getPromise(identifier, etag) {
		return new Promise((resolve, reject) => {
			this._cache.get(`${this._name}|${identifier}`, etag, (err, data) => {
				if (err) {
					reject(err);
				} else {
					resolve(data);
				}
			});
		});
	}
@template T
@param {string} identifier the cache identifier
@param {Etag | null} etag the etag
@returns {Promise<T>} promise with the data