Apache ZooKeeper is a distributed, open-source coordination service for distributed systems. It helps to manage large distributed systems by providing a centralized service that allows nodes to communicate and exchange information.
In this tutorial, we will learn how to use ZooKeeper to coordinate the actions of distributed nodes.
Before starting this tutorial, you should have the following:
- A Java development environment, including the
JAVA_HOMEenvironment variable set to the location of your Java installation.
- Apache ZooKeeper installed on your system. You can download the latest release from the ZooKeeper website.
Setting up a ZooKeeper Ensemble
A ZooKeeper ensemble is a group of ZooKeeper servers that work together to provide a distributed coordination service. In order to use ZooKeeper, you need to set up at least one ZooKeeper server in your ensemble.
To set up a ZooKeeper ensemble, follow these steps:
- Unpack the ZooKeeper distribution file and navigate to the
- Create a configuration file named
zoo.cfgand add the following configuration parameters:
tickTime=2000 initLimit=10 syncLimit=5 dataDir=/var/lib/zookeeper clientPort=2181
tickTime parameter specifies the length of a tick in milliseconds. A tick is the basic time unit used by ZooKeeper for timeouts and other time-based operations. The
syncLimit parameters control the amount of time that a follower can be behind a leader before it is considered to be out of sync. The
dataDir parameter specifies the directory where ZooKeeper will store its data files, and the
clientPort parameter specifies the port that clients will use to connect to the ensemble.
- Create a file named
dataDirdirectory and add the number
1to it. This file is used to identify each server in the ensemble, and the number you specify should be unique for each server in the ensemble. For example, if you are setting up a three-server ensemble, you would use the numbers
3for each server, respectively.
- Start the ZooKeeper server by running the following command:
To set up additional servers in the ensemble, repeat these steps, using a unique
myid number for each server and specifying the same
clientPort as the first server.
You can conversely use the stop command to stop Zookeeper
$ zkServer.sh stop JMX enabled by default Using config: /home/user1/zookeeper-3.4.6/bin/../conf/zoo.cfg Stopping zookeeper ... STOPPED
Checking the status of ZooKeeper when it has stopped or is not running will show the following result:
$ zkServer.sh status JMX enabled by default Using config: /home/user1/zookeeper-3.4.6/bin/../conf/zoo.cfg Error contacting service. It is probably not running
Connect to the ZooKeeper server using a client
You can use the zkCli.sh script to start a command-line client, or you can use one of the many client libraries available for various programming languages.
Create a new node in the ZooKeeper hierarchy. You can do this using the create command in the client, or by using the create method of a client library. For example:
create /app1 "Hello, World!"
This will create a new node called app1 with the value “Hello, World!”. As you can see from the following picture, Zookeeper organizes information using a hierarchical namespace:
Read the data stored in a node. You can use the get command in the client, or the getData method of a client library to read the data stored in a node. For example:
This will return the value of the app1 node, which is “Hello, World!”.
Update the data stored in a node. You can use the set command in the client, or the setData method of a client library to update the data stored in a node. For example:
set /app1 "Hello, ZooKeeper!"
This will update the value of the myapp node to “Hello, ZooKeeper!”.
Delete a node. You can use the delete command in the client, or the delete method of a client library to delete a node. For example:
This will delete the
That’s a basic tutorial on how to use ZooKeeper. There are many more features and capabilities available, such as the ability to create ephemeral nodes, watch nodes for changes, and manage ACLs (access control lists). You can learn more about these features in the ZooKeeper documentation.