Cinder Pagination Sorting Enhancements

Currently, the pagination support for Cinder allows the caller to specify a single sort key and sort direction. This blueprint enhances the pagination support for the /volumes and /volumes/detail APIs so that multiple sort keys and sort directions can be supplied on the request.

The database layer already supports multiple sort keys and directions. This blueprint will update the API layer to retrieve the sort information from the API request and pass that information down to the database layer.