[ovs-dev] [PATCH] netlink-notifier: Support multiple groups.

[Ben Pfaff]

On Thu, Apr 16, 2015 at 12:49:54PM -0700, Jarno Rajahalme wrote:
> A netlink notifier ('nln') already supports multiple notifiers.  This
> patch allows each of these notifiers to subscribe to a different
> multicast group.  Sharing a single socket for multiple event types
> (each on their own multicast group) provides serialization of events
> when reordering of different event types could be problematic.  For
> example, if a 'create' event and 'delete' event are on different
> netlink multicast group, we may want to process those events in the
> order in which kernel issued them, rather than in the order we happen
> to check for them.
> 
> Moving the multicast group argument from nln_create() to
> nln_notifier_create() allows each notifier to specify a different
> multicast group.  The parse callback needs to identify the group the
> message belonged to by returning the corresponding group number, or 0
> when an parse error occurs.
> 
> Signed-off-by: Jarno Rajahalme <jrajahalme at nicira.com>

What's the initial planned use case for this?  I am not sure that the
one described above makes sense to me yet, because normally all the
notifications related to a single object type have the same multicast
group.  For example, new OVS datapaths, OVS datapath deletions, and
changes to OVS datapaths all use the same multicast group.  I can
understand that one might want serialization for notifications on
different objects, but I don't have a good example in mind.