Blog

Zookeeper – CLI

ZooKeeper Command Line Interface (CLI) is used to interact with the ZooKeeper ensemble for development purposes. It is useful for debugging and working around different options.

To perform ZooKeeper CLI operations, first, turn on your ZooKeeper server (“bin/zkServer.sh start”) and then, ZooKeeper client (“bin/zkCli.sh”). Once the client starts, you can perform the following operation −

  • Create znodes
  • Get data
  • Watch znode for changes
  • Set data
  • Create children of a znode
  • List children of a znode
  • Check Status
  • Remove / Delete a znode

Now let us see the above command one by one with an example.

Create Znodes

Create a znode with the given path. The flag argument specifies whether the created znode will be ephemeral, persistent, or sequential. By default, all znodes are persistent.

  • Ephemeral znodes (flag: e) will be automatically deleted when a session expires or when the client disconnects.
  • Sequential znodes guaranty that the znode path will be unique.
  • ZooKeeper ensemble will add sequence number along with 10 digit padding to the znode path. 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.

Syntax

create /path /data

Sample

create /FirstZnode “Myfirstzookeeper-app”

Output

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

To create a Sequential znode, add -s flag as shown below.

Syntax

create -s /path /data

Sample

create -s /FirstZnode second-data

Output

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

To create an Ephemeral Znode, add -e flag as shown below.

Syntax

create -e /path /data

Sample

create -e /SecondZnode “Ephemeral-data”

Output

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

Remember when a client connection is lost, the ephemeral znode will be deleted. You can try it by quitting the ZooKeeper CLI and then re-opening the CLI.

Get Data

It returns the associated data of the znode and metadata of the specified znode. You will get information such as when the data was last modified, where it was modified, and information about the data. This CLI is also used to assign watches to show notifications about the data.

Syntax

get /path

Sample

get /FirstZnode

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

To access a sequential znode, you must enter the full path of the znode.

Sample

get /FirstZnode0000000023

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

Watch

Watches show a notification when the specified znode or znode’s children data changes. You can set a watch only in the get command.

Syntax

get /path [watch] 1

Sample

get /FirstZnode 1

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

The output is similar to the normal get command, but it will wait for znode changes in the background. <Start here>

Set Data

Set the data of the specified znode. Once you finish this set operation, you can check the data using the get CLI command.

Syntax

set /path /data

Sample

set /SecondZnode Data-updated

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 you assigned a watch option in get command (as in the previous command), then the output will be similar as shown below −

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

Create Children / Sub-znode

Creating children is similar to creating new znodes. The only difference is that the path of the child znode will have the parent path as well.

Syntax

create /parent/path/subnode/path /data

Sample

create /FirstZnode/Child1 firstchildren

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

List Children

This command is used to list and display the children of a znode.

Syntax

ls /path

Sample

ls /MyFirstZnode

Output

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

Check Status

Status describes the metadata of a specified znode. It contains details such as Timestamp, Version number, ACL, Data length, and Children znode.

Syntax

stat /path

Sample

stat /FirstZnode

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

Remove a Znode

Removes a specified znode and recursively all its children. This would happen only if such a znode is available.

Syntax

rmr /path

Sample

rmr /FirstZnode

Output

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

Delete (delete /path) command is similar to the remove command, except the fact that it works only on znodes with no children.

Leave a Reply