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https://issues.apache.org/jira/browse/ZOOKEEPER-1416?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15778863#comment-15778863
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ASF GitHub Bot commented on ZOOKEEPER-1416:
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GitHub user Randgalt opened a pull request:
https://github.com/apache/zookeeper/pull/136
[ZOOKEEPER-1416] Persistent Recursive Watch
Here is a completed implementation for a persistent, recursive watch
addition for ZK. These watches are set via a new method, `addPersistentWatch()`
and are removed via the existing watcher removal methods. Persistent, recursive
watches have these characteristics:
- Once set, they do not auto-remove when triggered
- They trigger for all event types (child, data, etc.) on the node they are
registered for and any child znode recursively.
- They are efficiently implemented by using the existing watch internals. A
new class `PathIterator` walks up the path parent-by-parent when checking if a
watcher applies.
Persistent watcher specific tests are in `PersistentWatcherTest.java`. I'd
appreciated feedback on other additional tests that should be added.
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/Randgalt/zookeeper ZOOKEEPER-1416
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/zookeeper/pull/136.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #136
----
commit 3c05c671d09e5b6df936af8f0a700995d5749e11
Author: randgalt <[email protected]>
Date: 2016-12-25T21:36:13Z
basic work done. Needs more testing, tuning, etc.
commit ca4a000dcf294aaebd09d3118ebc62cb0783f9cc
Author: randgalt <[email protected]>
Date: 2016-12-26T15:06:55Z
working on persistent watcher removal
commit bf13deda0b00ca67cd1fa963961d95a22634ed88
Author: randgalt <[email protected]>
Date: 2016-12-26T17:59:04Z
Support resetting persistent watches
commit 27d8d6cd45cb6adfabf50143f6de62a371447519
Author: randgalt <[email protected]>
Date: 2016-12-26T18:21:17Z
docs
commit 2766fb1020c600af579a0f701fa3c00ea92b7e22
Author: randgalt <[email protected]>
Date: 2016-12-26T18:44:42Z
containsWatcher() was broken for STANDARD watchers
commit 86fa1fbcb75021179f80588a2ea46aad2127fb4e
Author: randgalt <[email protected]>
Date: 2016-12-26T19:20:00Z
removed unused import
commit b490c84d1e56335ba66f9c56d64134886b144451
Author: randgalt <[email protected]>
Date: 2016-12-26T19:20:08Z
Updated doc for persistent watches
----
> 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: Thawan Kooburat
> 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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