Observing Node Events

ZooKeeper allows you to subscribe to a certain events happening to a node:

• Creation of node

• Deletion of node

• Changes of node data

• Adding and deleting of node's children

To do this, one can specify a NodeWatcher along with any GetRequest: ExistsRequest, GetChildrenRequest and GetDataRequest.

A watcher will be attached to a given node only if request results in success. Watcher will be triggered when a change occurs to this node only once.

While there is no guarantee that notification will be delivered to the client regardless of the state of connection, delivered events can never be out of order.

One should never call methods of client inside a watcher body as it would inevitably lead to a deadlock

INodeWatcher watcher = new AdHocNodeWatcher((NodeChangedEventType eventType, string changedNodePath) =>
{
    Console.WriteLine($"Something has changed with node {changedNodePath}...");
    Console.WriteLine(eventType switch
    {
        NodeChangedEventType.Created => "It was created!",
        NodeChangedEventType.Deleted => "It was deleted...",
        NodeChangedEventType.DataChanged => "Binary data stored in it has changed",
        NodeChangedEventType.ChildrenChanged => "There were children of this node added or deleted"
    });
});

client.Exists(new ExistsRequest("/path/to/watched/node") { Watcher = watcher });

Deduplication of watchers

By default, an event will be delivered to a watcher only once, no matter how many times the same watcher instance was passed to client. For example, if we attach the same watcher two times...

client.Exists(new ExistsRequest("/path/to/watched/node") { Watcher = watcher });
client.Exists(new ExistsRequest("/path/to/watched/node") { Watcher = watcher });

client.Create("/path/to/watched/node", CreateMode.Ephemeral);
// 'It was created!' is printed once

...it will be triggered exactly once.

That is because watcher instances are being cached inside a client. Sometimes it might be necessary not to cache a one-time watcher or receive a separate notification for every subscription.

To override caching behaviour, use IgnoreWatchersCache = true:

client.Exists(new ExistsRequest("/path/to/watched/node") { Watcher = watcher, IgnoreWatchersCache = true });
client.Exists(new ExistsRequest("/path/to/watched/node") { Watcher = watcher, IgnoreWatchersCache = true });

client.Create("/path/to/watched/node", CreateMode.Ephemeral);
// 'It was created!'
// 'It was created!' 

The total count of deduplicated watchers is limited by a WatchersCacheCapacity in ZooKeeperClientSettings.

Reattaching a Watcher

Any watcher is notified only of a single event. So, once it gets called, it may be necessary to attach a new watcher.

INodeWatcher watcher = null;
INodeWatcher GetWatcher() => watcher ??= new AdHocNodeWatcher((t, p) =>
{
    ...
    Task.Run(() => AttachWatcher());  // Avoiding deadlock by running request in a separate thread
});

void AttachWatcher() => client.Exists(new ExistsRequest("/path/to/watched/node") { Watcher = GetWatcher() });

AttachWatcher();