text: // kv_recovery_unit.h
  
  /**
  *    Copyright (C) 2014 MongoDB 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.
  */
  
  #include "mongo/db/record_id.h"
  #include "mongo/db/storage/recovery_unit.h"
  
  namespace mongo {
  
      /**
       * The KVDictionary layer takes care of capped id management (visible_id_tracker.h), which
       * requires a few hooks in to the RecoveryUnit.  You only need to inherit from this if your
       * engine supports doc locking (if it doesn't, hasSnapshot is sort of nonsensical).
       */
      class KVRecoveryUnit : public RecoveryUnit {
  
          /**
           * The lowest invisible RecordId for this transaction.  Used by multiple RecordIterators
           * once one is established.
           */
          RecordId _lowestInvisible;
  
      public:
          ~KVRecoveryUnit() {}
  
          /**
           * Check whether the recovery unit has a snapshot.  Used by the capped iterator to check
           * whether to set the lowest invisible id.
           */
          virtual bool hasSnapshot() const = 0;
  
          void setLowestInvisible(const RecordId& id) {
              _lowestInvisible = id;
          }
  
          const RecordId &getLowestInvisible() const {
              return _lowestInvisible;
          }
  
          /**
           * A KVRecoveryUnit may be asked to create a new RecoveryUnit
           * suitable for its engine (for capped deletes, see
           * KVRecordStoreCapped::deleteAsNeeded).
           */
          virtual KVRecoveryUnit *newRecoveryUnit() const = 0;
      };
  
  } // namespace mongo
  