What Yum Can Do

``Yum'' is really a set of python libraries with two primary command line user interface programs (yum and yum-arch) built to support installation and maintenance actions using RPMs provided in a repository. A repository can be any of the following:

• A website containing RPMs
• An FTP site containing RPMs
• A directory path (e.g. NFS mounted or local) containing RPMs.

(all of which can be thought of in generalized terms as Universal Resource Locator paths: URLs). A typical network-based repository might contain a mirror of a primary distribution and hence (in the case of Red Hat) directly support kickstart and network installation in addition to yum-based maintenance.

However, yum supports multiple repositories. Repositories can be added for many reasons:

• To extend the set of available RPMs. Two repositories can present the union of all the RPMs therein for installation and update purposes. It supports policy choices to use the order of the repositories in /etc/yum.conf to determine what to install or to always choose the latest revision.
• For robustness and redundancy. Yum supports a repository fallback scheme so if one repository intended to serve some purpose is unavailable, it automatically uses a fallback repository (perhaps a mirror of the first) again according to a controlled policy.
• As ordered layers. Yum can easily overlay, for example, a set of department-local RPMs on top of a larger set of organization-global RPMs). This permits proprietary packages to be made available only to selected clients for installation or maintenance purposes, or permits the overlay of RPMs containing private configuration information.
• Support different purposes in a maintenance and installation scheme. One department can be responsible for a repository that contains all the database rpms, another for all the graphical utilities, but both can be shared across the parent organization.

The ability to intelligently use multiple hierarchically ordered repositories to maintain client installations is one of yum's best features. Repository creation and maintenance is hence a major component of yum's value in a scalable administrative setting.