B4X

S3 Resume Download

See more Amazon S3 (new) Examples

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.

B4X

Dim success As Boolean = False

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

Dim rest As ChilkatRest
rest.Initialize("rest")

'  Connect to the Amazon AWS REST server.
Dim bTls As Boolean = True
Dim port As Int = 443
Dim bAutoReconnect As Boolean = True
success = 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 ChilkatAuthAws
authAws.Initialize
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 ChilkatFileAccess
fac.Initialize
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 Int = fac.FileSize(localFilepath)
If sz > 0 Then
    Log("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 ChilkatStringBuilder
    sbRange.Initialize
    sbRange.Append("bytes=")
    sbRange.AppendInt(sz)
    sbRange.Append("-")
    rest.AddHeader("Range", sbRange.GetAsString)
    Log("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
    Log(rest.LastErrorText)
    Return
End If


'  Read the response header.
Dim responseStatusCode As Int = rest.ReadResponseHeader
If responseStatusCode < 0 Then
    Log(rest.LastErrorText)
    Return
End If


Log("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 ChilkatStream
    bodyStream.Initialize("bodyStream")

    '  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
        Log(rest.LastErrorText)
        Return
    End If


    Log("Successfully downloaded the file.")

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

End If