in a distributed setting such an answer is impossible. especially given the theory of relativity and the speed of light. a machine may fail right after sending a heart beat or another may come online right after sending a report. even if zookeeper could provide this you would still have thread scheduling issues on a local machine that means that you are operating on old information.
to deal with this applications can use views, which allow clients to reconcile differences. for example, if two processes communicate and one has a different list of members than the other then they can both consult zookeeper to reconcile or use the membership list with the highest zxid. the other option is to count on eventually everyone converging. i would not develop a distributed system with the assumption that "all group members know *the exact number of members at all times*". ben On Fri, Mar 18, 2011 at 2:02 PM, Otis Gospodnetic <[email protected]> wrote: > Hi, > > Short version: > How can ZK be used to make sure that all group members know *the exact number > of > members at all times*? > > I have an app that can be run on 1 or more servers. New instances of the app > come and go, may die, etc. -- the number of the app instances is completely > dynamic. At any one time, as these apps come and go, each live instance of > the > app needs to know how many instances are there total. If a new instance of > the > app is started, all instances need to know the new total number of instances. > If an app is stopped or if it dies, the remaining apps need to know the new > number of app instances. > > Also, and this is critical, they need to know about these additions/removals > of > apps right away and they all need to find out them at the same time. > Basically, > all members of some group need to know *the exact number of members at all > times*. > > This sounds almost like we need to watch a "parent group znode" and monitor > the > number of its ephemeral children, which represent each app instance that is > watching the "parent groups znode". Is that right? If so, then all I'd need > to > know is the answer to "How many watchers are watching this znode?" of "How > many > kids does this znode have?". And I'd need ZK to notify all watchers whenever > the > answer to this question changes. Ideally it would send/push the answer (the > number of watchers) to all watchers, but if not, I assume any watcher that is > notified about the change would go poll ZK to get the number of ephemeral > kids. > > I think the above is essentially what's described on > http://eng.wealthfront.com/2010/01/actually-implementing-group-management.html > , > but doesn't answer the part that's critical for me (the very first Q up > above). > > Thanks, > Otis > ---- > Sematext :: http://sematext.com/ :: Solr - Lucene - Nutch > Lucene ecosystem search :: http://search-lucene.com/ > >
