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Apache ZooKeeper Simple Watch Example
The Apache ZooKeeper coordination service helps you with maintaining configuration information, naming, providing distributed synchronization, and providing group services for distributed applications.
This project was inspired by the Hacker School Paper of the Week on Google's Chubby Lock Service for Loosely-Coupled Distributed Systems.
Here I expand on the "Simple Watch Client" provided in the ZooKeeper docs, and turn it into a full running example.
Read These First:
- http://zookeeper.apache.org/doc/trunk/zookeeperStarted.html (Download, configure, start, and use the CLI)
- http://wiki.wsmoak.net/cgi-bin/wiki.pl?ZooKeeperGettingStarted (My notes on getting started)
- http://zookeeper.apache.org/doc/trunk/javaExample.html (Detailed explanation of the code in this example project)
Prerequisites
Download an Apache ZooKeeper .tar.gz file and unpack it.
(I used Apache ZooKeeper 3.5.0-alpha and put it under /Applications.)
You'll need a running ZooKeeper server and a way to connect and modify znodes.
The quickest way to get that is to use the zkServer.sh and zkCli.sh scripts in the bin directory of the .tar.gz distribution.
I suggest running bin/zkServer.sh start-foreground so you can see the output (and not forget that it's running).
For more information on the server and command line interface scripts, see http://zookeeper.apache.org/doc/trunk/zookeeperStarted.html
Running This Example
Assuming you have the server and CLI running, in this project:
Modify the build.sh and run.sh files with your path to the ZooKeeper install. Then execute
$ ./build.sh
which compiles the two source code files, and
$ ./run.sh
which sets the CLASSPATH and runs the Executor class. The command it uses is:
$ java Executor localhost /stuff output.txt ./count.sh
The arguments are:
-
'localhost' - This tells it that the server is on localhost (at the default 2181 port)
-
'/stuff' - This is the znode it is going to watch
-
'output.txt' - The name of the text file it will update with the contents of the znode. (It just writes to the current directory.)
-
'./count.sh' - The program it will start/stop/restart. This is a trivial script that simply counts up with a five second delay, and displays the contents of its "configuration file".
Once the Watcher/Executor is running, you'll need to use a client to modify the znode you told it to 'watch' (/stuff in this case).
In the terminal window with your CLI, execute:
] create /stuff my_data
You should see the Watcher/Executor example run your program, './count.sh' in this case, which prints consecutive integers starting with 1.
You should also see the output.txt file appear, with the contents 'my_data' (from above when you created it). Now modify the znode:
] set /stuff other_data
You should see the Watcher/Executor example stop and start your program. In this case it will stop counting and then start over at 1.
You should also see the contents of the output.txt file change to 'other_data', the new contents of the znode.
Try setting the znode to another string of text and see what happens.
] delete /stuff
You should see the Watcher/Executor example say "Killing process" and the script should stop counting.
For more information on the command line interface, see http://zookeeper.apache.org/doc/trunk/zookeeperStarted.html
Sample Output
In the command line interface
[zk: localhost:2181(CONNECTED) 16] create /stuff my_data
Created /stuff
[zk: localhost:2181(CONNECTED) 17] set /stuff other_data
[zk: localhost:2181(CONNECTED) 18] delete /stuff
In the running the watcher example
2014-11-01 11:05:43,908 [myid:] - INFO [main-SendThread(localhost:2181):ClientCnxn$SendThread@1346] -
Session establishment complete on server localhost/127.0.0.1:2181, sessionid =
0x1496bca44fe0002, negotiated timeout = 4000
Starting child
Count: 1 using my_data
Count: 2 using my_data
Count: 3 using my_data
Count: 4 using my_data
Stopping child
Starting child
Count: 1 using other_data
Count: 2 using other_data
Count: 3 using other_data
Killing process
Cleanup
$ ./clean.sh - deletes the *.class files and the output.txt file
Other Info
There is a Maven-ized version of this project in the sibling simple-watch-mvn directory.
If you're having classpath problems (NoClassDefFoundError) see http://docs.oracle.com/javase/7/docs/technotes/tools/windows/classpath.html