ZooKeeper：分布式进程协调指南

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《ZooKeeper：分布式进程协调指南》是一本由Flavio Junqueira和Benjamin Reed合著的专业书籍，于2014年出版，享有版权。该书详细介绍了ZooKeeper这一在分布式系统中广泛应用的工具，专为理解和实现分布式进程间的协调与管理提供深入的教学资源。 ZooKeeper是一个开源的分布式协调服务，主要用于解决分布式应用中的一致性问题，如配置管理、命名服务、领导者选举、会话管理和分布式锁等。它通过一个中心化的服务器集群来维护数据的状态，并确保所有节点对数据的访问保持同步。这本书深入探讨了ZooKeeper的工作原理，包括其数据模型（Znode）、数据一致性模型（Zookeeper Atomic Broadcast）以及客户端API的使用。作者Flavio Junqueira和Benjamin Reed在书中可能涵盖了以下关键知识点： 1. **ZooKeeper架构**：介绍ZooKeeper的基于领导者-跟随者模式的架构，以及其主要组件（如ZooKeeper Server、ZooKeeper Ensemble）的功能和交互。 2. **数据模型**：解释ZNode的概念，如何存储和组织数据，以及它们的角色在协调过程中是如何运作的。 3. **数据一致性保证**：阐述ZooKeeper如何通过顺序写入、原子性更新和观察者模式确保分布式环境下的数据一致性。 4. **API和客户端编程**：讲解如何使用Java API或其他语言的客户端库来与ZooKeeper交互，包括创建、读取、删除节点，以及执行交易操作。 5. **故障恢复与容错机制**：讨论ZooKeeper如何处理节点失效，以及其数据复制策略，以保证服务的高可用性。 6. **示例和实战**：书中可能包含丰富的案例研究和实践场景，帮助读者理解如何在实际项目中部署和优化ZooKeeper。 7. **最佳实践和局限性**：分析使用ZooKeeper时需要考虑的关键因素，以及在特定场景下可能遇到的问题及其解决方案。《ZooKeeper Nutshell Handbook》作为O'Reilly Media的作品，提供了简洁易懂的指导，适合对分布式系统有基础了解，希望进一步深入学习ZooKeeper的开发者、系统管理员和技术团队。无论是用于教育、企业培训还是个人研究，这本书都是深入了解和掌握ZooKeeper不可或缺的参考资料。同时，读者可以通过官方在线资源获取最新版本的更新和错误修正信息。

The ZooKeeper Mission

Trying to explain what ZooKeeper does for us is like trying to explain what a screwdriver

can do for us. In very basic terms, a screwdriver allows us to turn or drive screws, but

putting it this way does not really express the power of the tool. It enables us to assemble

pieces of furniture and electronic devices, and in some cases hang pictures on the wall.

By giving some examples like this, we can give a sense of what can be done, but it is

certainly not exhaustive.

The argument for what a system like ZooKeeper can do for us is along the same lines:

it enables coordination tasks for distributed systems. A coordination task is a task in‐

volving multiple processes. Such a task can be for the purposes of cooperation or to

regulate contention. Cooperation means that processes need to do something together,

and processes take action to enable other processes to make progress. For example, in

typical master-worker architectures, the worker informs the master that it is available

to do work. The master consequently assigns tasks to the worker. Contention is different:

it refers to situations in which two processes cannot make progress concurrently, so one

must wait for the other. Using the same master-worker example, we really want to have

a single master, but multiple processes may try to become the master. The multiple

processes consequently need to implement mutual exclusion. We can actually think of

the task of acquiring mastership as the one of acquiring a lock: the process that acquires

the mastership lock exercises the role of master.

If you have any experience with multithreaded programs, you will recognize that there

are a lot of similar problems. In fact, having a number of processes running in the same

computer or across computers is conceptually not different at all. Synchronization

primitives that are useful in the context of multiple threads are also useful in the context

of distributed systems. One important difference, however, stems from the fact that

different computers do not share anything other than the network in a typical shared-

nothing architecture. While there are a number of message-passing algorithms to im‐

plement synchronization primitives, it is typically much easier to rely upon a component

that provides a shared store with some special ordering properties, like ZooKeeper does.

Coordination does not always take the form of synchronization primitives like leader

election or locks. Configuration metadata is often used as a way for a process to convey

what others should be doing. For example, in a master-worker system, workers need to

know the tasks that have been assigned to them, and this information must be available

even if the master crashes.

Let’s look at some examples where ZooKeeper has been useful to get a better sense of

where it is applicable:

4 | Chapter 1: Introduction

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ZooKeeper：分布式进程协调指南

ambari 集成dolphinscheduler 报错: Caused by: java.lang.IllegalArgumentException: Could not resolve placeholder 'zookeeper.quorum' in value "${zookeeper.quorum}"

storm.zookeeper.topology.auth.payloa怎么配置

The node /hbase is not in ZooKeeper. It should have been written by the master. Check the value configured in 'zookeeper.znode.parent'. There could be a mismatch with the one configured in the master.

INFO ZooKeeper audit is disabled. (org.apache.zookeeper.audit.ZKAuditProvider)

hbase shell报错The node /hbase is not in ZooKeeper. It should have been written by the master. Check the value configured in 'zookeeper.znode.parent'. There could be a mismatch with the one configured in the master.

Failed to start zookeeper.service: Unit zookeeper.service not found.

Unit zookeeper.service could not be found.

hbase-site.xml，zookeeper超时设置（zookeeper.session.timeout）

hbase.zookeeper.quorum

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