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Re: [libvirt] RFC: virInterface change transaction API

Going back over this based on the comments I received... I will begin implementing it this week, starting from the bottom, by adapting the initscript that Dan Kenigsberg already wrote:


On 04/08/2011 03:31 PM, 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*()

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

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).

I think the libvirt layer can stand as described.

2) netcf

   The netcf api will have these same three APIs, just named slightly

        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)

Until/unless a solid problem is discovered that prevents it, I will implement option A.


            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) 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

               a) ifdown all interfaces
b) call initscript to restore previous config (rollback_config)
               c) ifup all interfaces.

(note that I had intended to switch A and B above, so that the following comment made sense).

I will initially implement (B), since it is very simple and will get things going, but will plan to switch it to (A) prior to commit, since there seems to be well-founded concerns that (B) may cause avoidable problems in some situations.

           (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 ;-)


   It will have three commands, one of which will be called
   automatically by "start" (the command called automatically at boot


     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.


     This will just remove all the files in the save directory.

Dan's script saves old versions of config with no limit. I will modify this to keep only a few old versions, so that it won't grow without limit.

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