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Zookeeper - CLI
ZooKeeper Command Line Interface (CLI) is used to interact with the ZooKeeper ensemble for development purpose. It is useful for debugging and working around with 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 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 notification 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 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 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 watch option in get command (as in 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 remove command, except the fact that it works only on znodes with no children.