Up to version 0.3 kazoo used the Python bindings to the Zookeeper C library. Unfortunately those bindings are fairly buggy and required a fair share of weird workarounds to interface with the native OS thread used in those bindings.
Starting with version 0.4 kazoo implements the entire Zookeeper wire protocol itself in pure Python. Doing so removed the need for the workarounds and made it much easier to implement the features missing in the C bindings.
Both the Kazoo handlers run 3 separate queues to help alleviate deadlock issues and ensure consistent execution order regardless of environment. The SequentialGeventHandler runs a separate greenlet for each queue that processes the callbacks queued in order. The SequentialThreadingHandler runs a separate thread for each queue that processes the callbacks queued in order (thus the naming scheme which notes they are sequential in anticipation that there could be handlers shipped in the future which don’t make this guarantee).
Callbacks are queued by type, the 3 types being:
This ensures that calls can be made to Zookeeper from any callback except for a state listener without worrying that critical session events will be blocked.
Its important to remember that if you write code that blocks in one of these functions then no queued functions of that type will be executed until the code stops blocking. If your code might block, it should run itself in a separate greenlet/thread so that the other callbacks can run.