/**
 * 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.hbase.client;

import org.apache.hadoop.hbase.classification.InterfaceAudience;
import org.apache.hadoop.hbase.client.Result;
import org.apache.hadoop.hbase.client.metrics.ScanMetrics;

/**
 * Receives {@link Result} for an asynchronous scan.
 * <p>
 * Notice that, the {@link #onNext(Result[], ScanController)} method will be called in the thread
 * which we send request to HBase service. So if you want the asynchronous scanner fetch data from
 * HBase in background while you process the returned data, you need to move the processing work to
 * another thread to make the {@code onNext} call return immediately. And please do NOT do any time
 * consuming tasks in all methods below unless you know what you are doing.
 */
@InterfaceAudience.Public
public interface RawScanResultConsumer {

  /**
   * Used to resume a scan.
   */
  @InterfaceAudience.Public
  interface ScanResumer {

    /**
     * Resume the scan. You are free to call it multiple time but only the first call will take
     * effect.
     */
    void resume();
  }

  /**
   * Used to suspend or stop a scan.
   * <p>
   * Notice that, you should only call the methods below inside onNext or onHeartbeat method. A
   * IllegalStateException will be thrown if you call them at other places.
   * <p>
   * You can only call one of the methods below, i.e., call suspend or terminate(of course you are
   * free to not call them both), and the methods are not reentrant. A IllegalStateException will be
   * thrown if you have already called one of the methods.
   */
  @InterfaceAudience.Public
  interface ScanController {

    /**
     * Suspend the scan.
     * <p>
     * This means we will stop fetching data in background, i.e., will not call onNext any more
     * before you resume the scan.
     * @return A resumer used to resume the scan later.
     */
    ScanResumer suspend();

    /**
     * Terminate the scan.
     * <p>
     * This is useful when you have got enough results and want to stop the scan in onNext method,
     * or you want to stop the scan in onHeartbeat method because it has spent too many time.
     */
    void terminate();
  }

  /**
   * Indicate that we have receive some data.
   * @param results the data fetched from HBase service.
   * @param controller used to suspend or terminate the scan. Notice that the {@code controller}
   *          instance is only valid within scope of onNext method. You can only call its method in
   *          onNext, do NOT store it and call it later outside onNext.
   */
  void onNext(Result[] results, ScanController controller);

  /**
   * Indicate that there is an heartbeat message but we have not cumulated enough cells to call
   * onNext.
   * <p>
   * This method give you a chance to terminate a slow scan operation.
   * @param controller used to suspend or terminate the scan. Notice that the {@code controller}
   *          instance is only valid within the scope of onHeartbeat method. You can only call its
   *          method in onHeartbeat, do NOT store it and call it later outside onHeartbeat.
   */
  default void onHeartbeat(ScanController controller) {
  }

  /**
   * Indicate that we hit an unrecoverable error and the scan operation is terminated.
   * <p>
   * We will not call {@link #onComplete()} after calling {@link #onError(Throwable)}.
   */
  void onError(Throwable error);

  /**
   * Indicate that the scan operation is completed normally.
   */
  void onComplete();

  /**
   * If {@code scan.isScanMetricsEnabled()} returns true, then this method will be called prior to
   * all other methods in this interface to give you the {@link ScanMetrics} instance for this scan
   * operation. The {@link ScanMetrics} instance will be updated on-the-fly during the scan, you can
   * store it somewhere to get the metrics at any time if you want.
   */
  default void onScanMetricsCreated(ScanMetrics scanMetrics) {
  }
}