org.hbase.async

Class HBaseClient.ZKClient

java.lang.Object

org.hbase.async.HBaseClient.ZKClient

All Implemented Interfaces:

org.apache.zookeeper.Watcher

Enclosing class:

HBaseClient

protected final class HBaseClient.ZKClient
extends Object
implements org.apache.zookeeper.Watcher

Helper to locate the -ROOT- region through ZooKeeper.

We don't watch the file of the -ROOT- region. We just asynchronously read it once to find -ROOT-, then we close our ZooKeeper session. There are a few reasons for this. First of all, the -ROOT- region doesn't move often. When it does, and when we need to use it, we'll realize that -ROOT- is no longer where we though it was and we'll find it again. Secondly, maintaining a session open just to watch the -ROOT- region is a waste of resources both on our side and on ZK's side. ZK is chatty, it will frequently send us heart beats that will keep waking its event thread, etc. Third, if the application we're part of already needs to maintain a session with ZooKeeper anyway, we can't easily share it with them anyway, because of ZooKeeper's API. Indeed, unfortunately the ZooKeeper API requires that the ZooKeeper object be re-created when the session is invalidated (due to a disconnection or a timeout), which means that it's impossible to share the ZooKeeper object. Ideally in an application there should be only one instance, but their poor API makes it impractical, since the instance must be re-created when the session is invalidated, which entails that one entity should own the reconnection process and have a way of giving everyone else the new instance. This is extremely cumbersome so I don't expect anyone to do this, which is why we manage our own instance.

Nested Class Summary

Nested classes/interfaces inherited from interface org.apache.zookeeper.Watcher

org.apache.zookeeper.Watcher.Event

Constructor Summary

Constructors

Constructor and Description
ZKClient(String quorum_spec, String base_path) Constructor.

Method Summary

Modifier and Type	Method and Description
void	disconnectZK() Disconnects from ZooKeeper.
com.stumbleupon.async.Deferred<Object>	getDeferredRoot() Returns a deferred that will be called back once we found -ROOT-.
void	process(org.apache.zookeeper.WatchedEvent event) Processes a ZooKeeper event.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

ZKClient

public ZKClient(String quorum_spec,
                String base_path)

Constructor.

Parameters:

quorum_spec - The specification of the quorum, e.g. "host1,host2,host3".

base_path - The base path under which is the znode for the -ROOT- region.

Method Detail

getDeferredRoot

public com.stumbleupon.async.Deferred<Object> getDeferredRoot()

Returns a deferred that will be called back once we found -ROOT-.

Returns:

A deferred which will be invoked with an unspecified argument once we know where -ROOT- is. Note that by the time you get called back, we may have lost the connection to the -ROOT- region again.

process

public void process(org.apache.zookeeper.WatchedEvent event)

Processes a ZooKeeper event.

This method is called back by ZooKeeper from its main event thread. So make sure you don't block.

Specified by:

process in interface org.apache.zookeeper.Watcher

Parameters:

event - The event to process.

disconnectZK

public void disconnectZK()

Disconnects from ZooKeeper.

This method is blocking. Unfortunately, ZooKeeper doesn't offer an asynchronous API to close a session at this time. It waits until the server responds to the closeSession RPC.