.ReplaceI("<range-end>",chunkSize - 1)
Debug.Print 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)
Debug.Print 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)
Debug.Print sbRange.GetAsString()
http3.SetRequestHeader "Range",sbRange.GetAsString()

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

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

Dim task2 As ChilkatTask
Set task2 = http2.DownloadAsync(url,"qa_output/chunk2.dat")
success = task2.Run()

Dim task3 As ChilkatTask
Set task3 = http3.DownloadAsync(url,"qa_output/chunk3.dat")
success = task3.Run()

Dim task4 As ChilkatTask
Set task4 = http4.DownloadAsync(url,"qa_output/chunk4.dat")
success = task4.Run()

' Wait for the downloads to complete.
Dim numLive As Long
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.
Dim numErrors As Long
numErrors = 0
If (task1.GetResultBool() = 0) Then
    Debug.Print task1.ResultErrorText
    numErrors = numErrors + 1
End If

If (task2.GetResultBool() = 0) Then
    Debug.Print task2.ResultErrorText
    numErrors = numErrors + 1
End If

If (task3.GetResultBool() = 0) Then
    Debug.Print task3.ResultErrorText
    numErrors = numErrors + 1
End If

If (task4.GetResultBool() = 0) Then
    Debug.Print 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 CkFileAccess
success = fac.ReassembleFile("qa_output","chunk","dat","qa_output/hamlet.xml")
If (success = 0) Then
    Debug.Print fac.LastErrorText
Else
    Debug.Print "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 Long
bSame = fac.FileContentsEqual("qa_output/hamlet.xml","qa_output/hamletRegular.xml")
Debug.Print "bSame = " & bSame