> On 31 Oct 2013, at 07:18, Bela Ban <[email protected]> wrote: > > > >> On 10/30/13 8:28 PM, William Burns wrote: >> Since it seems I can't comment on the wiki itself, I am just replying here. >> >> I wonder if the third option 'Primary partition' is desirable. I >> think availability in some cases would be harmed more than we would >> like. >> >> Lets say you have a 5 node cluster where 3 of the nodes are behind the >> same router and the remaining 2 are behind a different one. If the >> router crashes, power loss etc. for the 3 and are no longer >> addressable you have your 2 partitions (possibly 1 or even 4). When >> this occurs the other 2 nodes would go into read only mode since they >> lost the quorum check. > > Yes, this is intended. Actually, the minority partition {D,E} might even > become totally inaccessible, ie. rejecting *all* requests (also reads). > > This is in line with the Primary Partition approach where a majority > partition is allowed to make progress, and all minority partitions shut > down. In terms of CAP, we're sacrificing availabilty here in favor of > consistency. > >> But the 3 nodes that are "writable" can't be >> accessed any longer and thus no writes can be performed on the cluster. > > You mean some clients cannot access {A,B,C} ? Sure, then so be it, but > at least we don't have any inconsistent state. Again, PP is *one* tool > we give to th user to handle partitions. > >> It seems we would still want to allow writes to provide as >> high of availability as possible. > > PP is *not* about availability, it is about consistency.
I think it's about availability as well, as the primary partition is still available. And about consistency: the fact that PP is available doesn't mean it contains all the data in the original cluster(Unless we only allow PP iff the PP holds at least a reference to any pice of data in the original cluster.) > Good for some > apps, bad for others. If you pick PP, you lose availability. > >> Also if we did have read only, what criteria would cause those nodes >> to be writeable again? > > Once you become the primary partition, e.g. when a view is received > where view.size() >= N where N is a predefined threshold. Can be > different, as long as it is deterministic. > >> There is no guarantee when the other nodes >> will ever come back up or if there will ever be additional ones anytime soon. > > If a system picks the Primary Partition approach, then it can become > completely inaccessible (read-only). In this case, I envisage that a > sysadmin will be notified, who can then start additional nodes for the > system to acquire primary partition and become accessible again. > > -- > Bela Ban, JGroups lead (http://www.jgroups.org) > _______________________________________________ > infinispan-dev mailing list > [email protected] > https://lists.jboss.org/mailman/listinfo/infinispan-dev _______________________________________________ infinispan-dev mailing list [email protected] https://lists.jboss.org/mailman/listinfo/infinispan-dev
