Zif was a package manager for Fedora written in C that was designed to replace yum.
When designing PackageKit, I coded a framework for a distro-specific backend to be coupled with a distro-agnostic service that listened for user requests to create a package manager daemon.
In doing so, I became very familiar with about a dozen (!) package systems, with all their specific nuances and peculiarities. It also became bewildering obvious to me that most of these package systems were not a good fit, as much of the package system functionality was not used and usually what PackageKit was using had to hide some pretty hideous hacks.
Zif is now a dead project. A lot of the Zif code lives on in the hawkey backend in PackageKit, but there will not be any more standalone releases of Zif.