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Apache ZooKeeper Essentials
Apache ZooKeeper Essentials

Apache ZooKeeper Essentials: A fast-paced guide to using Apache ZooKeeper to coordinate services in distributed systems

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Apache ZooKeeper Essentials

Chapter 2. Understanding the Inner Workings of Apache ZooKeeper

In the previous chapter, we learned about the general definition of a distributed system and why implementing coordination among components of such a system is hard. We learned how Apache ZooKeeper solves this problem, followed by how to install and configure it. In this chapter, we will read more about the internals and architecture of ZooKeeper. As such, we will cover the following topics here in this second chapter:

  • The architecture of the ZooKeeper service
  • The data model behind ZooKeeper
  • The operations supported by the ZooKeeper data model
  • Probing the inner workings of ZooKeeper

A top-down view of the ZooKeeper service

As you are aware, Apache ZooKeeper is a coordination service for distributed applications. It aims to solve the tough problems associated with the coordination of components in a distributed application. It does this by exposing a simple yet powerful interface of primitives. Applications can be designed on these primitives implemented through ZooKeeper APIs to solve the problems of distributed synchronization, cluster configuration management, group membership, and so on.

ZooKeeper is, in itself, a replicated and distributed application, with the intention to be run as a service, similar to the way we run DNS or any other centralized service. A view of the ZooKeeper service is shown in the following diagram:

A top-down view of the ZooKeeper service

A ZooKeeper service and how clients connect to the service

From the preceding diagram (the image is referenced from http://zookeeper.apache.org/doc/r3.4.6/zookeeperOver.html), you will see the replicated set of servers on which the ZooKeeper service...

The ZooKeeper data model

As defined by the ZooKeeper wiki, ZooKeeper allows distributed processes to coordinate with each other through a shared hierarchical namespace of data registers. The namespace looks quite similar to a Unix filesystem. The data registers are known as znodes in the ZooKeeper nomenclature. You can see examples of znodes in the following image:

The ZooKeeper data model

A ZooKeeper's hierarchical namespace

Here, you can see that znodes are organized hierarchically, much like a tree, as a standard filesystem. Some important points to take note of are as follows:

  • The root node has one child znode called /zoo, which in turn has three znodes.
  • Every znode in the ZooKeeper tree is identified by a path, and the path elements are separated by /.
  • The znodes are called data registers because they can store data. Thus, a znode can have children as well as data associated with it. It's analogous to having a filesystem that allows a file to also be a path.

The data in a znode is typically stored in...

The ZooKeeper operations

ZooKeeper's data model and its API support the following nine basic operations:

Operation

Description

create

Creates a znode in a specified path of the ZooKeeper namespace

delete

Deletes a znode from a specified path of the ZooKeeper namespace

exists

Checks if a znode exists in the path

getChildren

Gets a list of children of a znode

getData

Gets the data associated with a znode

setData

Sets/writes data into the data field of a znode

getACL

Gets the ACL of a znode

setACL

Sets the ACL in a znode

sync

Synchronizes a client's view of a znode with ZooKeeper

Let's look at the ZooKeeper operations mentioned in the preceding table using ZooKeeper Java shell:

  1. Create a znode called root with ThisIsTheRootNode as its data:
    [zk: localhost(CONNECTED) 0] create /root "ThisIsTheRootNode"
    Created /root
    
  2. Get the content of the just created znode root:
    [zk: localhost(CONNECTED) 1] get /root
    "ThisIsTheRootNode"
    ...

Watches and ZooKeeper operations

The write operations in ZooKeeper are atomic and durable. There is the guarantee of a successful write operation if it has been written to persistent storage on a majority of ZooKeeper's servers. However, the eventual consistency model of ZooKeeper permits reads to log the latest state of the ZooKeeper service, and the sync operation allows a client to be up-to-date with the most recent state of the ZooKeeper service.

The read operations in znodes, such as exists, getChildren, and getData, allow watches to be set on them. On the other hand, the watches triggered by znode's write operations, such as create, delete, and setData ACL operations do not participate in watches.

The following are the types of watch events that might occur during a znode state change:

  • NodeChildrenChanged: A znode's child is created or deleted
  • NodeCreated: A znode is created in a ZooKeeper path
  • NodeDataChanged: The data associated with a znode is updated
  • NodeDeleted: A znode...

The ZooKeeper access control lists

ZooKeeper's data model provides a mechanism to control the access to znodes using ACL. While creating a znode, the ACLs determine the permissions with respect to the various operations that you can perform on the znodes. The ZooKeeper ACL model is similar to the Unix/Linux file permissions in terms of permitting or preventing operations being done on a znode by setting/unsetting permission bits. However, the ZooKeeper node doesn't have the concept of ownership, which is present in the Unix/Linux filesystem. ACLs are determined on the basis of the authentication mechanism of the client and the ZooKeeper service.

ZooKeeper provides the following built-in authentication mechanisms based on ACLs:

  • World: This represents anyone who is connecting to the ZooKeeper service
  • Auth: This represents any authenticated user, but doesn't use any ID
  • Digest: This represents the username and password way of authentication
  • IP address: This represents authentication...

