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ZooKeeper Programmer's Guide

Developing Distributed Applications that use ZooKeeper

Table of contents

1 Introduction........................................................................................................................ 3

2 The ZooKeeper Data Model.............................................................................................. 3

2.1 ZNodes..........................................................................................................................4

2.2 Time in ZooKeeper.......................................................................................................5

2.3 ZooKeeper Stat Structure............................................................................................. 6

3 ZooKeeper Sessions........................................................................................................... 7

4 ZooKeeper Watches......................................................................................................... 10

4.1 What ZooKeeper Guarantees about Watches............................................................. 11

4.2 Things to Remember about Watches..........................................................................11

5 ZooKeeper access control using ACLs............................................................................11

5.1 ACL Permissions........................................................................................................ 12

6 Pluggable ZooKeeper authentication............................................................................... 15

7 Consistency Guarantees................................................................................................... 17

8 Bindings............................................................................................................................18

8.1 Java Binding............................................................................................................... 18

8.2 C Binding....................................................................................................................19

9 Building Blocks: A Guide to ZooKeeper Operations......................................................21

9.1 Handling Errors...........................................................................................................21

9.2 Connecting to ZooKeeper...........................................................................................21

9.3 Read Operations..........................................................................................................21

ZooKeeper Programmer's Guide

Page 3

1 Introduction

This document is a guide for developers wishing to create distributed applications that

take advantage of ZooKeeper's coordination services. It contains conceptual and practical

information.

The first four sections of this guide present higher level discussions of various ZooKeeper

concepts. These are necessary both for an understanding of how ZooKeeper works as well

how to work with it. It does not contain source code, but it does assume a familiarity with the

problems associated with distributed computing. The sections in this first group are:

• The ZooKeeper Data Model

• ZooKeeper Sessions

• ZooKeeper Watches

• Consistency Guarantees

The next four sections provide practical programming information. These are:

• Building Blocks: A Guide to ZooKeeper Operations

• Bindings

• Program Structure, with Simple Example [tbd]

• Gotchas: Common Problems and Troubleshooting

The book concludes with an appendix containing links to other useful, ZooKeeper-related

information.

Most of information in this document is written to be accessible as stand-alone reference

material. However, before starting your first ZooKeeper application, you should probably at

least read the chaptes on the ZooKeeper Data Model and ZooKeeper Basic Operations. Also,

the Simple Programmming Example [tbd] is helpful for understanding the basic structure of

a ZooKeeper client application.

2 The ZooKeeper Data Model

ZooKeeper has a hierarchal name space, much like a distributed file system. The only

difference is that each node in the namespace can have data associated with it as well as

children. It is like having a file system that allows a file to also be a directory. Paths to nodes

are always expressed as canonical, absolute, slash-separated paths; there are no relative

reference. Any unicode character can be used in a path subject to the following constraints:

• The null character (\u0000) cannot be part of a path name. (This causes problems with the

C binding.)

• The following characters can't be used because they don't display well, or render in

confusing ways: \u0001 - \u0019 and \u007F - \u009F.

ZooKeeper Programmer's Guide

Page 4

• The following characters are not allowed: \ud800 -uF8FFF, \uFFF0-uFFFF, \uXFFFE -

\uXFFFF (where X is a digit 1 - E), \uF0000 - \uFFFFF.

• The "." character can be used as part of another name, but "." and ".." cannot alone be

used to indicate a node along a path, because ZooKeeper doesn't use relative paths. The

following would be invalid: "/a/b/./c" or "/a/b/../c".

• The token "zookeeper" is reserved.

2.1 ZNodes

Every node in a ZooKeeper tree is referred to as a znode. Znodes maintain a stat structure

that includes version numbers for data changes, acl changes. The stat structure also has

timestamps. The version number, together with the timestamp allow ZooKeeper to validate

the cache and to coordinate updates. Each time a znode's data changes, the version number

increases. For instance, whenever a client retrieves data, it also receives the version of the

data. And when a client performs an update or a delete, it must supply the version of the

data of the znode it is changing. If the version it supplies doesn't match the actual version

of the data, the update will fail. (This behavior can be overridden. For more information

see... )[tbd...]

Note:

In distributed application engineering, the word node can refer to a generic host machine, a server, a

member of an ensemble, a client process, etc. In the ZooKeeper documentation, znodes refer to the

data nodes. Servers refer to machines that make up the ZooKeeper service; quorum peers refer to

the servers that make up an ensemble; client refers to any host or process which uses a ZooKeeper

service.

Znodes are the main enitity that a programmer access. They have several characteristics that

are worth mentioning here.

2.1.1 Watches

Clients can set watches on znodes. Changes to that znode trigger the watch and then clear the

watch. When a watch triggers, ZooKeeper sends the client a notification. More information

about watches can be found in the section ZooKeeper Watches.

2.1.2 Data Access

The data stored at each znode in a namespace is read and written atomically. Reads get all the

data bytes associated with a znode and a write replaces all the data. Each node has an Access

Control List (ACL) that restricts who can do what.

ZooKeeper was not designed to be a general database or large object store. Instead,

it manages coordination data. This data can come in the form of configuration, status

ZooKeeper Programmer's Guide

Page 5

information, rendezvous, etc. A common property of the various forms of coordination data

is that they are relatively small: measured in kilobytes. The ZooKeeper client and the server

implementations have sanity checks to ensure that znodes have less than 1M of data, but the

data should be much less than that on average. Operating on relatively large data sizes will

cause some operations to take much more time than others and will affect the latencies of

some operations because of the extra time needed to move more data over the network and

onto storage media. If large data storage is needed, the usually pattern of dealing with such

data is to store it on a bulk storage system, such as NFS or HDFS, and store pointers to the

storage locations in ZooKeeper.

2.1.3 Ephemeral Nodes

ZooKeeper also has the notion of ephemeral nodes. These znodes exists as long as the

session that created the znode is active. When the session ends the znode is deleted. Because

of this behavior ephemeral znodes are not allowed to have children.

2.1.4 Sequence Nodes -- Unique Naming

When creating a znode you can also request that ZooKeeper append a monotonically

increasing counter to the end of path. This counter is unique to the parent znode. The counter

has a format of %010d -- that is 10 digits with 0 (zero) padding (the counter is formatted in

this way to simplify sorting), i.e. "<path>0000000001". See Queue Recipe for an example

use of this feature. Note: the counter used to store the next sequence number is a signed int

(4bytes) maintained by the parent node, the counter will overflow when incremented beyond

2147483647 (resulting in a name "<path>-2147483647").

2.2 Time in ZooKeeper

ZooKeeper tracks time multiple ways:

• Zxid

Every change to the ZooKeeper state receives a stamp in the form of a zxid (ZooKeeper

Transaction Id). This exposes the total ordering of all changes to ZooKeeper. Each

change will have a unique zxid and if zxid1 is smaller than zxid2 then zxid1 happened

before zxid2.

• Version numbers

Every change to a a node will cause an increase to one of the version numbers of that

node. The three version numbers are version (number of changes to the data of a znode),

cversion (number of changes to the children of a znode), and aversion (number of

changes to the ACL of a znode).

• Ticks

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内容反馈

ydw0603

2013-06-18

不错，可以作为zookeeper 参考感谢楼主
strongspeedmeihui12

2014-08-05

官方文档，多看看
javafan80130

2013-09-18

比较不错，可以看懂，要是文档再多一些更好啦
he1109he

2013-12-17

不错的参考资料，谢谢。
gengqi88

2013-11-15

官方文档。一般般

前往

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gavinh2008

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