>From the client POV, ZooKeeper is currently used to locate the currently >active master in addition to locating the current root region location (as you >describe).
This could be a "lookup" but any connection to ZK requires opening a session (that's my understanding at least). Rather than establishing a new connection/session on every lookup, it is kept open by the client. There should not be any measurable overhead to this idle connection, or at least I've never seen or heard of one. And there will only be one connection per process (well, per Configuration) so it would take a lot of client processes to cause any problems. And by retaining the connection, we actually get events in the client to point to the new master/root region when they do change, which is one benefit of keeping a session and watches open to ZK. The new master design in trunk moves region transitions into ZK, so it would actually be possible to send clients invalidations / updated region locations. There are also some who have thought about putting all assignment information into ZK at some point, but this is just a (contentious) idea for now. What exactly are your concerns about ZK sessions in the client? JG > -----Original Message----- > From: Naresh Rapolu [mailto:[email protected]] > Sent: Friday, October 01, 2010 7:52 PM > To: [email protected] > Subject: Re: Client`s cache invalidation > > If zookeeper isn`t invalidating the client`s cache, then what is the > actual use of a zookeeper session? Is it needed only to lookup > region-location of the "root-tablet" of META table, which is later used > to recursively scan the entire META table ? > Can`t that just be a lookup query to zookeeper, instead of a session ? > > I`ve looked into the Bigtable paper; the above use-case seems to be the > only one mentioned. Am I missing something ? > > Thanks, > Naresh. > > On 10/01/2010 09:40 PM, Jonathan Gray wrote: > > Yes. RegionServers will throw a NotServingRegionException. This, in > turn, will cause the client to grab the location from META again. > > > > > >> -----Original Message----- > >> From: Naresh Rapolu [mailto:[email protected]] > >> Sent: Friday, October 01, 2010 5:35 PM > >> To: [email protected] > >> Subject: Client`s cache invalidation > >> > >> Hello, > >> > >> How does the client`s cache of "region-location"( .META table), get > >> invalidated when a region-server splits regions ? Does Zookeeper > abort > >> the client session or inform it of staleness ? How is consistency > >> ensured in the time interval between splits being registered in > .META > >> table and client cache being refreshed ? I`m guessing, the region- > >> server > >> would reject operations on rows it isn`t responsible for. Am I > correct > >> ? > >> > >> Thanks, > >> Naresh. > >>
