Toy app to demonstrate that multiple clients taking part in a zookeeper leader election recipe can think they are leader at any one time.

Building

To build:

$ ../gradlew jar

This will create hanging-chad.jar in build/libs. This is a runnable jar containing all necessary dependencies.

Demonstrating the phenomena

Start a zookeeper server. The easiest way is to use docker to start one in a containers on your local machine.

$ docker run -p 2181:2181 jplock/zookeeper

Now open multiple terminals and start the hanging-chad program. One they should all connect to zookeeper and one of them will become leader. You will know which one is leader, because it will tell you so in a loop.

$ java -jar build/libs/hanging-chad.jar 
log4j:WARN No appenders could be found for logger (org.apache.curator.framework.imps.CuratorFrameworkImpl).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Leader selector started
I am leader
I am leader
I am leader
I am leader
I am leader
I am leader
...

Pause the leader process, using Ctrl-Z. The one of the other processes should take over leadership after the session timeout (30 seconds), and start printing out "I am leader". Now unpause the first process using fg. It will still print out "I am leader" until the zk thread can tell it otherwise.

I am leader
I am leader
^Z
[1]+  Stopped                 java -jar build/libs/hanging-chad.jar
$
$ fg
java -jar build/libs/hanging-chad.jar
I am leader
I am leader
I am leader
I am leader
I am leader
I am leader
I am leader
I am leader
I am leader
I am leader
Ooops, I'm no longer the leader