# Resource Primary Keys¶

This may be a GUID assigned to the resource, a user ID assigned in some database, or a MAC address of a network interface; whatever makes sense for that particular resource.

Primary keys may be composite — consists of a number of fields taken together. For example, the primary key for a particular disk device may be the “host ID” of the host the disk is attached to, and the “disk index” within that host, such as host-3, /dev/sdc