[sheepdog] [PATCH] sheep: remove master node

[MORITA Kazutaka]

At Thu, 18 Jul 2013 17:07:53 +0800,
Liu Yuan wrote:
> 
> On Thu, Jul 18, 2013 at 05:56:05PM +0900, MORITA Kazutaka wrote:
> > At Thu, 18 Jul 2013 16:45:40 +0800,
> > Liu Yuan wrote:
> > > 
> > > > > I noticed zookeeper just send one event to watcher on my test box even if there
> > > > > are multiple updater to one member of the queue. But I think there is problem
> > > > > like above example. I think we need to check if there someone updates the join
> > > > > event already in the queue inside push_join_response(), to allow only one
> > > > > updater thus one update event to watcher of all nodes.
> > > > 
> > > > It is not easy to make sure that there is only one updater in the
> > > > cluster.  I think of keeping the master of the zookeeper driver in
> > > > this patch.  This patch cannot remove the master in either way, and
> > > > introduces another complexity rather than simplifying the code.
> > > > 
> > > 
> > > Why hard? We can read the zk node and check ev->type == EVENT_ACCEPT or not. no?
> > 
> > While pushing a join response, how to prevent another node from
> > updating the same zk node?  For example,
> > 
> >  1. The node A reads the znode and ev->type is not EVENT_ACCEPT.
> >  2. The node B reads the znode before the node A updates it with
> >     EVENT_ACCEPT.
> >  3. The node A calls zk_set_data() with EVENT_ACCEPT.
> >  4. The node B calls zk_set_data() with EVENT_ACCEPT.
> 
> I can't testify it, but I notice following scenario:
> 
> 1 A and B try to update one zk node in a very short time window
> 2 the sequence might be (A, B) or (B, A), but it doesn't matter.
> 3 zookeeper will only send one update event!
> 
> I guess if the time windoe is less than a specific time in zk, it just
> notify one update event for that node.

Even if it is true, the problem still happens if all the nodes don't
call push_join_response() within the window.  I think we should
reconsider the way to remove the zookeeper master and it should be
discussed in another patch series.  I'll send the v2 after rebasing
and removing the zk codes.

Thanks,

Kazutaka