> > On reconnect, watches are reset. For Data watches, if the node no longer > exists, the watch will get NodeDeleted. If the node's zxId is different, > the watch will get NodeDataChanged. Exist and child nodes have similar > handling. Persistent watches, on the other hand, are merely reset.
Thanks for the nice summary, Jordan. I guess it can be argued that it isn't necessary or that it could result > if a _lot_ of persistent watch calls. Maybe the right thing to do is to > just document the difference and leave it as it's been this way for years. > The persistence recursive watch is a very nice feature to use, however, I do see the use case and benefits for the fix. I wonder how difficult it's to get it fixed? On Fri, Jul 25, 2025 at 2:06 PM Jordan Zimmerman <[email protected]> wrote: > Here's a summary: > > On reconnect, watches are reset. For Data watches, if the node no longer > exists, the watch will get NodeDeleted. If the node's zxId is different, > the watch will get NodeDataChanged. Exist and child nodes have similar > handling. Persistent watches, on the other hand, are merely reset. > > I no longer remember why we didn't mimic this for Persistent watches. I > guess it can be argued that it isn't necessary or that it could result if a > _lot_ of persistent watch calls. Maybe the right thing to do is to just > document the difference and leave it as it's been this way for years. > > -Jordan > > > On Jul 25, 2025, at 9:58 PM, Keith Turner <[email protected]> wrote: > > > > > > > > On 2025/07/25 19:23:41 Jordan Zimmerman wrote: > >> Hi, > >> > >> I took a look at the code (which I haven't looked at in 5 or more > years). It looks like the reconnection behavior _is_ different. Persistent > watches will miss some events that other watches are getting. This is > indeed a very long-standing bug. > > > > What events are missed for persistent recursive watchers that normal > watcher see? > > > >> > >> I'd be willing to work on this, but there's likely devs who are more > familiar with the code now who can do it. > >> > >> -JZ > >> > >>> On Jul 25, 2025, at 8:06 PM, Jordan Zimmerman < > [email protected]> wrote: > >>> > >>> Hi, > >>> > >>> Persistent watches are the same watch as every other watch. It all > goes through the same code. Let's look at the doc: > >>> > >>>> Because standard watches are one time triggers and there is latency > between getting the event and sending a new request to > >>>> get a watch you cannot reliably see every change that happens to a > node in ZooKeeper. Be prepared to handle the case where > >>>> the znode changes multiple times between getting the event and > setting the watch again. (You may not care, but at least realize it may > happen.) > >>> > >>> ZooKeeper does not keep any kind of queue of events. You cannot count > on seeing every event in ZooKeeper. Watchers are triggered as events happen. > >>> Again, it's been a very long time since I've looked at the code but > this is my memory of how it works. When I wrote Persistent watches, I used > all > >>> the existing watch code. A Persistent watch is the exact same code > path as all other watches. They only difference is that they don't get > deleted after > >>> firing. Also, recursive watches trigger for child nodes being watched. > But, again, same code path. > >>> > >>> I hope this helps. > >>> > >>> -JZ > >>> > >>> > >>>> On Jul 25, 2025, at 7:30 PM, Li Wang <[email protected]> wrote: > >>>> > >>>> Thanks for the input, Jordan. > >>>> > >>>> My understanding is that the standard watches do but persistent > watches > >>>> don't. Not sure if I miss anything or if this is a bug. Looking > forward to > >>>> any feedback/input on this. > >>>> > >>>> 1. We have the following in the standard watch section of Zookeeper > >>>> documentation and it looks like missing notifications are triggered. > >>>> > >>>> When a client reconnects, any previously registered watches will be > >>>>> reregistered and triggered if needed. > >>>> > >>>> > >>>> > >>>> > https://zookeeper.apache.org/doc/r3.9.3/zookeeperProgrammers.html#sc_WatchSemantics > >>>> > >>>> > >>>> 2. In the code base, Zookeeper client library maintains lastZXid in > memory > >>>> and sends it to the server when resetting watches upon reconnection. > The > >>>> server detects if any missing notifications need to be triggered > based on > >>>> the lastZxid. > >>>> > >>>> > https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/ClientCnxn.java#L1040-L1041 > >>>> > https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/server/DataTree.java#L1497 > >>>> > >>>> 3. The problem is that missing notifications seem only being > triggered for > >>>> standard watches but not for persistent watches when reconnecting. > >>>> > >>>> For example, for standard watches, watches.process() is invoked for > sending > >>>> missing notifications. > >>>> > >>>> for (String path : dataWatches) { > >>>>> DataNode node = getNode(path); > >>>>> if (node == null) { > >>>>> watcher.process(new WatchedEvent(EventType.NodeDeleted, > >>>>> KeeperState.SyncConnected, path)); > >>>>> } else if (node.stat.getMzxid() > relativeZxid) { > >>>>> watcher.process(new > >>>>> WatchedEvent(EventType.NodeDataChanged, KeeperState.SyncConnected, > path)); > >>>>> } else { > >>>>> this.dataWatches.addWatch(path, watcher); > >>>>> } > >>>>> } > >>>> > >>>> > >>>> > https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/server/DataTree.java#L1494-L1521 > >>>> > >>>> However, for persistence watches, we only register the watches, not > >>>> detecting and sending missing notifications. > >>>> > >>>> for (String path : persistentRecursiveWatches) { > >>>>> this.dataWatches.addWatch(path, watcher, > >>>>> WatcherMode.PERSISTENT_RECURSIVE); > >>>>> } > >>>> > >>>> > >>>> > https://github.com/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/server/DataTree.java#L1494-L1521 > >>>> > >>>> Thanks, > >>>> > >>>> Li > >>> > >> > >> > >
