class DatabaseStorage

class DatabaseStorage extends StorageBase {
    use DependencySerializationTrait;
    
    /**
     * The serialization class to use.
     *
     * @var \Drupal\Component\Serialization\SerializationInterface
     */
    protected $serializer;
    
    /**
     * The database connection.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $connection;
    
    /**
     * The name of the SQL table to use.
     *
     * @var string
     */
    protected $table;
    
    /**
     * Overrides Drupal\Core\KeyValueStore\StorageBase::__construct().
     *
     * @param string $collection
     *   The name of the collection holding key and value pairs.
     * @param \Drupal\Component\Serialization\SerializationInterface $serializer
     *   The serialization class to use.
     * @param \Drupal\Core\Database\Connection $connection
     *   The database connection to use.
     * @param string $table
     *   The name of the SQL table to use, defaults to key_value.
     */
    public function __construct($collection, SerializationInterface $serializer, Connection $connection, $table = 'key_value') {
        parent::__construct($collection);
        $this->serializer = $serializer;
        $this->connection = $connection;
        $this->table = $table;
    }
    
    /**
     * {@inheritdoc}
     */
    public function has($key) {
        try {
            return (bool) $this->connection
                ->query('SELECT 1 FROM {' . $this->connection
                ->escapeTable($this->table) . '} WHERE [collection] = :collection AND [name] = :key', [
                ':collection' => $this->collection,
                ':key' => $key,
            ])
                ->fetchField();
        } catch (\Exception $e) {
            $this->catchException($e);
            return FALSE;
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getMultiple(array $keys) {
        $values = [];
        try {
            $result = $this->connection
                ->query('SELECT [name], [value] FROM {' . $this->connection
                ->escapeTable($this->table) . '} WHERE [name] IN ( :keys[] ) AND [collection] = :collection', [
                ':keys[]' => $keys,
                ':collection' => $this->collection,
            ])
                ->fetchAllAssoc('name');
            foreach ($keys as $key) {
                if (isset($result[$key])) {
                    $values[$key] = $this->serializer
                        ->decode($result[$key]->value);
                }
            }
        } catch (\Exception $e) {
            // @todo: Perhaps if the database is never going to be available,
            // key/value requests should return FALSE in order to allow exception
            // handling to occur but for now, keep it an array, always.
        }
        return $values;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getAll() {
        try {
            $result = $this->connection
                ->query('SELECT [name], [value] FROM {' . $this->connection
                ->escapeTable($this->table) . '} WHERE [collection] = :collection', [
                ':collection' => $this->collection,
            ]);
        } catch (\Exception $e) {
            $this->catchException($e);
            $result = [];
        }
        $values = [];
        foreach ($result as $item) {
            if ($item) {
                $values[$item->name] = $this->serializer
                    ->decode($item->value);
            }
        }
        return $values;
    }
    
    /**
     * Saves a value for a given key.
     *
     * This will be called by set() within a try block.
     *
     * @param string $key
     *   The key of the data to store.
     * @param mixed $value
     *   The data to store.
     */
    protected function doSet($key, $value) {
        $this->connection
            ->merge($this->table)
            ->keys([
            'name' => $key,
            'collection' => $this->collection,
        ])
            ->fields([
            'value' => $this->serializer
                ->encode($value),
        ])
            ->execute();
    }
    
    /**
     * {@inheritdoc}
     */
    public function set($key, $value) {
        try {
            $this->doSet($key, $value);
        } catch (\Exception $e) {
            // If there was an exception, try to create the table.
            if ($this->ensureTableExists()) {
                $this->doSet($key, $value);
            }
            else {
                throw $e;
            }
        }
    }
    
    /**
     * Saves a value for a given key if it does not exist yet.
     *
     * This will be called by setIfNotExists() within a try block.
     *
     * @param string $key
     *   The key of the data to store.
     * @param mixed $value
     *   The data to store.
     *
     * @return bool
     *   TRUE if the data was set, FALSE if it already existed.
     */
    public function doSetIfNotExists($key, $value) {
        $result = $this->connection
            ->merge($this->table)
            ->insertFields([
            'collection' => $this->collection,
            'name' => $key,
            'value' => $this->serializer
                ->encode($value),
        ])
            ->condition('collection', $this->collection)
            ->condition('name', $key)
            ->execute();
        return $result == Merge::STATUS_INSERT;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setIfNotExists($key, $value) {
        try {
            return $this->doSetIfNotExists($key, $value);
        } catch (\Exception $e) {
            // If there was an exception, try to create the table.
            if ($this->ensureTableExists()) {
                return $this->doSetIfNotExists($key, $value);
            }
            else {
                throw $e;
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function rename($key, $new_key) {
        try {
            $this->connection
                ->update($this->table)
                ->fields([
                'name' => $new_key,
            ])
                ->condition('collection', $this->collection)
                ->condition('name', $key)
                ->execute();
        } catch (\Exception $e) {
            $this->catchException($e);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteMultiple(array $keys) {
        // Delete in chunks when a large array is passed.
        while ($keys) {
            try {
                $this->connection
                    ->delete($this->table)
                    ->condition('name', array_splice($keys, 0, 1000), 'IN')
                    ->condition('collection', $this->collection)
                    ->execute();
            } catch (\Exception $e) {
                $this->catchException($e);
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteAll() {
        try {
            $this->connection
                ->delete($this->table)
                ->condition('collection', $this->collection)
                ->execute();
        } catch (\Exception $e) {
            $this->catchException($e);
        }
    }
    
    /**
     * Check if the table exists and create it if not.
     *
     * @return bool
     *   TRUE if the table exists, FALSE if it does not exists.
     */
    protected function ensureTableExists() {
        try {
            $database_schema = $this->connection
                ->schema();
            $database_schema->createTable($this->table, $this->schemaDefinition());
        } catch (DatabaseException $e) {
        } catch (\Exception $e) {
            return FALSE;
        }
        return TRUE;
    }
    
    /**
     * Act on an exception when the table might not have been created.
     *
     * If the table does not yet exist, that's fine, but if the table exists and
     * yet the query failed, then the exception needs to propagate if it is not
     * a DatabaseException. Due to race conditions it is possible that another
     * request has created the table in the meantime. Therefore we can not rethrow
     * for any database exception.
     *
     * @param \Exception $e
     *   The exception.
     *
     * @throws \Exception
     */
    protected function catchException(\Exception $e) {
        if (!$e instanceof DatabaseException && $this->connection
            ->schema()
            ->tableExists($this->table)) {
            throw $e;
        }
    }
    
    /**
     * Defines the schema for the key_value table.
     */
    public static function schemaDefinition() {
        return [
            'description' => 'Generic key-value storage table. See the state system for an example.',
            'fields' => [
                'collection' => [
                    'description' => 'A named collection of key and value pairs.',
                    'type' => 'varchar_ascii',
                    'length' => 128,
                    'not null' => TRUE,
                    'default' => '',
                ],
                'name' => [
                    'description' => 'The key of the key-value pair. As KEY is a SQL reserved keyword, name was chosen instead.',
                    'type' => 'varchar_ascii',
                    'length' => 128,
                    'not null' => TRUE,
                    'default' => '',
                ],
                'value' => [
                    'description' => 'The value.',
                    'type' => 'blob',
                    'not null' => TRUE,
                    'size' => 'big',
                ],
            ],
            'primary key' => [
                'collection',
                'name',
            ],
        ];
    }

}