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https://issues.apache.org/jira/browse/ZOOKEEPER-1416?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16118992#comment-16118992
]
ASF GitHub Bot commented on ZOOKEEPER-1416:
-------------------------------------------
Github user afine commented on a diff in the pull request:
https://github.com/apache/zookeeper/pull/136#discussion_r132006551
--- Diff:
src/java/test/org/apache/zookeeper/test/PersistentWatcherTest.java ---
@@ -0,0 +1,228 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.zookeeper.test;
+
+import org.apache.zookeeper.AsyncCallback;
+import org.apache.zookeeper.CreateMode;
+import org.apache.zookeeper.KeeperException;
+import org.apache.zookeeper.WatchedEvent;
+import org.apache.zookeeper.Watcher;
+import org.apache.zookeeper.ZooDefs;
+import org.apache.zookeeper.ZooKeeper;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.TimeUnit;
+
+public class PersistentWatcherTest extends ClientBase {
+ private static final Logger LOG =
LoggerFactory.getLogger(PersistentWatcherTest.class);
+ private BlockingQueue<WatchedEvent> events;
+ private Watcher persistentWatcher;
+
+ @Override
+ @Before
+ public void setUp() throws Exception {
+ super.setUp();
+
+ events = new LinkedBlockingQueue<>();
+ persistentWatcher = new Watcher() {
+ @Override
+ public void process(WatchedEvent event) {
+ events.add(event);
+ }
+ };
+ }
+
+ @Test
+ public void testBasic()
+ throws IOException, InterruptedException, KeeperException {
+ ZooKeeper zk = null;
+ try {
+ zk = createClient(new CountdownWatcher(), hostPort);
+
+ zk.addPersistentWatch("/a/b", persistentWatcher);
+ internalTestBasic(zk);
+ } finally {
+ if (zk != null) {
+ zk.close();
--- End diff --
ZooKeeper supports try-with-resources so using that may make this code
cleaner
> Persistent Recursive Watch
> --------------------------
>
> Key: ZOOKEEPER-1416
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1416
> Project: ZooKeeper
> Issue Type: Improvement
> Components: c client, documentation, java client, server
> Reporter: Phillip Liu
> Assignee: Jordan Zimmerman
> Attachments: ZOOKEEPER-1416.patch, ZOOKEEPER-1416.patch
>
> Original Estimate: 504h
> Remaining Estimate: 504h
>
> h4. The Problem
> A ZooKeeper Watch can be placed on a single znode and when the znode changes
> a Watch event is sent to the client. If there are thousands of znodes being
> watched, when a client (re)connect, it would have to send thousands of watch
> requests. At Facebook, we have this problem storing information for thousands
> of db shards. Consequently a naming service that consumes the db shard
> definition issues thousands of watch requests each time the service starts
> and changes client watcher.
> h4. Proposed Solution
> We add the notion of a Persistent Recursive Watch in ZooKeeper. Persistent
> means no Watch reset is necessary after a watch-fire. Recursive means the
> Watch applies to the node and descendant nodes. A Persistent Recursive Watch
> behaves as follows:
> # Recursive Watch supports all Watch semantics: CHILDREN, DATA, and EXISTS.
> # CHILDREN and DATA Recursive Watches can be placed on any znode.
> # EXISTS Recursive Watches can be placed on any path.
> # A Recursive Watch behaves like a auto-watch registrar on the server side.
> Setting a Recursive Watch means to set watches on all descendant znodes.
> # When a watch on a descendant fires, no subsequent event is fired until a
> corresponding getData(..) on the znode is called, then Recursive Watch
> automically apply the watch on the znode. This maintains the existing Watch
> semantic on an individual znode.
> # A Recursive Watch overrides any watches placed on a descendant znode.
> Practically this means the Recursive Watch Watcher callback is the one
> receiving the event and event is delivered exactly once.
> A goal here is to reduce the number of semantic changes. The guarantee of no
> intermediate watch event until data is read will be maintained. The only
> difference is we will automatically re-add the watch after read. At the same
> time we add the convience of reducing the need to add multiple watches for
> sibling znodes and in turn reduce the number of watch messages sent from the
> client to the server.
> There are some implementation details that needs to be hashed out. Initial
> thinking is to have the Recursive Watch create per-node watches. This will
> cause a lot of watches to be created on the server side. Currently, each
> watch is stored as a single bit in a bit set relative to a session - up to 3
> bits per client per znode. If there are 100m znodes with 100k clients, each
> watching all nodes, then this strategy will consume approximately 3.75TB of
> ram distributed across all Observers. Seems expensive.
> Alternatively, a blacklist of paths to not send Watches regardless of Watch
> setting can be set each time a watch event from a Recursive Watch is fired.
> The memory utilization is relative to the number of outstanding reads and at
> worst case it's 1/3 * 3.75TB using the parameters given above.
> Otherwise, a relaxation of no intermediate watch event until read guarantee
> is required. If the server can send watch events regardless of one has
> already been fired without corresponding read, then the server can simply
> fire watch events without tracking.
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