[libvirt] RFC: virInterface change transaction API

[Dan Kenigsberg]

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.

> 
> 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?

I'm fine with gradual changes, so (A) is good enough for me, particularly as
long as netcf is shipping the initscripts (3).

> 
> 
>         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?

(A) is simpler? Or did you mean (B)? I'm slightly worried about (B) causing
disconnection of completely unrelated interfaces; this may break concurrent
applications, or even the user of netcf.

> 
> 
> 
>         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.html
> 
>    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.

I sleep better at night when there are backups... Obviously, I should not have
kept them beyond a certain limit (last 20). And I'd understand if you think that
it is the business of a backup system, or conf management system, to take theses
backups.

> 
>    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.)
> 
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