/*-
 * -\-\-
 * Helios Services
 * --
 * Copyright (C) 2016 Spotify AB
 * --
 * Licensed 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 com.spotify.helios.master.reaper;

import com.google.common.annotations.VisibleForTesting;
import com.spotify.helios.common.descriptors.Job;
import com.spotify.helios.common.descriptors.JobId;
import com.spotify.helios.master.MasterModel;
import com.spotify.helios.servicescommon.coordination.Paths;
import com.spotify.helios.servicescommon.coordination.ZooKeeperClient;
import java.util.Collections;
import java.util.List;
import java.util.Random;
import java.util.concurrent.TimeUnit;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.KeeperException.NoNodeException;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Removes job histories whose corresponding jobs don't exist anymore.
 * There are two race conditions where jobs can be deleted but their histories are
 * left behind in ZooKeeper:
 *
 * <p>1. The master deletes the job (in {@link com.spotify.helios.master.ZooKeeperMasterModel}
 * and then deletes its history. During this deletion, the agent creates a znode. The master's
 * deletion operations fail.
 *
 * <p>2. The master deletes all relevant history znodes successfully. The agent still hasn't
 * undeployed its job and continues writing history to ZooKeeper. This will recreate deleted history
 * znodes via {@link com.spotify.helios.agent.TaskHistoryWriter}.
 *
 * <p>Solve both of these cases by scheduling an instance of this class. It runs once a day once
 * scheduled.
 */
public class JobHistoryReaper extends RateLimitedService<String> {

  private static final double PERMITS_PER_SECOND = 0.2; // one permit every 5 seconds
  private static final int DELAY = 60 * 24; // 1 day in minutes
  private static final TimeUnit TIME_UNIT = TimeUnit.MINUTES;

  private static final Logger log = LoggerFactory.getLogger(JobHistoryReaper.class);

  private final MasterModel masterModel;
  private final ZooKeeperClient client;

  public JobHistoryReaper(final MasterModel masterModel,
                          final ZooKeeperClient client) {
    this(masterModel, client, PERMITS_PER_SECOND, new Random().nextInt(DELAY));
  }

  @VisibleForTesting
  JobHistoryReaper(final MasterModel masterModel,
                   final ZooKeeperClient client,
                   final double permitsPerSecond,
                   final int initialDelay) {
    super(permitsPerSecond, initialDelay, DELAY, TIME_UNIT);
    this.masterModel = masterModel;
    this.client = client;
  }

  @Override
  Iterable<String> collectItems() {
    final String path = Paths.historyJobs();
    List<String> jobIds = Collections.emptyList();

    try {
      jobIds = client.getChildren(path);
    } catch (KeeperException e) {
      log.warn("Failed to get children of znode {}", path, e);
    }

    return jobIds;
  }

  @Override
  void processItem(final String jobId) {
    log.info("Deciding whether to reap job history for job {}", jobId);
    final JobId id = JobId.fromString(jobId);
    final Job job = masterModel.getJob(id);
    if (job == null) {
      try {
        client.deleteRecursive(Paths.historyJob(id));
        log.info("Reaped job history for job {}", jobId);
      } catch (NoNodeException ignored) {
        // Something deleted the history right before we got to it. Ignore and keep going.
      } catch (KeeperException e) {
        log.warn("error reaping job history for job {}", jobId, e);
      }
    }
  }
}