How to watch on descendant child nodes in Python using kazoo?

Question

I recently started working with Python for Zookeeper. I am using kazoo library for Zookeeper.

I have a very simple use case using kazoo for Zookeeper. I have a root node which is - /root. Now I need to keep a watch on the root node /root and if the new node which gets added to root node /root is /root/testing then I will only keep a watch on the /root/testing node. I don't want to keep a watch on any other node apart from the testing node. And then if any new children get added up to the /root/testing node, then I will keep a watch on them as well.

Let's say the child below that gets added up -

`/root/testing/test1`

Then I will keep a watch on test1 node as well.

Is this possible to do using Kazoo in Zookeeper? I am only able to keep a watch on one Zookeeper node (/root) with the code below:

#!/usr/bin/python
import time

from kazoo.client import KazooClient

zk = KazooClient(hosts='127.0.0.1:2181')
zk.start()

########################################
@zk.ChildrenWatch("/root/testing")
def watch_children(children):
   print(children)

########################################

while True:
    time.sleep(5)

Can anyone help me with this in making multiple watches on the child node?

score 2 · Answer 1 · answered Nov 25 '13 at 21:52

Try something like this.

import time
from kazoo.client import KazooClient

zk = KazooClient(hosts='127.0.0.1:2181')
zk.start()

children = zk.get_children("/root/",)
if "/testing" in children:
    children_testing = zk.get_children("/root/testing/")
        if children_testing != []: 
            @zk.ChildrenWatch("/root/testing")
            def watch_children(children):
                print(children)
        else:
            @zk.ChildrenWatch("/root/")
            def watch_children(children):
                print(children)
while True:
    time.sleep(5)

score 0 · Answer 2 · answered Nov 23 '13 at 22:27

If you are trying to watch over multiple nodes, rather than one, you could use multiple threads (basically it runs different code simultaneously on different "threads"). Python has a threading module for doing simultaneous actions, which may be exactly what you want. Here is an example of code with threads implemented.

import threading

def watch_node(node):
    #implement your node watching script here

def watch_in_background(node):
    watcher = threading.Thread(target=watch_node,args=(node))   #initializes a thread that can run the watch_node function in the background, passing the node argument to it
    watcher.start()                                             #tells the thread to start running
    return watcher                                              #returns the thread object just in case you want to use it for something

watch_in_background(<node>)                                     #so this code will now watch the node in the background, allowing you to do other things, like watch multiple nodes

Thanks for the help but my question is specifically related to kazoo library for zookeeper, I am not able to understand how to put a watches on parent child, and then its child as well. — arsenal, Nov 24 '13 at 05:45
The script that watches the nodes, using Kazoo, can be executed many times at once using threading. The use of Kazoo should not influence threading. — trevorKirkby, Nov 25 '13 at 00:22

How to watch on descendant child nodes in Python using kazoo?

2 Answers2