text: /*
   * Copyright 2017-present Facebook, 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.
   */
  #pragma once
  
  #include <folly/Optional.h>
  #include <string>
  
  namespace wangle {
  
  /**
   * Interface for a persistent cache that backs up the cache on
   * storage so that it can be reused. This desribes just the key
   * operations common to any cache. Loading from and syncing to
   * the storage is in the actual implementation of the class and
   * the clients should not have to worry about it.
   */
  template <typename K, typename V>
  class PersistentCache {
    public:
      virtual ~PersistentCache() {}
  
      /**
       * Get a value corresponding to a key
       * @param key string, the key to lookup
       *
       * @returns value associated with key
       */
      virtual folly::Optional<V> get(const K& key) = 0;
  
      /**
       * Set a value corresponding to a key
       * @param key string, the key to set
       * @param val string, the value to set
       *
       * overwrites value if key has a value associated in the cache
       */
      virtual void put(const K& key, const V& val) = 0;
  
      /**
       * Clear a cache entry associated with a key
       * @param key string, the key to lookup and clear
       *
       * @return boolean true if any elements are removed, else false
       */
      virtual bool remove(const K& key) = 0;
  
      /**
       * Empty the contents of the cache
       */
      virtual void clear(bool clearPersistence = false) = 0;
  
      /**
       * return the size of the cache
       *
       * @returns size_t, the size of the cache
       */
  
      virtual size_t size() = 0;
  };
  
  }
  