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Re: [Libvir] RFC: get/set properties



Daniel P. Berrange wrote:
On Tue, Apr 15, 2008 at 11:31:32AM -0700, Ryan Scott wrote:
  I'd like to get some comments on the following...

We would like to use libvirt to store some properties related to a domain. This can be done by adding a simple get/set API as follows:

What are the properties ?  I'm not really very enthusiastic about the
idea of adding API / XML for generic key,value properties. I think it
will quickly be abused as a way to add arbitrary, non-standardized hypervisor / driver specific configuration which would be better represented with explicit schema.

One thing we have in mind is driver/software version numbers. For example, the control tools may change the domain configuration based on whether a certain driver has the support for a new feature. If we create the domain's xml with the driver information, we can make this decision quickly, on the fly, in dom0.

While this information is related to domain configuration, it's not hypervisor-specific, so I'm not sure where else we would put it in the domain's XML description.

This gives the control tools someplace to store that information, rather than having to create some separate storage for each domain.


/*
 * Domain property get/set interfaces
 *
 * For GetProp, the return value must be freed by the caller.
 */
char *          virDomainGetProp    (virDomainPtr domain,
                        const char *propName);

int         virDomainSetProp    (virDomainPtr domain,
                        const char *propName,
                        const char *value);

  The XML would look something like this:

virsh # dumpxml solaris-pv-0
<domain type='xen' id='-1'>
[ snip ]
  <properties>
    <blah value="3"/>
    <foo value="bar"/>
  </properties>
</domain>

One of the key ideas behind libvirt is that we try to provide a consistent
set of configuration options across all drivers. NB, I'm not saying we need
the lowest-common denominator - just that we try to formalize a way to
represent every configuration option in such a away that it could be applied
to any driver.  I don't think simple  key,value pairs are sufficient in the
general case.

This wasn't meant to be a generic solution to handle storage of all domain information. For your example below, if someone wants more information on the console, the API should be extended to allow that.

However, extending the API to every piece of information for a domain seems to be overkill. That's why we just want a simple name/value pair.

-Ryan


As an example, you could define a property 'console' which takes a pty path
as its value. But  if you want to generalize this to different types of
console access, then a single value becomes insufficient.eg need a host+port
for a console accessed over TCP. The more explicit specialized <console>
tag allows us the flexibility of adding attributes and sub-elements as needed, not restricting ourselves to key,value pairs.

Dan,


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