Persistent Recursive Watch
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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
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 Observers.
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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