ZooKeeper: The Four Letter Commands

Datetime:2016-08-23 00:31:56          Topic: ZooKeeper           Share

ZooKeeper responds to a small set of commands. Each command is composed of four letters. When we issue the commands to ZooKeeper via telnet or nc, we would get corresponding response.

Three of the more interesting commands: “stat” gives some general information about the server and connected clients, while “srvr” and “cons” give extended details on server and connections respectively.

conf: Print details about serving configuration.

cons: List full connection/session details for all clients connected to this server. Includes information on numbers of packets received/sent, session id, operation latencies, last operation performed, etc…

crst: Reset connection/session statistics for all connections.

dump: Lists the outstanding sessions and ephemeral nodes. This only works on the leader.

envi: Print details about serving environment

ruok: Tests if server is running in a non-error state. The server will respond with imok if it is running. Otherwise it will not respond at all.

A response of “imok” does not necessarily indicate that the server has joined the quorum, just that the server process is active and bound to the specified client port. Use “stat” for details on state wrt quorum and client connection information.

srst: Reset server statistics.

srvr: Lists full details for the server.

stat: Lists brief details for the server and connected clients.

wchs: Lists brief information on watches for the server.

wchc: Lists detailed information on watches for the server, by session. This outputs a list of sessions(connections) with associated watches (paths). Note, depending on the number of watches this operation may be expensive (ie impact server performance), use it carefully.

wchp: Lists detailed information on watches for the server, by path. This outputs a list of paths (znodes) with associated sessions. Note, depending on the number of watches this operation may be expensive (ie impact server performance), use it carefully.

mntr: Outputs a list of variables that could be used for monitoring the health of the cluster.

$ echo mntr | nc localhost 2181

zk_version 3.4.6-1569965, built on 02/20/2014 09:09 GMT

zk_avg_latency 0

zk_max_latency 209

zk_min_latency 0

zk_packets_received 2313012

zk_packets_sent 2331630

zk_num_alive_connections 1

zk_outstanding_requests 0

zk_server_state follower

zk_znode_count 17

zk_watch_count 0

zk_ephemerals_count 0

zk_approximate_data_size 1084

zk_open_file_descriptor_count 32

zk_max_file_descriptor_count 655360

The output is compatible with java properties format and the content may change over time (new keys added). Your scripts should expect changes.

ATTENTION: Some of the keys are platform specific and some of the keys are only exported by the Leader.

The output contains multiple lines with the following format: key \t value

Here’s an example of the ruok command:

$ echo ruok | nc 127.0.0.1 2181imok





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