class KeyValueEntityStorage

class KeyValueEntityStorage extends EntityStorageBase {
    
    /**
     * Length limit of the entity ID.
     */
    const MAX_ID_LENGTH = 128;
    
    /**
     * The key value store.
     *
     * @var \Drupal\Core\KeyValueStore\KeyValueStoreInterface
     */
    protected $keyValueStore;
    
    /**
     * The UUID service.
     *
     * @var \Drupal\Component\Uuid\UuidInterface
     */
    protected $uuidService;
    
    /**
     * The language manager.
     *
     * @var \Drupal\Core\Language\LanguageManagerInterface
     */
    protected $languageManager;
    
    /**
     * Constructs a new KeyValueEntityStorage.
     *
     * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
     *   The entity type.
     * @param \Drupal\Core\KeyValueStore\KeyValueStoreInterface $key_value_store
     *   The key value store.
     * @param \Drupal\Component\Uuid\UuidInterface $uuid_service
     *   The UUID service.
     * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
     *   The language manager.
     * @param \Drupal\Core\Cache\MemoryCache\MemoryCacheInterface $memory_cache
     *   The memory cache.
     */
    public function __construct(EntityTypeInterface $entity_type, KeyValueStoreInterface $key_value_store, UuidInterface $uuid_service, LanguageManagerInterface $language_manager, MemoryCacheInterface $memory_cache) {
        parent::__construct($entity_type, $memory_cache);
        $this->keyValueStore = $key_value_store;
        $this->uuidService = $uuid_service;
        $this->languageManager = $language_manager;
        // Check if the entity type supports UUIDs.
        $this->uuidKey = $this->entityType
            ->getKey('uuid');
    }
    
    /**
     * {@inheritdoc}
     */
    public static function createInstance(ContainerInterface $container, EntityTypeInterface $entity_type) {
        return new static($entity_type, $container->get('keyvalue')
            ->get('entity_storage__' . $entity_type->id()), $container->get('uuid'), $container->get('language_manager'), $container->get('entity.memory_cache'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function doCreate(array $values = []) {
        // Set default language to site default if not provided.
        $values += [
            $this->getEntityType()
                ->getKey('langcode') => $this->languageManager
                ->getDefaultLanguage()
                ->getId(),
        ];
        $entity_class = $this->getEntityClass();
        $entity = new $entity_class($values, $this->entityTypeId);
        // @todo This is handled by ContentEntityStorageBase, which assumes
        //   FieldableEntityInterface. The current approach in
        //   https://www.drupal.org/node/1867228 improves this but does not solve it
        //   completely.
        if ($entity instanceof FieldableEntityInterface) {
            foreach ($entity as $name => $field) {
                if (isset($values[$name])) {
                    $entity->{$name} = $values[$name];
                }
                elseif (!array_key_exists($name, $values)) {
                    $entity->get($name)
                        ->applyDefaultValue();
                }
                unset($values[$name]);
            }
        }
        return $entity;
    }
    
    /**
     * {@inheritdoc}
     */
    public function doLoadMultiple(array $ids = NULL) {
        if (empty($ids)) {
            $entities = $this->keyValueStore
                ->getAll();
        }
        else {
            $entities = $this->keyValueStore
                ->getMultiple($ids);
        }
        return $this->mapFromStorageRecords($entities);
    }
    
    /**
     * {@inheritdoc}
     */
    public function loadRevision($revision_id) {
        return NULL;
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteRevision($revision_id) {
        return NULL;
    }
    
    /**
     * {@inheritdoc}
     */
    public function doDelete($entities) {
        $entity_ids = array_keys($entities);
        $this->keyValueStore
            ->deleteMultiple($entity_ids);
    }
    
    /**
     * {@inheritdoc}
     */
    public function save(EntityInterface $entity) {
        $id = $entity->id();
        if ($id === NULL || $id === '') {
            throw new EntityMalformedException('The entity does not have an ID.');
        }
        // Check the entity ID length.
        // @todo This is not config-specific, but serial IDs will likely never hit
        //   this limit. Consider renaming the exception class.
        if (strlen($entity->id()) > static::MAX_ID_LENGTH) {
            throw new ConfigEntityIdLengthException("Entity ID {$entity->id()} exceeds maximum allowed length of " . static::MAX_ID_LENGTH . ' characters.');
        }
        return parent::save($entity);
    }
    
    /**
     * {@inheritdoc}
     */
    protected function doSave($id, EntityInterface $entity) {
        $is_new = $entity->isNew();
        // Save the entity data in the key value store.
        $this->keyValueStore
            ->set($entity->id(), $entity->toArray());
        // If this is a rename, delete the original entity.
        if ($this->has($id, $entity) && $id !== $entity->id()) {
            $this->keyValueStore
                ->delete($id);
        }
        return $is_new ? SAVED_NEW : SAVED_UPDATED;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function has($id, EntityInterface $entity) {
        return $this->keyValueStore
            ->has($id);
    }
    
    /**
     * {@inheritdoc}
     */
    public function hasData() {
        return (bool) $this->keyValueStore
            ->getAll();
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getQueryServiceName() {
        return 'entity.query.keyvalue';
    }

}