[libvirt] RFC: virInterface change transaction API

[Daniel P. Berrange]

On Thu, Apr 14, 2011 at 04:03:03PM +0300, Dan Kenigsberg wrote:
> On Fri, Apr 08, 2011 at 03:31:05PM -0400, Laine Stump wrote:
> > I've been asked to implement what some people have termed as a
> > "transaction-oriented" API for host interface configuration (ie
> > virInterface*()).
> > The basic intent is to allow rollback to a known-good config if
> > anything goes
> > wrong when changing around the host network config with virInterface*()
> > functions.
> > 
> > The most straightforward way to achieve this is that prior to calling
> > virInterfaceDefine/virInterfaceUndefine, the current state of the
> > host's network configuration (ie the /etc/sysconfig/network-scripts/ifcfg-*
> > files in the case of Fedora and RHEL) would be saved off somewhere, and
> > kept around until we're sure the new config is good; once we know that,
> > we can just eliminate the backup. If, however, the user of virInterface*()
> > explicitly requests, we could copy the files back; alternately if the system
> > is rebooted without these known-good files being erased, we would assume
> > that something went wrong and restore the original config.
> > 
> > As with all other virInterface functions, the details of all this will
> > be handled by netcf (and below), but since libvirt is the main consumer
> > of netcf, I figure this is the appropriate place to discuss how it
> > gets done,
> > so please let me know any opinions on any piece of this. I plan to start
> > the implementation "soon", as I want to be finished before the end of
> > May.
> 
> I like the idea, and think that virtInterface* users will benefit from it.
> Few comments are inline.
> 
> > 
> > I see 3 layers to this:
> > 
> > 1) libvirt
> > 
> >    At the libvirt layer, this feature just requires 3 new APIs, which
> >    are directly passed through to netcf:
> > 
> >        virInterfaceChangeStart(virConnectPtr conn, unsigned int flags);
> >        virInterfaceChangeCommit(virConnectPtr conn, unsigned int flags);
> >        virInterfaceChangeRollback(virConnectPtr conn, unsigned int flags);
> > 
> >    For the initial implementation, these will be simple passthroughs
> >    to similarly named netcf functions. (in the future, it would be
> >    useful for the server side of libvirt to determine if client<->server
> >    connectivity was lost due to the network changes, and automatically
> >    tell netcf to do a rollback).
> 
> When such a feature is added, we should make it dependent on FLAG_AUTO_ROLLBACK
> passed to ChangeStart. Higher levels on the management stack may want full
> controll over when rollback happens.

I don't think a AUTO_ROLLBACK flag is sufficient. You'd almost certainly
want to pass some info such as hostname/port number to test, and perhaps
a test timeout in milliseconds, and perhaps a retry count. I suggest a
separate API that is invoked just after starting the TXN, to provide
auto-rollback data.

Regards,
Daniel
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