Re: [libvirt] RFC: virInterface change transaction API

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.