(VB.NET) S3 Resume Download

Suppose an S3 download of a very large file failed for some reason and you have a partial file on disk. Rather than restart the entire download, you wish to download the remaining portion. This example demonstrates how to finish a previously failed download.

Note: This example requires Chilkat v9.5.0.83 or above.

Chilkat .NET Downloads Chilkat .NET Framework Chilkat for .NET Core

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

Dim rest As New Chilkat.Rest

' Connect to the Amazon AWS REST server.
Dim bTls As Boolean = True
Dim port As Integer = 443
Dim bAutoReconnect As Boolean = True
Dim success As Boolean = rest.Connect("s3.amazonaws.com",port,bTls,bAutoReconnect)

' ---------------------------------------------------------------------------
' Important: For buckets created in regions outside us-east-1,
' there are three important changes that need to be made.
' See Working with S3 Buckets in Non-us-east-1 Regions for the details.
' ---------------------------------------------------------------------------

' Provide AWS credentials for the REST call.
Dim authAws As New Chilkat.AuthAws
authAws.AccessKey = "AWS_ACCESS_KEY"
authAws.SecretKey = "AWS_SECRET_KEY"
authAws.ServiceName = "s3"
success = rest.SetAuthAws(authAws)

' Set the bucket name via the HOST header.
' In this case, the bucket name is "chilkat100".
rest.Host = "chilkat100.s3.amazonaws.com"

' We want to continue downloading a file.
' The relative local filepath of our previously partially downoaded file is: qa_output/hamlet.xml
' Let's find out how many bytes are already downloaded.
Dim fac As New Chilkat.FileAccess
Dim localFilepath As String = "qa_output/hamlet.xml"
' Note: The FileSize method returns a signed 32-bit integer.  If the file is potentially larger than 2GB, call FileSizeStr instead to return
' the size of the file as a string, then convert to an integer value.
Dim sz As Integer = fac.FileSize(localFilepath)
If (sz > 0) Then
    Debug.WriteLine("sz = " & sz)

    ' If the sz equals 42375 bytes, then we want to add a Range header that looks like this:
    ' Range: bytes=42375-
    Dim sbRange As New Chilkat.StringBuilder
    sbRange.Append("bytes=")
    sbRange.AppendInt(sz)
    sbRange.Append("-")
    rest.AddHeader("Range",sbRange.GetAsString())
    Debug.WriteLine("Added Range: " & sbRange.GetAsString())
End If


' Send the request to download the remainder of the file.
success = rest.SendReqNoBody("GET","/hamlet.xml")
If (success <> True) Then
    Debug.WriteLine(rest.LastErrorText)
    Exit Sub
End If


' Read the response header.
Dim responseStatusCode As Integer = rest.ReadResponseHeader()
If (responseStatusCode < 0) Then
    Debug.WriteLine(rest.LastErrorText)
    Exit Sub
End If


Debug.WriteLine("Response status code = " & responseStatusCode)

' We expect a 200 or 206 response status if the file data is coming.
' Otherwise, we'll get a string response body with an error message(or no response body).
If ((responseStatusCode = 200) Or (responseStatusCode = 206)) Then

    Dim bodyStream As New Chilkat.Stream

    ' The stream's sink will be a file.
    ' We will append to the file..
    bodyStream.SinkFile = localFilepath

    ' Indicate that we wish to append to the output file.
    ' The SinkFileAppend property was added in Chilkat v9.50.83
    bodyStream.SinkFileAppend = True

    ' Read the response body to the stream.  Given that we've
    ' set the stream's sink to a file, it will stream directly
    ' to the file.
    success = rest.ReadRespBodyStream(bodyStream,True)
    If (success <> True) Then
        Debug.WriteLine(rest.LastErrorText)
        Exit Sub
    End If


    Debug.WriteLine("Successfully downloaded the file.")

Else
    Dim errResponse As String = rest.ReadRespBodyString()
    If (rest.LastMethodSuccess <> True) Then
        Debug.WriteLine(rest.LastErrorText)
    Else
        Debug.WriteLine(errResponse)
    End If

End If