DataFlex

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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Use ChilkatAx-win32.pkg

Procedure Test
    Boolean iSuccess
    Handle hoSftp
    Integer iPort
    String sPassword
    String sTemp1

    Move False To iSuccess

    //  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.

    Get Create (RefClass(cComChilkatSFtp)) To hoSftp
    If (Not(IsComObjectCreated(hoSftp))) Begin
        Send CreateComObject of hoSftp
    End

    //  Connect, authenticate, and initialize the SFTP subsystem.
    Move 22 To iPort
    Get ComConnect Of hoSftp "sftp.example.com" iPort To iSuccess
    If (iSuccess = False) Begin
        Get ComLastErrorText Of hoSftp To sTemp1
        Showln sTemp1
        Procedure_Return
    End

    //  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.
    Move "mySshPassword" To sPassword

    Get ComAuthenticatePw Of hoSftp "mySshLogin" sPassword To iSuccess
    If (iSuccess = False) Begin
        Get ComLastErrorText Of hoSftp To sTemp1
        Showln sTemp1
        Procedure_Return
    End

    Get ComInitializeSftp Of hoSftp To iSuccess
    If (iSuccess = False) Begin
        Get ComLastErrorText Of hoSftp To sTemp1
        Showln sTemp1
        Procedure_Return
    End

    //  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.
    Get ComResumeUploadFileByName Of hoSftp "subdir/largefile.zip" "qa_data/largefile.zip" To iSuccess
    If (iSuccess = False) Begin
        Get ComLastErrorText Of hoSftp To sTemp1
        Showln sTemp1
        Procedure_Return
    End

    Showln "Upload resumed and completed."

    Send ComDisconnect To hoSftp


End_Procedure