Introducing the libvirt-glib, a mapping of the libvirt API and XML to GLib/GObject
The historical philosophy of libvirt is for all our core libraries to be written in C and then create bindings to other programming languages or mappings to alternative object models. Thus far we have bindings to Python, Perl, Ruby, OCaml, Php, C#, Java and mappings to the QMF (Matahari), CIM and SNMP object models. The virt-install and virt-manager applications use the python binding to libvirt, but even very early in development of virt-manager it was clear that the libvirt python API is not a natural fit for an application using GTK, since it does not integrate with GObject and in particular GObject signals. Thus virt-manager wraps the libvirt python objects to create real GObjects it then works with. This has been quite successful, but because all the virt-manager code is in python other applications have not been able to take advantage of the higher level libvirt API virt-manager has evolved. In addition the virt-install code (which is called internally by virt-manager) contains a set of Python objects which represent the various libvirt XML schemas as plain old objects with properties and setters/getters. If you’ve developed applications against libvirt, you’ll likely appreciate just how useful such an API would be. Again though, because the API is in Python and (technically) internal to the virt-install codebase, it is not accessible to many other applications
There was clearly space for an independent library mapping the libvirt API and XML schemas to GObject, which could then be used by any application. The task of creating a libvirt GObject library API is large enough, without considering the task of also ensuring it is accessible from all the non-C programming languages. Fortunately, with the release of GNOME-3, GObject introspection has now matured to the point where it can really be used in anger for real application development. The upshot is that it is now feasible to attempt development of a proper libvirt GObject API.
The libvirt-glib package is the result, and it actually contains three related libraries
- libvirt-gib – non-object based glue code between GLib and libvirt. In particular this has APIs to convert libvirt virErrorPtr instances into GError instances, and provides an implementation of the libvirt event loop contract, using the GLib GMain APIs.
- libvirt-gconfig – object based APIs which map libvirt XML documents/schemas into GObject classes. This library explicitly has no direct link to the libvirt API, solely concerning itself with XML management. This is to allow use of libvirt-gconfig from applications which are using one of the object mappings like QMF/CIM/SNMP, instead of the direct libvirt API. This where the current virt-install XML handling objects will be replicated
- libvirt-gobject – object based APIs which map libvirt types and APIs into GObject classes. This library depends on libirt-glib and libvirt-gconfig, and is where the current virt-manager object mapping APIs will be replicated. This library is also adopting the GIO paradigm for allowing asynchronous API invocation & completion, for long running applications. This eliminates much of the need for applications to explicitly use threads (thread usage is hidden behind the async API impl).
From day 1, all the APIs are being developed with GObject introspection in mind., so all methods are fully annotated, and we are generating the glue layer for Vala bindings as standard in order to support the GNOME Boxes application. It is still very early days for development and very little of the libvirt API has been mapped into GObject thus far and work is only just starting on the XML object mappings. The overall target, however, is to develop the library to the state where it can support the aforementioned GNOME Boxes application in Fedora 17, as well as an application sandbox framework I am developing for Fedora 17 (more on that in a later blog post).
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