The ZooKeeper stat structure

Every znode in ZooKeeper's namespace has a stat structure associated with it, which is analogous to the stat structure of files in a Unix/Linux filesystem. The fields in the stat structure of a znode are shown as follows with their respective meanings:

  • cZxid: This is the transaction ID of the change that caused this znode to be created.
  • mZxid: This is the transaction ID of the change that last modified this znode.
  • pZxid: This is the transaction ID for a znode change that pertains to adding or removing children.
  • ctime: This denotes the creation time of a znode in milliseconds from epoch.
  • mtime: This denotes the last modification time of a znode in milliseconds from epoch.
  • dataVersion: This denotes the number of changes made to the data of this znode.
  • cversion: This denotes the number of changes made to the children of this znode.
  • aclVersion: This denotes the number of changes made to the ACL of this znode.
  • ephemeralOwner: This is the session ID of the znode&apos...

A top-down view of the ZooKeeper service


As you are aware, Apache ZooKeeper is a coordination service for distributed applications. It aims to solve the tough problems associated with the coordination of components in a distributed application. It does this by exposing a simple yet powerful interface of primitives. Applications can be designed on these primitives implemented through ZooKeeper APIs to solve the problems of distributed synchronization, cluster configuration management, group membership, and so on.

ZooKeeper is, in itself, a replicated and distributed application, with the intention to be run as a service, similar to the way we run DNS or any other centralized service. A view of the ZooKeeper service is shown in the following diagram:

A ZooKeeper service and how clients connect to the service

From the preceding diagram (the image is referenced from http://zookeeper.apache.org/doc/r3.4.6/zookeeperOver.html), you will see the replicated set of servers on which the ZooKeeper service...

The ZooKeeper data model


As defined by the ZooKeeper wiki, ZooKeeper allows distributed processes to coordinate with each other through a shared hierarchical namespace of data registers. The namespace looks quite similar to a Unix filesystem. The data registers are known as znodes in the ZooKeeper nomenclature. You can see examples of znodes in the following image:

A ZooKeeper's hierarchical namespace

Here, you can see that znodes are organized hierarchically, much like a tree, as a standard filesystem. Some important points to take note of are as follows:

  • The root node has one child znode called /zoo, which in turn has three znodes.

  • Every znode in the ZooKeeper tree is identified by a path, and the path elements are separated by /.

  • The znodes are called data registers because they can store data. Thus, a znode can have children as well as data associated with it. It's analogous to having a filesystem that allows a file to also be a path.

The data in a znode is typically stored in a byte format...

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Description

Whether you are a novice to ZooKeeper or already have some experience, you will be able to master the concepts of ZooKeeper and its usage with ease. This book assumes you to have some prior knowledge of distributed systems and high-level programming knowledge of C, Java, or Python, but no experience with Apache ZooKeeper is required.

Who is this book for?

Whether you are a novice to ZooKeeper or already have some experience, you will be able to master the concepts of ZooKeeper and its usage with ease.
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Publication date, Length, Edition, Language, ISBN-13
Publication date : Jan 28, 2015
Length: 168 pages
Edition : 1st
Language : English
ISBN-13 : 9781784391324

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Estimated delivery fee Deliver to Philippines

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(Includes tracking information)

Product Details

Publication date : Jan 28, 2015
Length: 168 pages
Edition : 1st
Language : English
ISBN-13 : 9781784391324

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Table of Contents

8 Chapters
1. A Crash Course in Apache ZooKeeper Chevron down icon Chevron up icon
2. Understanding the Inner Workings of Apache ZooKeeper Chevron down icon Chevron up icon
3. Programming with Apache ZooKeeper Chevron down icon Chevron up icon
4. Performing Common Distributed System Tasks Chevron down icon Chevron up icon
5. Administering Apache ZooKeeper Chevron down icon Chevron up icon
6. Decorating ZooKeeper with Apache Curator Chevron down icon Chevron up icon
7. ZooKeeper in Action Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

Customer reviews

Rating distribution
Full star icon Full star icon Full star icon Full star icon Half star icon 4.3
(4 Ratings)
5 star 50%
4 star 25%
3 star 25%
2 star 0%
1 star 0%
Oliver Draese Jan 08, 2019
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Good book to start ZK based development. Everything you need to know about ZK and surrounding APIs.
Amazon Verified review Amazon
The GuN Man Feb 03, 2016
Full star icon Full star icon Full star icon Full star icon Full star icon 5
This is a very nice book for a starter who is exploring Zookeeper. It gets you up and running in few hours of skimming through the pages with so much simplicity. Being new to ZK, I started reading O'Reilly book and it seemed pretty dry and soon lost interest. But, when I started with this book, ZK seemed soo much simpler and got a more practical overview of ZK after reading first few pages. So, I continued reading till the end.Now that I've read this book, the O'Reilly book started making much more sense and I use it as a quick reference.I sort of agree with the @Anon's review in that, the O'reilly book is more authoritative and can be used as a reference. However, if you are new to the subject, read "Apache ZooKeeper Essentials" first.
Amazon Verified review Amazon
Dick Dowdell Oct 03, 2017
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
It's a decent book, but pretty thin on programming techniques and examples. Not a lot more than the ZooKeeper Programming section on the Hadoop Web site.
Amazon Verified review Amazon
Anon Feb 27, 2015
Full star icon Full star icon Full star icon Empty star icon Empty star icon 3
Overall I think this is a worthy book. It is well written without a lot of filler. However the O'Reilly book is much better and more authoritative. Reading this one I was struck with déjà vu repeatedly since it reads like a summary of existing materials.
Amazon Verified review Amazon
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