Hey Mahadev, I had a question about that. In my application I am using ZooKeeper for several different unrelated purposes, e.g. to generate unique ids, for distributed locks, and as a property store. I have implemented generic black box classes that use ZooKeeper to provide that functionality, and when an object of one of those classes is instantiated it internally creates a ZooKeeper instance for its personal use. So, for example, one of my apps has a property store and an id generator, and so it ends up using 2 ZooKeeper client objects. In principle I could create a single ZooKeeper client object and pass it in to the objects, so then I would only have a single ZooKeeper instance. However, if receive a Watcher.Event.KeeperState.Expired event, a fresh ZooKeeper client instance has to be created. If I were sharing the ZooKeeper instance, then somehow my objects would have to be notified that they should switch to using the new ZooKeeper instance. That means somewhere in my app I would need to maintain a list of all objects using ZooKeeper. Is this the recommended approach? Is there some other more elegant way to do this?
Satish On Sun, Jun 7, 2009 at 11:58 AM, Mahadev Konar <maha...@yahoo-inc.com>wrote: > HI Grant, > I agree with Ted but just to elaborate a little more. > > Its good to have a single zookeeper instance connected to the server. > Zookeeper client are supposed to be long lived client and the expected > idiom > to use a zookeeper client is to have a long lived single zookeeper client > per application instance. Most of the zookeeper recipes use zookeeper > session capabilities for implementing those recipes. So in that case, it > becomes necessary to have just a single client per app instance. Even if > you > don't plan to use zookeeper session capabilities (like ephemeral nodes and > watches) it would be good to just use a single zookeeper instance. > > A zookeeper client if not being used in an application would just be > sending > pings every one third of the timeout values you set. We are working on an > opimization in 3.3 wherein we wont be even sending these pings if the > client > does not use ephemeral nodes and watches. ZOOKEEPER-321 is the jira if you > want to track that. Hope this helps. > > mahadev > > > On 6/6/09 12:09 AM, "Ted Dunning" <ted.dunn...@gmail.com> wrote: > > > It is a common idiom to have a single Zookeeper instance. One reason for > > this is that it can be hard to keep track of which instance has which > > watches if you have lots of them around. > > > > Instantiating several Zookeeper structures and then discarding them also > > eliminates the utility of ephemeral philes. > > > > Watches and ephemerals are two of the key characteristics of ZK, so they > are > > quite a loss. > > > > That said, keeping a single zookeeper as a static in a single class isn't > > such a strange thing to do, especially if you can't imagine closing the > ZK > > instance. That gives you some scope but can keep the existence and use > of > > ZK a secret. > > > > You do have to worry a bit about how to initialize the ZK. For that > reason > > and for mocking purposes, it is pretty good practice to always inject the > ZK > > instance into your classes a la spring. > > > > On Fri, Jun 5, 2009 at 8:56 PM, Grant Ingersoll <gsing...@apache.org> > wrote: > > > >> What's the overhead of connecting to a Server? In other words, if I'm > in a > >> multi-threaded web-app server environment, should I cache my ZooKeeper > >> instance and set a larger timeout value or should I just construct them > as I > >> need them? > >> > >> > >> > > > >