I think Hao is pointing out that there is no way to see every change (delta) that happened to a znode. Consider 2 changes A,B in quick succession. When client gets notified of A and before setting the watch the change B has occurred on the server side. This means the client cannot know the delta A. Client can only read the state after change B is applied.
Implementing the concept of Persistent watcher guarantees that client if notified after every change. This is a nice to have feature but I dont understand the requirement in Hbase where this is needed. Hao, can you shed more light on how this would be useful for HBase (to act like state machine) thanks, Kishore G On Fri, Jan 17, 2014 at 5:18 AM, FPJ <[email protected]> wrote: > But you don't really miss events, you'll see them when you read the ZK > state. If you follow the pattern I described, you're supposed to observe > all > changes. Perhaps I'm missing some concrete use case you have mind. > > -Flavio > > > -----Original Message----- > > From: 陈迪豪 [mailto:[email protected]] > > Sent: 17 January 2014 13:03 > > To: [email protected] > > Subject: RE: Where are we in ZOOKEEPER-1416 > > > > No, it's not complicated. But for the people who don't understand zk > deeply, > > they would easily ignore the fact that they would miss events in some > way. > > Moreover, I think providing persistent watch is good for developers to > build > > the "state-machine" application. Actually, HBase suffer from missing the > > intermediate state when use zk to store the data. > > > > If the feature is implemented, I would like to see the patch and consider > if it > > can be used for us. > > > > ________________________________________ > > From: Flavio Junqueira [[email protected]] > > Sent: Friday, January 17, 2014 8:12 PM > > To: [email protected] > > Subject: RE: Where are we in ZOOKEEPER-1416 > > > > My take is that persistent subscriptions add complexity and are not > strictly > > necessary. You can follow this pattern of setting a watch, reading the > state > > upon a notification and setting a new watch. Why do you feel that's > > complicated? > > > > -Flavio > > > > -----Original Message----- > > From: 陈迪豪 [mailto:[email protected]] > > Sent: Friday, January 17, 2014 3:13 AM > > To: [email protected] > > Subject: Where are we in ZOOKEEPER-1416 > > > > > > > > Persistent watch and implementing the feature to act like "state machine" > > which is mentioned in > > ZOOKEEPER-153<https://issues.apache.org/jira/browse/ZOOKEEPER-153>, > > would be great for ZooKeeper user. I think HBase would like to know all > the > > change in zk rather than missing kind of events. > > > > So, would we continue developing these features? It's also a little > > complicated to develop with zk and I think there're lots of things to > improve. > > >
