Apache ZooKeeper CLI (Command Line Interface) Operations

1. Objective

In our last Apache ZooKeeper Tutorial, we discussed ZooKeeper Leader Election. Today, we will see ZooKeeper CLI. In this Zookeeper article, we will learn the whole about ZooKeeper Command Line Interface. First, we will see what is CLI in ZooKeeper. Also, we will discuss commands in ZooKeeper CLI. Also, there are several operations we can perform with the ZooKeeper ensemble for development purpose, we will also discuss all in detail.
So, let’s start Apache ZooKeeper CLI.

ZooKeeper CLI

Apache ZooKeeper CLI (Command Line Interface) Operations

2. What is ZooKeeper Command Line Interface?

For development purpose, we can interact with the ZooKeeper ensemble, so, for that, we use ZooKeeper CLI (Command Line Interface). Its main purpose is for debugging and working around with different options.
Do you know about ZooKeeper pros and cons
In order to perform ZooKeeper CLI operations, at very first we have to turn on our ZooKeeper server (“bin/zkServer.sh start”). Afterward, we will also turn on ZooKeeper client (“bin/zkCli.sh”). Hence, we can perform the following operation, once the client starts:

  1. Create znodes.
  2. Get data.
  3. Watch znode for changes.
  4. Set data.
  5. Create children of a znode.
  6. List children of a znode.
  7. Check Status.
  8. Remove / Delete a znode.
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3. Commands in ZooKeeper CLI

Let’s discuss ZooKeeper CLI Commands in detail:

ZooKeeper CLI

Commands in ZooKeeper CLI

i. Create Znodes

At very first, with the given path, create a znode. Basically, the flag argument specifies whether the created Znode will be ephemeral, persistent, or sequential. However, all znodes are persistent, by default.
In addition, when a session expires or when the client disconnects, Ephemeral znodes (flag:e) will be automatically deleted.
Let’s revise ZooKeeper Terminologies
Although, Sequential znodes make sure that the Znode path will be unique.
Along with 10 digit padding to the znode path, ZooKeeper ensemble will add the sequence number. For example, the znode path /myapp will be converted to /myapp0000000001 and the next sequence number will be /myapp0000000002. If no flags are specified, then the znode is considered as persistent.
a. Syntax

create /path /data

b. Sample

create /FirstZnode “Myfirstzookeeper-app”

c. Output

[zk: localhost:2181(CONNECTED) 0] create /FirstZnode “Myfirstzookeeper-app”
Created /FirstZnode

Now, add -s flag, in order to create a Sequential znode:
a. Syntax

create -s /path /data

b. Sample

create -s /FirstZnode second-data

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c. Output

[zk: localhost:2181(CONNECTED) 2] create -s /FirstZnode “second-data”
Created /FirstZnode0000000023

Further, add -e flag, to create an Ephemeral Znode:
a. Syntax

create -e /path /data

b. Sample

create -e /SecondZnode “Ephemeral-data”

c. Output

[zk: localhost:2181(CONNECTED) 2] create -e /SecondZnode “Ephemeral-data”
Created /SecondZnode

Although, make sure the ephemeral znode will be deleted when a client connection is lost. So, by quitting the ZooKeeper CLI and then re-opening the CLI, we can try it.

ii. Get Data

In order to get the associated data of the znode as well as metadata of the specified znode, we use this operation. Information like when the data was last modified, where it was modified, and all the information about the data, we can get from it. In addition, to assign watches to show notification about the data, we use this ZooKeper CLI Command.
Let’s explore main features of ZooKeeper
a. Syntax

get /path

b. Sample

get /FirstZnode

c. Output

[zk: localhost:2181(CONNECTED) 1] get /FirstZnode
“Myfirstzookeeper-app”
cZxid = 0x7f
ctime = Tue Sep 29 16:15:47 IST 2015
mZxid = 0x7f
mtime = Tue Sep 29 16:15:47 IST 2015
pZxid = 0x7f
cversion = 0
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 22
numChildren = 0

