How to programmatically detect which server in ensemble client is connected to?

Question

How to programmatically detect which server in a ZooKeeper ensemble a client is connected to?

I'm using the Apache Curator API and I am listening for state changes in connection by registering ConnectionStateListener. I would like to know which server in the ensemble a client is connected to when the client reconnects if the server it was connected to goes down.

Shalna

Answer 1

You can see this in the logs produced by Curator. In the example output below, the CuratorFramework client has been given 4 different ZooKeeper instances in the connectionString that it can connect to. As can be seen in the log, it choses the first:

21:13:45.384 [main] INFO  org.apache.curator.framework.imps.CuratorFrameworkImpl - Starting
21:13:45.386 [main] INFO  org.apache.zookeeper.ZooKeeper - Initiating client connection, connectString=127.0.0.1:2181,127.0.0.1:2182,127.0.0.1:2183,127.0.0.1:2184 sessionTimeout=60000 watcher=org.apache.curator.ConnectionState@2876f0c
21:13:45.388 [main-SendThread(127.0.0.1:2181)] INFO  org.apache.zookeeper.ClientCnxn - Opening socket connection to server 127.0.0.1/127.0.0.1:2181. Will not attempt to authenticate using SASL (unknown error)
21:13:45.388 [main-SendThread(127.0.0.1:2181)] INFO  org.apache.zookeeper.ClientCnxn - Socket connection established to 127.0.0.1/127.0.0.1:2181, initiating session
21:13:45.392 [main-SendThread(127.0.0.1:2181)] INFO  org.apache.zookeeper.ClientCnxn - Session establishment complete on server 127.0.0.1/127.0.0.1:2181, sessionid = 0x14aac461eb70004, negotiated timeout = 40000

In case the ZooKeeper server that the client has connected to crashes, you will also see the new server that the client connects to in the logs:

21:23:03.675 [main-SendThread(127.0.0.1:2182)] INFO  org.apache.zookeeper.ClientCnxn - Opening socket connection to server 127.0.0.1/127.0.0.1:2182. Will not attempt to authenticate using SASL (unknown error)
21:23:03.677 [main-SendThread(127.0.0.1:2182)] INFO  org.apache.zookeeper.ClientCnxn - Socket connection established to 127.0.0.1/127.0.0.1:2182, initiating session
21:23:03.697 [main-SendThread(127.0.0.1:2182)] INFO  org.apache.zookeeper.ClientCnxn - Session establishment complete on server 127.0.0.1/127.0.0.1:2182, sessionid = 0x14aac461eb70004, negotiated timeout = 40000
21:23:03.697 [main-EventThread] INFO  org.apache.curator.framework.state.ConnectionStateManager - State change: RECONNECTED

Minim

But, how do I get that information programmatically, using Curator and by extension Zookeeper API? – Shalna