VB.NET

Resume an Interrupted SFTP Upload

Demonstrates the Chilkat SFtp.ResumeUploadFileByName method, which resumes an interrupted upload. The first argument is the remote file path and the second is the local file path. Chilkat uses the existing remote file size as the local starting offset.

Note: This example uses relative local paths, which are resolved against the application's current working directory. Absolute local paths may also be used. Supply the paths appropriate to your own environment.

Background: For large transfers over unreliable links, restarting from zero after a dropped connection is wasteful. Resume picks up where it left off by comparing sizes: if the remote file is missing or empty it does a full upload, if it is smaller it appends the remaining local bytes, and if it is already the same size or larger it does nothing. The size-only check is simple and fast, but it assumes the partial remote file is a valid prefix of the local one — true for an interrupted transfer of the same file, but not a substitute for content verification if the file may have changed.

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Dim success As Boolean = False

'  Demonstrates the SFtp.ResumeUploadFileByName method, which resumes an interrupted upload.  The
'  1st argument is the remote file path and the 2nd is the local file path.

Dim sftp As New Chilkat.SFtp

'  Connect, authenticate, and initialize the SFTP subsystem.
Dim port As Integer = 22
success = sftp.Connect("sftp.example.com",port)
If (success = False) Then
    Debug.WriteLine(sftp.LastErrorText)
    Exit Sub
End If


'  Normally you would not hard-code the password in source.  You should instead obtain it
'  from an interactive prompt, environment variable, or a secrets vault.
Dim password As String = "mySshPassword"

success = sftp.AuthenticatePw("mySshLogin",password)
If (success = False) Then
    Debug.WriteLine(sftp.LastErrorText)
    Exit Sub
End If


success = sftp.InitializeSftp()
If (success = False) Then
    Debug.WriteLine(sftp.LastErrorText)
    Exit Sub
End If


'  Resume uploading the local file.  Chilkat uses the existing remote file size as the local
'  starting offset, so only the remaining bytes are sent.  If the remote file is missing or
'  empty, a normal full upload is performed.
success = sftp.ResumeUploadFileByName("subdir/largefile.zip","qa_data/largefile.zip")
If (success = False) Then
    Debug.WriteLine(sftp.LastErrorText)
    Exit Sub
End If

Debug.WriteLine("Upload resumed and completed.")

sftp.Disconnect()