This document provides an overview of Apache Zookeeper, a distributed coordination service. It discusses Zookeeper's data model, use of ZAB protocol for consensus, and common use cases such as naming, configuration management, and leader election. It also provides a brief Java client API example.

1. Binu George
www.globinch.com
www.java.globinch.com

2. Agenda
 Coordination headache in distributed applications
 What is zookeeper?
 Zookeeper Overview
 Zookeeper Service & Data Model
 ZAB (Zookeeper Atomic Broadcasting Protocol)
 Zookeeper Read, Write and Watch
 Zookeeper Use Cases
 Zookeeper Client API Example

3. Coordination headache in distributed
applications
 Problem of Detection
 Cluster Size Estimation
 Leader Election
 Group Membership
 Configuration Management
 Consensus among multiple parties is difficult in
dynamic distributed systems as members may
leave/fail/join anytime
 Consistency, Availability and Partition tolerance

4. What is Zookeeper?
 Apache Zookeeper is a highly consistent, scalable and
reliable cluster co-ordination service.
 Zookeeper is a CP system where Consistency and
Partition tolerance are guaranteed.
 Off-the-shelf support for implementing consensus,
group management, leader election etc.
 Zookeeper cluster is called Ensemble
 Ensemble has one leader and remaining followers
 Zookeeper is a Distributed Coordination Service for
Distributed Applications.
 Zookeeper is originally developed by Yahoo research
Team and became a top level Apache project.
Zookeeper is used by many enterprises including
Rackspace, Yahoo! and eBay.

5. Zookeeper Overview
 Zookeeper use Zookeeper Broadcasting Protocol (ZAB) for
ordering and integrity.
 Achieves High availability and performance via replication.
 Low latency and high throughput via in memory image of
data tree ,transaction logs and snapshots.
 Uses “quorum” model and will only be available if a
majority of servers are alive (by a strict majority of nodes).
 Zookeeper relies on a quorum for durability as well.
 Pipelined architecture of Zookeeper enables the execution
of operations from a single client in FIFO order.
 Ordered updates uses ZAB protocol (a leader-based atomic
broadcast protocol) for Ordering, Integrity , exactly once.
 Election of a new leader is followed by a synchronization
phase before any new changes are accepted.

6. Zookeeper Service & Data Model
 At any given time, one Zookeeper client is connected
to one Zookeeper server.
 If the node (to which client connected) fails, client
automatically connect to another server in ensemble
with transparent session transfer.
 Znode is an in-memory data node in Zookeeper Data
organized in a hierarchical namespace (data tree)
 The data model of ZooKeeper (znode) is essentially a
file system with a simplified API and associated meta
data and versions.
 Regular, Ephemeral and Sequential Znode are
supported.
 Each ZNode is using UNIX style notation. For
example, /A/B/C to denote the path to znode C, where
C has B as its parent and B has A as its parent.
 All znodes store data, and all znodes, except for
ephemeral znodes, can have children.

7. Zookeeper read/write/watch
 Clients make use of Zookeeper API to update/write
Zookeeper the data tree.
 All Updates are ordered and managed by leader with
unique zxid and are considered complete when a
quorum confirms the update.
 The leader executes update request and broadcasts
the change to the ZooKeeper state through Zab.
 Zab by default uses a simple majority quorums to
decide on a proposal.
 Read and Watch requests on the znode sent by a client
to a ZooKeeper server are processed locally.
 Watch –Allows client to get notification if an already
returned information gets updated.

8. Zookeeper read/write/watch

9. Zookeeper read/write/watch
 Read and Watch requests on the znode are
processed locally but, write requests are
propagated to leader server and go through a
consensus before a response is generated.
 Throughput of read request increases with
number of servers and throughput of write
requests, decreases with number of servers.

10. Zookeeper use cases
 ZooKeeper offers the library to create and manage
synchronization primitives.
 ZooKeeper avoids the single-point-of-failure. Typical
use cases ,
 Naming service
 Configuration management
 Synchronization
 Leader election
 Message Queue
 Notification system

11. Zookeeper Java Client API Examples
 A simple example.
 To check the complete example see at the below link.
 http://java.globinch.com/enterprise-
services/zookeeper/apache-zookeeper-explained-
tutorial-cases-zookeeper-java-api-examples/