Zookeeper Atomic Broadcast

Zookeeper

Before we dive into the Zookeeper Atomic Broadcast Protocol (ZAB), let’s take a look at some of the core constructs of Zookeeper.

Zookeeper is a distributed system used to implement coordination services for applications. Some of the examples of coordination services include leader election and config management. It provides a file system-based hierarchical key-value store storing data in a tree-like structure.

znodes

znodes provide the primary abstraction in Zookeeper to represent a node in the key-value store. A znode stores data in a byte array and can have child nodes. Each znode also includes an additional data structure called a stat, which consists of the following metadata:

• A transaction identifier (zxid) that created or modified the znode.

• A transaction identifier (zxid) that created or modified its children.

• Timestamp and version of the data change and child node change.

• The owner of the znode.

There are three types of znodes:

• Persistent znodes: These znodes remain permanent even after the client that created them dies.

• Ephemeral znodes: These znodes remain transient and remain valid only during the active session of the client. When the client dies, Zookeeper automatically deletes ephemeral znodes.

• Sequential znodes: Zookeeper automatically appends a sequential integer to ...