(Delphi ActiveX) 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 for Delphi Downloads Chilkat ActiveX DLL for Delphi Chilkat non-ActiveX DLL for Delphi * The examples here use the ActiveX DLL.

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Chilkat_TLB;

...

procedure TForm1.Button1Click(Sender: TObject);
var
rest: TChilkatRest;
bTls: Integer;
port: Integer;
bAutoReconnect: Integer;
success: Integer;
authAws: TChilkatAuthAws;
fac: TCkFileAccess;
localFilepath: WideString;
sz: Integer;
sbRange: TChilkatStringBuilder;
responseStatusCode: Integer;
bodyStream: TChilkatStream;
errResponse: WideString;

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

rest := TChilkatRest.Create(Self);

// Connect to the Amazon AWS REST server.
bTls := 1;
port := 443;
bAutoReconnect := 1;
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.
authAws := TChilkatAuthAws.Create(Self);
authAws.AccessKey := 'AWS_ACCESS_KEY';
authAws.SecretKey := 'AWS_SECRET_KEY';
authAws.ServiceName := 's3';
success := rest.SetAuthAws(authAws.ControlInterface);

// 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.
fac := TCkFileAccess.Create(Self);
localFilepath := '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.
sz := fac.FileSize(localFilepath);
if (sz > 0) then
  begin
    Memo1.Lines.Add('sz = ' + IntToStr(sz));

    // If the sz equals 42375 bytes, then we want to add a Range header that looks like this:
    // Range: bytes=42375-
    sbRange := TChilkatStringBuilder.Create(Self);
    sbRange.Append('bytes=');
    sbRange.AppendInt(sz);
    sbRange.Append('-');
    rest.AddHeader('Range',sbRange.GetAsString());
    Memo1.Lines.Add('Added Range: ' + sbRange.GetAsString());
  end;

// Send the request to download the remainder of the file.
success := rest.SendReqNoBody('GET','/hamlet.xml');
if (success <> 1) then
  begin
    Memo1.Lines.Add(rest.LastErrorText);
    Exit;
  end;

// Read the response header.
responseStatusCode := rest.ReadResponseHeader();
if (responseStatusCode < 0) then
  begin
    Memo1.Lines.Add(rest.LastErrorText);
    Exit;
  end;

Memo1.Lines.Add('Response status code = ' + IntToStr(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
  begin

    bodyStream := TChilkatStream.Create(Self);

    // 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 := 1;

    // 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.ControlInterface,1);
    if (success <> 1) then
      begin
        Memo1.Lines.Add(rest.LastErrorText);
        Exit;
      end;

    Memo1.Lines.Add('Successfully downloaded the file.');

  end
else
  begin
    errResponse := rest.ReadRespBodyString();
    if (rest.LastMethodSuccess <> 1) then
      begin
        Memo1.Lines.Add(rest.LastErrorText);
      end
    else
      begin
        Memo1.Lines.Add(errResponse);
      end;
  end;
end;