Xojo Plugin

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. Xojo Plugin

Dim success As Boolean
success = 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
url = "https://www.chilkatsoft.com/hamlet.xml"

Dim resp As New Chilkat.HttpResponse
success = http.HttpNoBody("HEAD",url,resp)
If (success = False) Then
    System.DebugLog(http.LastErrorText)
    Return
End If

Dim remoteFileSize As Int32
remoteFileSize = resp.ContentLength

System.DebugLog("Downloading " + Str(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 Int32
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 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
success = sbRange.SetString("bytes=<range-start>-<range-end>")
Dim numReplaced As Int32
numReplaced = sbRange.ReplaceI("<range-start>",0)
numReplaced = sbRange.ReplaceI("<range-end>",chunkSize - 1)
System.DebugLog(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)
System.DebugLog(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)
System.DebugLog(sbRange.GetAsString())
http3.SetRequestHeader "Range",sbRange.GetAsString()

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

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

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

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

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

// Wait for the downloads to complete.
Dim numLive As Int32
numLive = 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

Wend

// All should be downloaded now..
// Examine the result of each Download.
Dim numErrors As Int32
numErrors = 0
If (task1.GetResultBool() = False) Then
    System.DebugLog(task1.ResultErrorText)
    numErrors = numErrors + 1
End If

If (task2.GetResultBool() = False) Then
    System.DebugLog(task2.ResultErrorText)
    numErrors = numErrors + 1
End If

If (task3.GetResultBool() = False) Then
    System.DebugLog(task3.ResultErrorText)
    numErrors = numErrors + 1
End If

If (task4.GetResultBool() = False) Then
    System.DebugLog(task4.ResultErrorText)
    numErrors = numErrors + 1
End If

If (numErrors > 0) Then

    Return
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
    System.DebugLog(fac.LastErrorText)
Else
    System.DebugLog("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
bSame = fac.FileContentsEqual("qa_output/hamlet.xml","qa_output/hamletRegular.xml")
System.DebugLog("bSame = " + Str(bSame))