preserve time order
	 * and minimise the possibility of collisions between multiple processes working
	 * at the same time.
	 *
	 * @return string
	 */
	protected function generate_option_name_timestamp() {
		return sprintf( '%.6f', microtime( true ) );
	}

	/**
	 * Gets the checkout ID.
	 *
	 * @return bool|string
	 */
	private function get_checkout_id() {
		global $wpdb;
		$checkout_value = $wpdb->get_var(
			$wpdb->prepare(
				"SELECT option_value FROM $wpdb->options WHERE option_name = %s",
				$this->get_lock_option_name()
			)
		);

		if ( $checkout_value ) {
			list( $checkout_id, $timestamp ) = explode( ':', $checkout_value );
			if ( (int) $timestamp > time() ) {
				return $checkout_id;
			}
		}

		return false;
	}

	/**
	 * Sets the checkout id.
	 *
	 * @param string $checkout_id The ID of the checkout.
	 *
	 * @return bool|int
	 */
	private function set_checkout_id( $checkout_id ) {
		global $wpdb;

		$expires     = time() + Defaults::$default_sync_queue_lock_timeout;
		$updated_num = $wpdb->query(
			$wpdb->prepare(
				"UPDATE $wpdb->options SET option_value = %s WHERE option_name = %s",
				"$checkout_id:$expires",
				$this->get_lock_option_name()
			)
		);

		if ( ! $updated_num ) {
			$updated_num = $wpdb->query(
				$wpdb->prepare(
					"INSERT INTO $wpdb->options ( option_name, option_value, autoload ) VALUES ( %s, %s, 'no' )",
					$this->get_lock_option_name(),
					"$checkout_id:$expires"
				)
			);
		}

		return $updated_num;
	}

	/**
	 * Deletes the checkout ID.
	 *
	 * @return bool|int
	 */
	private function delete_checkout_id() {
		global $wpdb;
		// Rather than delete, which causes fragmentation, we update in place.
		return $wpdb->query(
			$wpdb->prepare(
				"UPDATE $wpdb->options SET option_value = %s WHERE option_name = %s",
				'0:0',
				$this->get_lock_option_name()
			)
		);

	}

	/**
	 * Return the lock option name.
	 *
	 * @return string
	 */
	private function get_lock_option_name() {
		return "jpsq_{$this->id}_checkout";
	}

	/**
	 * Return the next data row option name.
	 *
	 * @return string
	 */
	private function get_next_data_row_option_name() {
		$timestamp = $this->generate_option_name_timestamp();

		// Row iterator is used to avoid collisions where we're writing data waaay fast in a single process.
		if ( PHP_INT_MAX === $this->row_iterator ) {
			$this->row_iterator = 0;
		} else {
			$this->row_iterator += 1;
		}

		return 'jpsq_' . $this->id . '-' . $timestamp . '-' . $this->random_int . '-' . $this->row_iterator;
	}

	/**
	 * Return the items in the queue.
	 *
	 * @param null|int $limit Limit to the number of items we fetch at once.
	 *
	 * @return array|object|null
	 */
	private function fetch_items( $limit = null ) {
		global $wpdb;

		if ( $limit ) {
			$items = $wpdb->get_results(
				$wpdb->prepare(
					"SELECT option_name AS id, option_value AS value FROM $wpdb->options WHERE option_name LIKE %s ORDER BY option_name ASC LIMIT %d",
					"jpsq_{$this->id}-%",
					$limit
				),
				OBJECT
			);
		} else {
			$items = $wpdb->get_results(
				$wpdb->prepare(
					"SELECT option_name AS id, option_value AS value FROM $wpdb->options WHERE option_name LIKE %s ORDER BY option_name ASC",
					"jpsq_{$this->id}-%"
				),
				OBJECT
			);
		}

		return $this->unserialize_values( $items );

	}

	/**
	 * Return items with specific ids.
	 *
	 * @param array $items_ids Array of event ids.
	 *
	 * @return array|object|null
	 */
	private function fetch_items_by_id( $items_ids ) {
		global $wpdb;

		// return early if $items_ids is empty or not an array.
		if ( empty( $items_ids ) || ! is_array( $items_ids ) ) {
			return null;
		}

		$ids_placeholders        = implode( ', ', array_fill( 0, count( $items_ids ), '%s' ) );
		$query_with_placeholders = "SELECT option_name AS id, option_value AS value
				FROM $wpdb->options
				WHERE option_name IN ( $ids_placeholders )";
		$items                   = $wpdb->get_results(
			$wpdb->prepare(
				$query_with_placeholders, // phpcs:ignore WordPress.DB.PreparedSQL.NotPrepared
				$items_ids
			),
			OBJECT
		);

		return $this->unserialize_values( $items );
	}

	/**
	 * Unserialize item values.
	 *
	 * @param array $items Events from the Queue to be serialized.
	 *
	 * @return mixed
	 */
	private function unserialize_values( $items ) {
		array_walk(
			$items,
			function ( $item ) {
				$item->value = maybe_unserialize( $item->value );
			}
		);

		return $items;

	}

	/**
	 * Return true if the buffer is still valid or an Error other wise.
	 *
	 * @param Automattic\Jetpack\Sync\Queue_Buffer $buffer The Queue_Buffer.
	 *
	 * @return bool|\WP_Error
	 */
	private function validate_checkout( $buffer ) {
		if ( ! $buffer instanceof Queue_Buffer ) {
			return new \WP_Error( 'not_a_buffer', 'You must checkin an instance of Automattic\\Jetpack\\Sync\\Queue_Buffer' );
		}

		$checkout_id = $this->get_checkout_id();

		if ( ! $checkout_id ) {
			return new \WP_Error( 'buffer_not_checked_out', 'There are no checked out buffers' );
		}

		// TODO: change to strict comparison.
		if ( $checkout_id != $buffer->id ) { // phpcs:ignore WordPress.PHP.StrictComparisons.LooseComparison
			return new \WP_Error( 'buffer_mismatch', 'The buffer you checked in was not checked out' );
		}

		return true;
	}
}