ANNOUNCE: New libvirt project Go XML parser model

Posted: January 5th, 2017 | Author: | Filed under: Coding Tips, Fedora, libvirt, OpenStack, Virt Tools | Tags: , , , | No Comments »

Shortly before christmas, I announced the availability of new Go bindings for the libvirt API. This post announces a companion package for dealing with XML parsing/formatting in Go. The master repository is available on the libvirt GIT server, but it is expected that Go projects will consume it via an import of the github mirror, since the Go ecosystem is heavilty github focused (e.g. godoc.org can’t produce docs for stuff hosted on libvirt.org git)

import (
  libvirtxml "github.com/libvirt/libvirt-go-xml"
  "encoding/xml"
)

domcfg := &libvirtxml.Domain{Type: "kvm", Name: "demo",
                             UUID: "8f99e332-06c4-463a-9099-330fb244e1b3",
                             ....}
xmldoc, err := xml.Marshal(domcfg)

API documentation is available on the godoc website.

When dealing with the libvirt API, most applications will find themselves needing to either parse or format XML documents describing configuration of various libvirt objects. Traditionally this task has been left upto the application to deal with and as a result most applications end up creating some kind of structure / object model to represent the XML document in a more easily accessible manner. To try to reduce this duplicate effort, libvirt has already created the libvirt-glib package, which contains a libvirt-gconfig library mapping libvirt XML documents into the GObject world. This library is accessible to many programming languages via the magic of GObject Introspection, and while there is some work to support this in Go, it is not particularly mature at this time.

In the Go world, there is a package “encoding/xml” which is able to transform between XML documents and Go structs, given suitable annotations on the struct fields. It is very easy to deal with, simply requiring someone to define a bit set of structs with annotated fields to map to the XML document. There’s no real “code” to write as it is really a data definition task.  Looking at applications using libvirt in Go, we see quite a few have already go down this route for dealing with libvirt XML. It should be readily apparent that every application using libvirt in Go is essentially going to end up writing an identical set of structs to deal with the XML handling. This duplication of effort makes no sense at all, and as such, we have started this new libvirt-go-xml package to provide a standard set of Go structs to deal with libvirt XML. The current level of schema support is pretty minimal supporting the capabilities XML, secrets XML and a small amount of the domain XML, so we’d encourage anyone interested in this to contribute patches to expand the XML schema coverage.

The following illustrates a further example of its usage in combination with the libvirt-go library (with error checking omitted for brevity):

import (
  libvirt "github.com/libvirt/libvirt-go"
  libvirtxml "github.com/libvirt/libvirt-go-xml"
  "encoding/xml"
  "fmt"
)

conn, err := libvirt.NewConnect("qemu:///system")
dom := conn.LookupDomainByName("demo")
xmldoc, err := dom.GetXMLDesc(0)

domcfg := &libvirtxml.Domain{}
err := xml.Unmarshal([]byte(xmldocC), domcfg)

fmt.Printf("Virt type %s", domcfg.Type)

 

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