Dart

S3 Get Object Metadata, such as ETag

See more Amazon S3 (new) Examples

Demonstrates how to get an S3 object's metadata using a HEAD request. 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 in the desired region.
  // In this case we connect to S3 in the "us-west-2" region.
  final bTls = true;
  final port = 443;
  final bAutoReconnect = true;
  rest.connect('s3-us-west-2.amazonaws.com', port, bTls, bAutoReconnect);

  // Provide AWS credentials.
  final authAws = CkAuthAws();
  authAws.accessKey = 'AWS_ACCESS_KEY';
  authAws.secretKey = 'AWS_SECRET_KEY';
  authAws.serviceName = 's3';

  // This particular bucket is in the Oregon region, as opposed to the US Standard,
  // therefore the Region must be set appropriately.
  // Also note that we connected to "s3-us-west-2.amazonaws.com".
  authAws.region = 'us-west-2';

  rest.setAuthAws(authAws);

  // Note: The above REST connection and setup of the AWS credentials
  // can be done once.  After connecting, any number of REST calls can be made.
  // The "auto reconnect" property passed to rest.Connect indicates that if
  // the connection is lost, a REST method call will automatically reconnect
  // if needed.
  // --------------------------------------------------------------------------

  // Set the bucket name via the HOST header.
  // In this case, the bucket name is "chilkat.ocean".
  // Note that the Host header should use "bucketName.s3.amazonaws.com", not "bucketName.s3-us-west-2.amazonaws.com"
  rest.host = 'chilkat.ocean.s3.amazonaws.com';

  // Our object is "starfish.jpg".
  // Get the metadata.
  // Since we are sending a HEAD request, we shouldn't use any of the Full* methods.
  // Instead, send the request, then just read the response header.
  try {
    rest.sendReqNoBody('HEAD', '/starfish.jpg');
  } on ChilkatException catch (e) {
    print(e.lastErrorText);
    return;
  }

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

  // When successful, the S3 Storage service will respond with a 200 response code,
  // with an XML body.  
  if (statusCode != 200) {
    // Examine the request/response to see what happened.
    print('response status code = $statusCode');
    print('response status text = ${rest.responseStatusText}');
    print('response header: ${rest.responseHeader}');
    return;
  }

  // The response header contains information about the S3 object.
  // For example:

  // 	HTTP/1.1 200 OK
  // 	x-amz-id-2: GrV/hrB2EUkBlWc0h3JDdmVIrYDXfPC7CGvo/qSVRmYRPwilAOry8+4r2YWl4xJfa1HRYXrP808=
  // 	x-amz-request-id: 4BDC6D6AEA0BDEF3
  // 	Date: Mon, 13 May 2019 22:03:13 GMT
  // 	Last-Modified: Mon, 10 Nov 2014 16:10:23 GMT
  // 	ETag: "2e9c59dbf2662367dc97dfdda85da048"
  // 	Accept-Ranges: bytes
  // 	Content-Type: image/jpg
  // 	Content-Length: 6229
  // 	Server: AmazonS3

  // Get the ETag response header value:
  final etag = rest.responseHdrByName('ETag');
  print('ETag: $etag');
}