Delphi DLL

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.

Delphi DLL

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, FileAccess, AuthAws, Rest, StringBuilder, Stream;

...

procedure TForm1.Button1Click(Sender: TObject);
var
success: Boolean;
rest: HCkRest;
bTls: Boolean;
port: Integer;
bAutoReconnect: Boolean;
authAws: HCkAuthAws;
fac: HCkFileAccess;
localFilepath: PWideChar;
sz: Integer;
sbRange: HCkStringBuilder;
responseStatusCode: Integer;
bodyStream: HCkStream;
errResponse: PWideChar;

begin
success := False;

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

rest := CkRest_Create();

// Connect to the Amazon AWS REST server.
bTls := True;
port := 443;
bAutoReconnect := True;
success := CkRest_Connect(rest,'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 := CkAuthAws_Create();
CkAuthAws_putAccessKey(authAws,'AWS_ACCESS_KEY');
CkAuthAws_putSecretKey(authAws,'AWS_SECRET_KEY');
CkAuthAws_putServiceName(authAws,'s3');
success := CkRest_SetAuthAws(rest,authAws);

// Set the bucket name via the HOST header.
// In this case, the bucket name is "chilkat100".
CkRest_putHost(rest,'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 := CkFileAccess_Create();
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 := CkFileAccess_FileSize(fac,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 := CkStringBuilder_Create();
    CkStringBuilder_Append(sbRange,'bytes=');
    CkStringBuilder_AppendInt(sbRange,sz);
    CkStringBuilder_Append(sbRange,'-');
    CkRest_AddHeader(rest,'Range',CkStringBuilder__getAsString(sbRange));
    Memo1.Lines.Add('Added Range: ' + CkStringBuilder__getAsString(sbRange));
  end;

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

// Read the response header.
responseStatusCode := CkRest_ReadResponseHeader(rest);
if (responseStatusCode < 0) then
  begin
    Memo1.Lines.Add(CkRest__lastErrorText(rest));
    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 := CkStream_Create();

    // The stream's sink will be a file.
    // We will append to the file..
    CkStream_putSinkFile(bodyStream,localFilepath);

    // Indicate that we wish to append to the output file.
    // The SinkFileAppend property was added in Chilkat v9.50.83
    CkStream_putSinkFileAppend(bodyStream,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 := CkRest_ReadRespBodyStream(rest,bodyStream,True);
    if (success <> True) then
      begin
        Memo1.Lines.Add(CkRest__lastErrorText(rest));
        Exit;
      end;

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

  end
else
  begin
    errResponse := CkRest__readRespBodyString(rest);
    if (CkRest_getLastMethodSuccess(rest) <> True) then
      begin
        Memo1.Lines.Add(CkRest__lastErrorText(rest));
      end
    else
      begin
        Memo1.Lines.Add(errResponse);
      end;
  end;

CkRest_Dispose(rest);
CkAuthAws_Dispose(authAws);
CkFileAccess_Dispose(fac);
    CkStringBuilder_Dispose(sbRange);
    CkStream_Dispose(bodyStream);

end;