text: //@file balance.h
  
  /**
  *    Copyright (C) 2008 10gen Inc.
  *
  *    This program is free software: you can redistribute it and/or  modify
  *    it under the terms of the GNU Affero General Public License, version 3,
  *    as published by the Free Software Foundation.
  *
  *    This program is distributed in the hope that it will be useful,
  *    but WITHOUT ANY WARRANTY; without even the implied warranty of
  *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  *    GNU Affero General Public License for more details.
  *
  *    You should have received a copy of the GNU Affero General Public License
  *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  *    As a special exception, the copyright holders give permission to link the
  *    code of portions of this program with the OpenSSL library under certain
  *    conditions as described in each individual source file and distribute
  *    linked combinations including the program with the OpenSSL library. You
  *    must comply with the GNU Affero General Public License in all respects
  *    for all of the code used other than as permitted herein. If you modify
  *    file(s) with this exception, you may extend this exception to your
  *    version of the file(s), but you are not obligated to do so. If you do not
  *    wish to do so, delete this exception statement from your version. If you
  *    delete this exception statement from all source files in the program,
  *    then also delete it in the license file.
  */
  
  #pragma once
  
  #include "mongol/s/catalog/forwarding_catalog_manager.h"
  #include "mongol/util/background.h"
  
  namespace mongol {
  
  class BalancerPolicy;
  struct MigrateInfo;
  class OperationContext;
  struct WriteConcernOptions;
  
  /**
   * The balancer is a background task that tries to keep the number of chunks across all
   * servers of the cluster even. Although every mongols will have one balancer running, only one
   * of them will be active at the any given point in time. The balancer uses a distributed lock
   * for that coordination.
   *
   * The balancer does act continuously but in "rounds". At a given round, it would decide if
   * there is an imbalance by checking the difference in chunks between the most and least
   * loaded shards. It would issue a request for a chunk migration per round, if it found so.
   */
  class Balancer : public BackgroundJob {
  public:
      Balancer();
      virtual ~Balancer();
  
      // BackgroundJob methods
  
      virtual void run();
  
      virtual std::string name() const {
          return "Balancer";
      }
  
  private:
      // hostname:port of my mongols
      std::string _myid;
  
      // time the Balancer started running
      time_t _started;
  
      // number of moved chunks in last round
      int _balancedLastTime;
  
      // decide which chunks to move; owned here.
      std::unique_ptr<BalancerPolicy> _policy;
  
      /**
       * Checks that the balancer can connect to all servers it needs to do its job.
       *
       * @return true if balancing can be started
       *
       * This method throws on a network exception
       */
      bool _init(OperationContext* txn);
  
      /**
       * Gathers all the necessary information about shards and chunks, and decides whether there are
       * candidate chunks to be moved.
       *
       * @param conn is the connection with the config server(s)
       * @param candidateChunks (IN/OUT) filled with candidate chunks, one per collection, that could
       *                          possibly be moved
       */
      void _doBalanceRound(OperationContext* txn,
                           ForwardingCatalogManager::ScopedDistLock* distLock,
                           std::vector<std::shared_ptr<MigrateInfo>>* candidateChunks);
  
      /**
       * Issues chunk migration request, one at a time.
       *
       * @param candidateChunks possible chunks to move
       * @param writeConcern detailed write concern. NULL means the default write concern.
       * @param waitForDelete wait for deletes to complete after each chunk move
       * @return number of chunks effectively moved
       */
      int _moveChunks(OperationContext* txn,
                      const std::vector<std::shared_ptr<MigrateInfo>>& candidateChunks,
                      const WriteConcernOptions* writeConcern,
                      bool waitForDelete);
  
      /**
       * Marks this balancer as being live on the config server(s).
       */
      void _ping(OperationContext* txn, bool waiting = false);
  
      /**
       * @return true if all the servers listed in configdb as being shards are reachable and are
       *         distinct processes
       */
      bool _checkOIDs(OperationContext* txn);
  };
  
  extern Balancer balancer;
  }
  