/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.hadoop.fs.s3a;

import com.amazonaws.auth.InstanceProfileCredentialsProvider;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;

/**
 * A subclass of {@link InstanceProfileCredentialsProvider} that enforces
 * instantiation of only a single instance.
 * This credential provider calls the EC2 instance metadata service to obtain
 * credentials.  For highly multi-threaded applications, it's possible that
 * multiple instances call the service simultaneously and overwhelm it with
 * load.  The service handles this by throttling the client with an HTTP 429
 * response or forcibly terminating the connection.  Forcing use of a single
 * instance reduces load on the metadata service by allowing all threads to
 * share the credentials.  The base class is thread-safe, and there is nothing
 * that varies in the credentials across different instances of
 * {@link S3AFileSystem} connecting to different buckets, so sharing a singleton
 * instance is safe.
 *
 * As of AWS SDK 1.11.39, the SDK code internally enforces a singleton.  After
 * Hadoop upgrades to that version or higher, it's likely that we can remove
 * this class.
 */
@InterfaceAudience.Private
@InterfaceStability.Stable
public final class SharedInstanceProfileCredentialsProvider
    extends InstanceProfileCredentialsProvider {

  private static final SharedInstanceProfileCredentialsProvider INSTANCE =
      new SharedInstanceProfileCredentialsProvider();

  /**
   * Returns the singleton instance.
   *
   * @return singleton instance
   */
  public static SharedInstanceProfileCredentialsProvider getInstance() {
    return INSTANCE;
  }

  /**
   * Default constructor, defined explicitly as private to enforce singleton.
   */
  private SharedInstanceProfileCredentialsProvider() {
    super();
  }
}