[libvirt] RFC: virInterface change transaction API

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 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). 2) netcf The netcf api will have these same three APIs, just named slightly differently: ncf_change_start(struct netcf *ncf, unsigned int flags); There are two possibilities for this. Either: A) call the initscript described below to save all config files that might possibly be changed (snapshot_config) or B) set a flag in *ncf indicating that all future calls to netcf that would end up modifying a particular config file should save off that file *if it hasn't already been saved*. (A) is simpler, but relies on the initscript having exact/complete matching knowledge of what files netcf may change. Should we worry about that and deal with the complexities of (B), or is (A) good enough for now? ncf_change_rollback(struct netcf *ncf, unsigned int flags); Again, two possbilities: A) a) save the config of all current interfaces (in memory) b) call the initscript below to restore the config to its original state. c) compare the new config to the old, and: * bring down any interfaces that no longer exist (PROBLEM: once an interface has no config files, you can no longer operate on it with "ifdown") * bounce any interfaces that have changed * bring up any interfaces that have been re-added or B) a) ifdown all interfaces b) call initscript to restore previous config (rollback_config) c) ifup all interfaces. (A) is much simpler, but may lead to unnecessary difficulties when we bounce interfaces that didn't really need it. So, the same question oas for ncf_change_start() - is the more exact operation worth the extra complexity? ncf_change_commit(struct netcf *ncf, unsigned int flags); The simplest function - this will just call the initscript to erase the backup (commit_config). 3) initscript This initscript will at first live in (be installed by) netcf (called /etc/init.d/networking-config?), but hopefully it will eventually be accepted by the initscripts package (which includes the networking-related initscripts), as it is of general use. (Dan Kenigsberg already already took a stab at this script last year, but received no reply from the initscripts maintainers, implying they may not be too keen on the idea right now - it might take some convincing ;-) https://fedorahosted.org/pipermail/initscripts-devel/2010-February/000025... It will have three commands, one of which will be called automatically by "start" (the command called automatically at boot time): snapshot_config This will save a copy of (what the script believes are - is this problematic?) all network-config related files. It may or may not be called by netcf (see the notes in ncf_start_change() above. If this function finds that a snapshot has already been taken, it should fail. rollback_config (automatically called from "start" at boottime) This will move back (from the saved copies) all files that were changed/removed since snapshot, *and delete any files that have been added*. Note that this command doesn't need to worry about ifup/ifdown, because it will be called prior to any other networking startup (part of the reason that netcf will need to deal with that). I notice that Dan K's version saves the modified files to a "rollback-${date}" directory. Does this seem like a good idea? It's nice to not lose anything, but there is no provision for eliminating old versions, so it could grow without bound. commit_config This will just remove all the files in the save directory. So, the two problems I have right now: 1) Do we accept the inexact method of just saving all files that match a list of patterns during *start(), then in *rollback() erasing all files matching that pattern and copying the old file back? Or do we need to keep track of what files have been changed/removed and added, and copy back / delete only those files during rollback? (A version control system would keep track of this rather nicely, but that's too complex for something that's intended to be a failsafe (and that we would also like to eventually be in the base OS install). Dan B. at one point suggested using patchfiles if I wanted the save info to keep exact track of which files would need to be replaced/deleted on rollback, but on further thought this turns out to not be workable, since we would need to run diff (to create the patchfile) after all changes had been made, and any outside changes to any of the files would leave the patchfile un-appliable, thus causing our "failsafe" to fail :-( ). Therefore, we will need to rely on the list of globs to tell us what files need to be deleted, or keep our own list in a separate file.) 2) Is it going to be okay to ifdown all interfaces prior to the rollback, and ifup all interfaces afterwards? Or must we compare the new config to the original, and ifdown only those interfaces that had been previously added/changed, then ifup only those interfaces that had been previously removed/changed? 3) If anyone has ideas on making the initscript more palatable to the initscripts people, please speak up! :-) (one comment from an initscripts person was that 1) for the general case it would be difficult to draw the line on what parts of network connectivity should be included in this rollback functionality, and 2) at some point this becomes a general system config problem, and would really be better addressed by a general system wide config management system. These are both concerns that need well qualified answers. (I tend to think that this is intended as a failsafe to prevent unreachable systems, so it should be as simple as possible, and thus shouldn't be burdened with the complexity of a full system config management system (which could also co-exist at a higher level), but better answers are welcome.)