Though, we must enter the full path of the znode, to access a sequential znode:
a. Sample

get /FirstZnode0000000023

b. Output

[zk: localhost:2181(CONNECTED) 1] get /FirstZnode0000000023
“Second-data”
cZxid = 0x80
ctime = Tue Sep 29 16:25:47 IST 2015
mZxid = 0x80
mtime = Tue Sep 29 16:25:47 IST 2015
pZxid = 0x80
cversion = 0
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 13
numChildren = 0

iii. Watch

when the specified znode or znode’s children data changes, the operatipon “Watches” show a notification. However, only in get command, we can set a watch.
Do you Know How ZooKeeper actually Works
a. Syntax

get /path [watch] 1

b. Sample

get /FirstZnode 1

c. Output

[zk: localhost:2181(CONNECTED) 1] get /FirstZnode 1
“Myfirstzookeeper-app”
cZxid = 0x7f
ctime = Tue Sep 29 16:15:47 IST 2015
mZxid = 0x7f
mtime = Tue Sep 29 16:15:47 IST 2015
pZxid = 0x7f
cversion = 0
dataVersion = 0
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 22
numChildren = 0

Although, make sure the output for watch operation is same as normal get command, though it will wait for Znode changes in the background. <Start here>

iv. Set Data

As per its name, this operations sets the data of the specified znode. We can check the data using the get CLI command in ZooKeeper, once we finish this ZooKeeper set operation.
a. Syntax

set /path /data

b. Sample

set /SecondZnode Data-updated

c. Output

[zk: localhost:2181(CONNECTED) 1] get /SecondZnode “Data-updated”
cZxid = 0x82
ctime = Tue Sep 29 16:29:50 IST 2015
mZxid = 0x83
mtime = Tue Sep 29 16:29:50 IST 2015
pZxid = 0x82
cversion = 0
dataVersion = 1
aclVersion = 0
ephemeralOwner = 0x15018b47db00000
dataLength = 14
numChildren = 0

If we assign watch option in ZooKeeper CLI getting command, its output will be as same as below −
Let’s revise applications of ZooKeeper
d. Output

[zk: localhost:2181(CONNECTED) 1] get /FirstZnode “Mysecondzookeeper-app”
WATCHER: :
WatchedEvent state:SyncConnected type:NodeDataChanged path:/FirstZnode
cZxid = 0x7f
ctime = Tue Sep 29 16:15:47 IST 2015
mZxid = 0x84
mtime = Tue Sep 29 17:14:47 IST 2015
pZxid = 0x7f
cversion = 0
dataVersion = 1
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 23
numChildren = 0

v. Create Children / Sub-znode

Basically, creating children is as same as creating new znodes. However, the only difference in both is that the path of the child znode will have the parent path as well.
a. Syntax

create /parent/path/subnode/path /data

b. Sample

create /FirstZnode/Child1 firstchildren

c. Output

[zk: localhost:2181(CONNECTED) 16] create /FirstZnode/Child1 “firstchildren”
created /FirstZnode/Child1
[zk: localhost:2181(CONNECTED) 17] create /FirstZnode/Child2 “secondchildren”
created /FirstZnode/Child2

vi. List Children

In order to list and display the children of a znode, we use this List Children ZooKeeper CLI command.
a. Syntax

ls /path

b. Sample

ls /MyFirstZnode

c. Output

[zk: localhost:2181(CONNECTED) 2] ls /MyFirstZnode
[mysecondsubnode, myfirstsubnode]

vii. Check Status

Here, Status refers to the metadata of a specified znode. It consists of several details like Timestamp, Version number, ACL, Data length, and Children znode as well.
a. Syntax

stat /path

b. Sample

stat /FirstZnode

c. Output

[zk: localhost:2181(CONNECTED) 1] stat /FirstZnode
cZxid = 0x7f
ctime = Tue Sep 29 16:15:47 IST 2015
mZxid = 0x7f
mtime = Tue Sep 29 17:14:24 IST 2015
pZxid = 0x7f
cversion = 0
dataVersion = 1
aclVersion = 0
ephemeralOwner = 0x0
dataLength = 23
numChildren = 0

viii. Remove a Znode

The operation, “Remove a Znode”, removes a specified znode and recursively all its children as well. However, only if such a znode is available, this would happen.
a. Syntax

rmr /path

b. Sample

rmr /FirstZnode

c. Output

[zk: localhost:2181(CONNECTED) 10] rmr /FirstZnode
[zk: localhost:2181(CONNECTED) 11] get /FirstZnode
Node does not exist: /FirstZnode

Though we can say except for the fact that it works only on znodes with no children, Delete (delete /path) command is same as remove command in Apache ZooKeeper Command-Line Interface.
So, this was all in ZooKeeper CLI. Hope you like our explanation of ZooKeeper Command-Line Operations.

4. Conclusion

Hence, in this ZooKeeper CLI tutorial, we discussed all the operations and the concept of ZooKeeper Command Line Interface. Hope it helps. Still, if any doubt regarding Apache ZooKeeper CLI, ask in the comment tab.
For reference

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