text: ////////////////////////////////////////////////////////////////////////////////
  /// DISCLAIMER
  ///
  /// Copyright 2014-2021 ArangoDB GmbH, Cologne, Germany
  /// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
  ///
  /// 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.
  ///
  /// Copyright holder is ArangoDB GmbH, Cologne, Germany
  ///
  /// @author Dr. Frank Celler
  ////////////////////////////////////////////////////////////////////////////////
  
  #pragma once
  
  #include "ApplicationFeatures/ApplicationFeature.h"
  #include "ProgramOptions/ProgramOptions.h"
  
  namespace arangodb {
  
  class StorageEngine;
  
  class EngineSelectorFeature final : public application_features::ApplicationFeature {
   public:
    explicit EngineSelectorFeature(application_features::ApplicationServer& server);
  
    void collectOptions(std::shared_ptr<options::ProgramOptions>) override final;
    void prepare() override final;
    void start() override final;
    void unprepare() override final;
  
    // return the names of all available storage engines
    static std::unordered_set<std::string> availableEngineNames();
  
    // whether the engine has been selected yet
    bool selected() const { return _selected.load(); }
  
    StorageEngine& engine();
  
    template <typename As, typename std::enable_if<std::is_base_of<StorageEngine, As>::value, int>::type = 0>
    As& engine();
  
    std::string const& engineName();
  
    static std::string const& defaultEngine();
  
    /// @brief note that this will return true for the ClusterEngine too, in case
    /// the underlying engine is the RocksDB engine.
    bool isRocksDB();
  
  #ifdef ARANGODB_USE_GOOGLE_TESTS
    void setEngineTesting(StorageEngine*);
  #endif
  
   private:
    StorageEngine* _engine;
    std::string _engineName;
    std::string _engineFilePath;
    std::atomic<bool> _selected;
    bool _allowDeprecatedDeployments;
  };
  
  }  // namespace arangodb
  
  