[sheepdog] [PATCH] sheep: remove master node

[Liu Yuan]

On Thu, Jul 18, 2013 at 05:29:06PM +0900, MORITA Kazutaka wrote:
> At Tue, 16 Jul 2013 17:37:55 +0800,
> Liu Yuan wrote:
> > 
> > >
> > > > push_join_response() of zk driver are called on all the nodes too. So if
> > > > following case happens, can sheep handle it?
> > > > 
> > > > 2 nodes in the cluster {A, B}. And C is joining the cluster.
> > > > 
> > > > A -> push_join_response() and quickly return, watcher of A, B, C is called
> > > >      to handle EVENT_ACCEPT from A.
> > > > B -> push_join_response() slowly return because of network, A, B, C handles
> > > >      EVENT_ACCEPT from B.
> > > > 
> > > > Simply put, can sheep hanle multiple EVENT_ACCEPT of the same node?
> > > 
> > > I think the answer is yes.
> > > 
> > >  - local: The event queue is a mmapped file and guared by flock, so
> > >    concurrent sd_accept_handler() calls don't happen.
> > > 
> > >  - corosync: cdrv_cpg_deliver() ignores the arriving
> > >    COROSYNC_MSG_TYPE_ACCEPT() if there is no JOIN event in the queue.
> > 
> > Corosync actually never try to send EVENT_ACCEPT more than once for current code
> > So no worries about corosync.
> 
> This is a timing problem and it actually happens that multiple nodes
> send COROSYNC_MSG_TYPE_ACCEPT events on my environment.  In either
> way, the corosync driver can handle this problem because it ignores
> the second or later COROSYNC_MSG_TYPE_ACCEPT event.

This was what I meant, corosync driver ignored redundant ACCEPT event.

> 
> > 
> > > 
> > >  - zookeeper: push_join_response() just overwrites the znode with
> > >    EVENT_ACCEPT, and multiple calls of push_join_response() is no
> > >    problem.
> > 
> > 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?

Thanks
Yuan