[libvirt] An introduction to the "libvirt-designer" library

People might have noticed Michal's patches[1] for the "libvirt-designer" and wondered wtf.com that is. This is an attempt to explain it... If you are familiar with the code in virt-install (also used by virt-manager) you'll know it has a bunch of internal classes / APIs for dealing with libvirt configuration, particularly wrt guest domains. If you're also familiar with other apps like GNOME Boxes, VDSM/oVirt, OpenStack, then you'll know that these apps have roughly the same needs as virt-install and virt-manager wrt libvirt configuration. The virt-install internal APIs though cannot practically be used by these apps since they were never really designed with broad re-use in mind. Obviously in such scenarios it is desirable to share code across apps instead of having them all reinvent the wheel. The libvirt-gconfig library was the first part in an attempt to address this problem by providing a formal API for creating & reading libvirt configurations without having to know anything about XML. This library is completely policy-free, restricting itself to direct/explicit manipulation of the configuration attributes. This deals with some of functionality found in the virt-install internal APIs. The libosinfo library [2] provides an API for discovering supportable and optimal hardware for operating systems & hypervisors. This replaces the OS configuration data tables found inside virt-install. The next step is to provide a policy-driven API for configurating virtual machines. When configuring a virtual machine, parameters can be roughly split into two groups - guest hardware config and host interaction config. The idea behind the libvirt-designer API is that when creating or changing a guest config, application developers should only need to care about the host interaction config. The guest hardware config can be figured out automatically on their behalf using data obtained from the libosinfo library. As an example, consider adding a disk to a guest. Traditionally an app developer needs to - Choose what virtual hardware to use - Figure out guest device name - Set the host filename - Choose optimal I/O parameters (cache mode, driver type, etc, etc) With the libvirt-designer APIs, an app developer will need to - Set the host filename The rest of the points will be filled in automatically, though the app developer will still get the opportunity to further customize the defaults via the existing libvirt-gconfig APIs. As mentioned above the libvirt-designer library is binding together the libvirt-gconfig and libosinfo APIs. It explicitly does *not* directly depend on libvirt or libvirt-gobject. This is to try to allow the libvirt-designer library to be used by a wide variety of applications, no matter how they are interacting with libvirt. eg if they use the CIM, SNMP or QMF bindings to libvirt they can still use libvirt-designer to build their XML configurations. The intent is that there is yet another library, so far called libvirt-builder, which binds together the libvirt-designer and libvirt-gobject APIs, to actually automate some of the lifecycle management and VM provisioning tasks. Since it will use libvirt-gobject, this final library would not be accessible to apps using CIM/SNMP/QMF bindings. With this library, we would have more or less complete coverage of all the important tasks found in the virt-install internal APIs, which can be reused by other apps. There is of course still quite a long way to go before we get there, since we're aiming to build this all up incrementally from the bottom up. The hope is that in the near term, GNOME Boxes will be able to start taking advantage of some of the code provided by libvirt-designer, since it already uses libvirt-gconfig & libosinfo. In addition as more functionality appears we'll be able to start changing virt-install to switch it over to these APIs in favour of its internal code. Regards, Daniel [1] https://www.redhat.com/archives/libvir-list/2012-September/msg00214.html [2] https://fedorahosted.org/libosinfo -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|