Re: [infinispan-dev] design for cluster events (wiki page)

On 31 Oct 2013, at 07:18, Bela Ban <bban(a)redhat.com&gt; 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.

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 infinispan-dev(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/infinispan-dev