[libvirt] Questions about the schemas

[Gene Czarcinski]

On 02/18/2013 09:12 AM, Daniel P. Berrange wrote:
> On Mon, Feb 18, 2013 at 08:54:35AM -0500, Gene Czarcinski wrote:
>> This is not in any way critical but simply trying to get a better
>> idea of the purposes of docs/schemas/*
>>
>> I recently submitted a patch which needed me to look at and modify
>> some of the schema files [adding a client-id specification to
>> dhcp/host].  As I looked them over I wondered what the purpose was
>> of the schemas.  If you ran virt-xml-validate (which uses xmllint)
>> and the result is that your file is not validated, then there is a
>> problem but it is not clear just what the problem is.  On the other
>> hand, if you run virt-xml-validate and it says the your file is OK
>> (validated), that only means that it did not find a problem but not
>> that the file is OK.
>>
>> For example, the schema says it is valid to specify an IPv6 address
>> in an IPv4 definition.  The implementing software if kick it out but
>> the schema says it is OK.
>>
>> Another example is the (host) name under the dhcp/host definition.
>> The current schema says this is required but dnsmasq does not
>> require a host-name specification [I submitted a trivial patch to
>> make it optional].
>>
>> So, if the purpose is to kick out obvious errors, this it is OK as
>> is.  If the purpose to to be definitiive, then more work needs to be
>> done.
>>
>> My opinion is that the current situation is "good enough" as long as
>> valid elements/specifications are not treated as errors.
> The schemas are intended to provide a simple "syntax check" for XML
> documents that an app has created. eg detect typos in element/attribute
> names, or use of elements which don't use or incorrect nesting of
> elements. Even for syntax checks though, the schemas a not going to
> provide 100% coverage - todo so would make the schemas far more complicated
> and it is just not worth the effort. The XML parser is the ultimate
> place where 100% complete syntax checks are performed.
>
> What they generally do not do, is provide any kind of "semantic check"
> about whether the configuration actually makes sense. For example, the
> schema won't tell you that requesting virtio disks with a vmware guest
> is semantically invalid. This is left to the virt driver.
>
>
Thank you.  This is what I thought was the intent.

BTW, coming up with the schema specifications for the duid hex-character 
strings was "interesting."

Gene