Dart

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.

Dart

import 'package:chilkat/chilkat.dart';

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

  final rest = CkRest();

  // Connect to the Amazon AWS REST server.
  final bTls = true;
  final port = 443;
  final bAutoReconnect = true;
  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.
  final authAws = CkAuthAws();
  authAws.accessKey = 'AWS_ACCESS_KEY';
  authAws.secretKey = 'AWS_SECRET_KEY';
  authAws.serviceName = 's3';
  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.
  final fac = CkFileAccess();
  final 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.
  final sz = fac.fileSize(localFilepath);
  if (sz > 0) {
    print('sz = $sz');

    // If the sz equals 42375 bytes, then we want to add a Range header that looks like this:
    // Range: bytes=42375-
    final sbRange = CkStringBuilder();
    sbRange.append('bytes=');
    sbRange.appendInt(sz);
    sbRange.append('-');
    rest.addHeader('Range', sbRange.getAsString());
    print('Added Range: ${sbRange.getAsString()}');
  }

  // Send the request to download the remainder of the file.
  try {
    rest.sendReqNoBody('GET', '/hamlet.xml');
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

  // Read the response header.
  final responseStatusCode = rest.readResponseHeader();
  if (responseStatusCode < 0) {
    print(rest.lastErrorText);
    return;
  }

  print('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) || (responseStatusCode == 206)) {

    final bodyStream = CkStream();

    // 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.
    try {
      rest.readRespBodyStream(bodyStream, true);
    } on ChilkatException catch (e) {
      print(e.lastErrorText);
      return;
    }

    print('Successfully downloaded the file.');
  } else {
    try {
      final errResponse = rest.readRespBodyString();
      print(errResponse);
    } on ChilkatException catch (e) {
      print(e.lastErrorText);
    }
  }
}