ZooKeeper ServiceGroup driver

The ZooKeeper ServiceGroup driver works by using ZooKeeper ephemeral nodes. ZooKeeper, in contrast to databases, is a distributed system. Its load is divided among several servers. At a compute worker node, after establishing a ZooKeeper session, it creates an ephemeral znode in the group directory. Ephemeral znodes have the same lifespan as the session. If the worker node or the nova-compute daemon crashes, or a network partition is in place between the worker and the ZooKeeper server quorums, the ephemeral znodes are removed automatically. The driver gets the group membership by running the ls command in the group directory.

To use the ZooKeeper driver, you must install ZooKeeper servers and client libraries. Setting up ZooKeeper servers is outside the scope of this article. For the rest of the article, assume these servers are installed, and their addresses and ports are 192.168.2.1:2181, 192.168.2.2:2181, 192.168.2.3:2181.

To use ZooKeeper, you must install client-side Python libraries on every nova node: python-zookeeper – the official Zookeeper Python binding and evzookeeper – the library to make the binding work with the eventlet threading model.

The relevant configuration snippet in the /etc/nova/nova.conf file on every node is:

servicegroup_driver="zk"

[zookeeper]
address="192.168.2.1:2181,192.168.2.2:2181,192.168.2.3:2181"