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Re: [Libvir] [RFC] Host and guest capabilities

Daniel Veillard wrote:
On Wed, Mar 07, 2007 at 02:56:47PM +0000, Richard W.M. Jones wrote:
Daniel Veillard wrote:
 This sounds too variable, adding an entry point per capability of
some of the hypervisor available will lead to just too many entry points
once the set of virtualization engines and associated benefits increase.
That's one of the places where I feel wy more comfortable returning an
XML description which can then be augmented as more features are added.
But this API is _precisely_ designed to be extensible. The virCapabilities structure is not accessible to callers (unlike, say, virNodeInfo), except through accessor functions. We can add accessor functions in future.

Returning XML just punts the problem elsewhere. Now clients need to worry about parsing the XML, and there's no real guarantee that the XML won't change in a way which is incompatible with the clients. Whereas by using ordinary functions we have that guarantee.

  It's easier to make that garantee at the XML level in my opinion.

Well one can certainly be careful about future changes to the XML, but there is no guarantee for clients, not even the (very minimal) guarantees that C linking makes. After all "char *getXMLBlob ()" will still link perfectly fine no matter what's in the XML.

And adding pile of accessor functions for a struct that you don't feel
you can define well enough to export is not the way I like to define APIs.

I would gladly make the structure public. The original proposal which unfortunately didn't make it beyond some private chat was to prototype the call so it would be called as:

  struct virCapabilities caps;
  virConnectGetCapabilities (conn, &caps, sizeof caps);

This provides forwards compatibility because libvirt can add further fields to the structure, and libvirt can always tell which version of the structure that the client was linked against through the size field.

Future elements of the structure can even override earlier elements, for future clients which understand this.

The participants in the chat decided they preferred accessor functions instead, and since those are still type-safe I did not demur.

Anyway, to be honest I'm not that bothered. If you think XML is better, I'm quite happy to return XML blobs. My main concern is to get virt-manager working sensibly in the remote case.


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