/*
 * Copyright (c) 2013 Google Inc.
 *
 * 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.google.cloud.backend.pushnotification;

import com.google.appengine.api.ThreadManager;
import com.google.appengine.api.datastore.DatastoreService;
import com.google.appengine.api.datastore.DatastoreServiceFactory;
import com.google.appengine.api.datastore.Entity;
import com.google.appengine.api.datastore.FetchOptions;
import com.google.appengine.api.datastore.Key;
import com.google.appengine.api.datastore.PreparedQuery;
import com.google.appengine.api.datastore.Query;
import com.google.appengine.api.datastore.Query.FilterOperator;
import com.google.appengine.api.datastore.Query.FilterPredicate;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.List;
import java.util.TimeZone;
import java.util.logging.Level;
import java.util.logging.Logger;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * HttpServlet for removing no longer needed records of processed notifications.
 *
 * It is intended to be called by Push Task Queue or Cron requests that target a backend 
 * as the time needed to delete all obsolete records just once a day may exceed the HTTP time limit
 * for the front end instances.
 */
public class NotificationCleanupServlet extends HttpServlet {
  private static final long serialVersionUID = 1L;
  private static final Logger log =
      Logger.getLogger(NotificationCleanupServlet.class.getCanonicalName());
  private static final int HOURS_TO_KEEP_RECORDS_OF_PROCESSED_NOTIFICATIONS = 12;

  @Override
  public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException {

    Thread thread = ThreadManager.createBackgroundThread(new Runnable() {
      @Override
      public void run() {
        doCleanup();
      }
    });

    thread.start();
  }

  private void doCleanup() {
    log.log(Level.INFO, "Starting a job to clean up processed notification records");

    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();

    Calendar cutoffTime = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
    cutoffTime.add(Calendar.HOUR, -HOURS_TO_KEEP_RECORDS_OF_PROCESSED_NOTIFICATIONS);

    Query query = new Query(Worker.PROCESSED_NOTIFICATION_TASKS_ENTITY_KIND)
      .setFilter(new FilterPredicate("processedAt", FilterOperator.LESS_THAN, cutoffTime.getTime()))
      .setKeysOnly();

    PreparedQuery preparedQuery = datastore.prepare(query);

    // Delete in batches
    List<Entity> entitiesToBeDeleted = null;
    do {
      entitiesToBeDeleted = preparedQuery.asList(FetchOptions.Builder.withLimit(5));

      List<Key> keys = new ArrayList<Key>();

      for (Entity entity : entitiesToBeDeleted) {
        keys.add(entity.getKey());
      }

      datastore.delete(keys);
    } while (entitiesToBeDeleted.size() > 0);

    log.log(Level.INFO, "Finished a job to clean up processed notification records");
  }
}