ZooKeeper Client Tutorial

ZooKeeper is a distributed configuration and synchronization service for large distributed systems. ZooKeeper's architecture supports high availability through redundant services. In the following tutorial we are going to start a group of ZooKeeper servers, then interact with them from both the command line and a GUI client. First download the zip file which contains everything you need:

Download ZooKeeper Demo

ZooKeeper Architecture
Run ZooKeeper Servers - locally start three servers
Start a Client and connect to server
ZooKeeper GUI
Create/Watch/Delete zNodes

ZooKeeper Tutorial

ZooKeeper Architecture

First it would be best to introduce what ZooKeeper is. ZooKeeper is a replicated service spread over a number of servers called an ensemble. It provides a set of reliable basic services, similar to a filesystem to allow coordination. It's architecture looks like this:

ZooKeeper nodes store their data in a hierarchical name space, much like a file system or a tree data structure. Clients can read from and write to the nodes and in this way have a shared configuration service. Updates are totally ordered.

Run ZooKeeper Servers

First download the demo files. Once unzipped you will see it has a number of folders s1/s2/s3 that contains the config for three servers and a number of .bat files for launching. To run a single server we could specify the command: java -jar ../zookeeper-3.3.3-fatjar.jar server s.cfg within the .cfg file is the config which looks like this:

dataDir=.
clientPort=2181
initLimit=3
syncLimit=3
server.1:127.0.0.1:2221:3331
server.2:127.0.0.1:2222:3332
server.3:127.0.0.1:2223:3333

We are going to run three servers to demonstrate failover, to do this use start-servers.bat. These three servers run based on config in directories s1/s2/s3 respectively like so:

Start a Client and connect to server

To start a client that will connect to our servers we can either run start-client.bat or the command java -jar zookeeper-3.3.3-fatjar.jar client -server. Below you can see me starting a command line client and then using the ls and create commands. I use create /demo1 "example of zk" to create a new node demo1 with some string data:

C:\temp\zookeeper-demo>java -jar zookeeper-3.3.3-fatjar.jar client -server 127.0.0.1:2181,127.0.0.1:2182,127.0.0.1:2183
Connecting to 127.0.0.1:2181,127.0.0.1:2182,127.0.0.1:2183
...
Welcome to ZooKeeper!
....
[zk: 127.0.0.1:2181,127.0.0.1:2182,127.0.0.1:2183(CONNECTED) 0] ls /
[zookeeper]
[zk: 127.0.0.1:2181,127.0.0.1:2182,127.0.0.1:2183(CONNECTED) 2] create /demo1 "example of zk"
Created /demo1
[zk: 127.0.0.1:2181,127.0.0.1:2182,127.0.0.1:2183(CONNECTED) 3] ls /
[zookeeper, demo1]
[zk: 127.0.0.1:2181,127.0.0.1:2182,127.0.0.1:2183(CONNECTED) 4]

ZooKeeper GUI Client

To start the zooinspector client run javaw -cp * org.apache.zookeeper.inspector.ZooInspector:

Create/Watch/Delete zNodes

Some Commands available in the cli client include:

ZooKeeper Command Line Commands

Command	Description
help	Display possible commands
ls path [watch]	List the contents of a zNode and optionally watch it for changes.
create path data	Create a new zNode containing the data specified.
set path data	Set the data for an existing zNode.
delete path	Delete an existing path.