The asynchronous Kazoo API relies on the IAsyncResult object which is returned by all the asynchronous methods. Callbacks can be added with the rawlink() method which works in a consistent manner whether threads or an asynchronous framework like gevent is used.
Kazoo utilizes a pluggable IHandler interface which abstracts the callback system to ensure it works consistently.
Creating a connection:
from kazoo.client import KazooClient from kazoo.handlers.gevent import SequentialGeventHandler zk = KazooClient(handler=SequentialGeventHandler()) # returns immediately event = zk.start_async() # Wait for 30 seconds and see if we're connected event.wait(timeout=30) if not zk.connected: # Not connected, stop trying to connect zk.stop() raise Exception("Unable to connect.")
In this example, the wait method is used on the event object returned by the start_async() method. A timeout is always used because its possible that we might never connect and that should be handled gracefully.
The SequentialGeventHandler is used when you want to use gevent. Kazoo doesn’t rely on gevents monkey patching and requires that you pass in the appropriate handler, the default handler is SequentialThreadingHandler.
All kazoo _async methods except for start_async() return an IAsyncResult instance. These instances allow you to see when a result is ready, or chain one or more callback functions to the result that will be called when it’s ready.
The callback function will be passed the IAsyncResult instance and should call the get() method on it to retrieve the value. This call could result in an exception being raised if the asynchronous function encountered an error. It should be caught and handled appropriately.
import sys from kazoo.exceptions import ConnectionLossException from kazoo.exceptions import NoAuthException def my_callback(async_obj): try: children = async_obj.get() do_something(children) except (ConnectionLossException, NoAuthException): sys.exit(1) # Both these statements return immediately, the second sets a callback # that will be run when get_children_async has its return value async_obj = zk.get_children_async("/some/node") async_obj.rawlink(my_callback)
The following CRUD methods all work the same as their synchronous counterparts except that they return an IAsyncResult object.