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ZooKeeper
Learn about how you can use ZooKeeper to solve a large number of problems in System Design.
Intro
Coordinating distributed systems is hard. While processing power and scaling techniques have evolved dramatically, the fundamental problem remains: how do you orchestrate dozens or hundreds of servers to work together seamlessly? When these machines need to elect leaders, maintain consistent configurations, and detect failures in real time, you face the exact problems that ZooKeeper was designed to solve.
Released in 2008, ZooKeeper has aged, and numerous alternatives have emerged. Nevertheless, it remains central to the Apache ecosystem in particular.
Despite its age, understanding ZooKeeper teaches essential distributed systems concepts that apply even if you never use it directly. By learning how ZooKeeper handles coordination through simple primitives (hierarchical namespace, data nodes, and watches), you gain insights into solving universal problems like consensus, leader election, and configuration management.
Let's walk through how ZooKeeper works, when you should use it, and how it's evolving in today's landscape of distributed systems.
A Motivating Example
To understand why coordination is tough, let's start with an example. Imagine you're building a chat application.
Initially, your chat app runs on a single server. Life is simple. When Alice sends a message to Bob, both users are connected to the same server. The server knows exactly where to deliver the message - it's all in-memory, low latency, no coordination needed.
Single server chat app
ZooKeeper Basics
Data Model: ZNodes
Server Roles and Ensemble
Watches: Knowing When Things Change
Key Capabilities
ZooKeeper for Configuration Management
ZooKeeper for Service Discovery
ZooKeeper for Leader Election
ZooKeeper for Distributed Locks
How ZooKeeper Works
Consensus with ZAB
Strong Consistency Guarantees
Read and Write Operations
Sessions and Connection Management
Storage Architecture
Handling Failures
ZooKeeper in the Modern World
Current Usage in Major Distributed Systems
Alternatives to Consider
Limitations
So when should you use ZooKeeper then?
Smart Routing
Certain Infrastructure Design Problems
Durable Distributed Locks
Summary
References
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