/**
 * Copyright (C) 2014-2016 LinkedIn Corp. (pinot-core@linkedin.com)
 *
 * 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.linkedin.pinot.core.query.scheduler;

import com.google.common.base.Preconditions;
import com.linkedin.pinot.common.query.QueryExecutor;
import java.lang.reflect.Constructor;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import org.apache.commons.configuration.Configuration;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;


public class QuerySchedulerFactory {
  private static final String FCFS_ALGORITHM = "fcfs";
  private static final String DEFAULT_QUERY_SCHEDULER_ALGORITHM = FCFS_ALGORITHM;
  private static final String ALGORITHM_NAME_CONFIG_KEY = "name";
  private static Logger LOGGER = LoggerFactory.getLogger(QuerySchedulerFactory.class);

  /**
   * Static factory to instantiate query scheduler based on scheduler configuration.
   * 'name' configuration in the scheduler will decide which scheduler instance to create
   * Besides known instances, 'name' can be a classname
   * @param schedulerConfig scheduler specific configuration
   * @param queryExecutor QueryExecutor to use
   * @return
   */
  public static @Nonnull  QueryScheduler create(@Nonnull Configuration schedulerConfig,
      @Nonnull QueryExecutor queryExecutor) {
    Preconditions.checkNotNull(schedulerConfig);
    Preconditions.checkNotNull(queryExecutor);

    String schedulerName = schedulerConfig.getString(ALGORITHM_NAME_CONFIG_KEY,
        DEFAULT_QUERY_SCHEDULER_ALGORITHM).toLowerCase();

    if (schedulerName.equals(FCFS_ALGORITHM)) {
      LOGGER.info("Using FCFS query scheduler");
      return new FCFSQueryScheduler(schedulerConfig, queryExecutor);
    }

    // didn't find by name so try by classname
    QueryScheduler scheduler = getQuerySchedulerByClassName(schedulerName, schedulerConfig, queryExecutor);
    if (scheduler != null) {
      return scheduler;
    }

    // if we don't find the configured algorithm we warn and use the default one
    // because it's better to execute with poor algorithm than completely fail.
    // Failure on bad configuration will cause outage vs an inferior algorithm that
    // will provide degraded service

    LOGGER.warn("Scheduler {} not found. Using default FCFS query scheduler", schedulerName);
    return new FCFSQueryScheduler(schedulerConfig, queryExecutor);
  }

  private static @Nullable QueryScheduler getQuerySchedulerByClassName(String className, Configuration schedulerConfig,
      QueryExecutor queryExecutor) {
    try {
      Constructor<?> constructor =
          Class.forName(className).getDeclaredConstructor(Configuration.class, QueryExecutor.class);
      constructor.setAccessible(true);
      return (QueryScheduler) constructor.newInstance(new Object[]{schedulerConfig, queryExecutor});
    } catch (Exception e) {
      LOGGER.error("Failed to instantiate scheduler class by name: {}", className, e);
      return null;
    }
  }
}