January 2018
Beginner to intermediate
234 pages
7h 8m
Chinese
Content preview from Kafka权威指南
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4
｜
第
1
章
1.2
Kafka
登场
Kafka
就是为了解决上述问题而设计的一款基于发布与订阅的消息系统。它一般被称为
“分布式提交日志”或者“分布式流平台”。文件系统或数据库提交日志用来提供所有事务
的持久记录，通过重放这些日志可以重建系统的状态。同样地，
Kafka
的数据是按照一定
顺序持久化保存的，可以按需读取。此外，
Kafka
的数据分布在整个系统里，具备数据故
障保护和性能伸缩能力。
1.2.1
消息和批次
Kafka
的数据单元被称为
消息
。如果你在使用
Kafka
之前已经有数据库使用经验，那么可
以把消息看成是数据库里的一个“数据行”或一条“记录”。消息由字节数组组成，所以
对于
Kafka
来说，消息里的数据没有特别的格式或含义。消息可以有一个可选的元数据，
也就是键。键也是一个字节数组，与消息一样，对于
Kafka
来说也没有特殊的含义。当消
息以一种可控的方式写入不同的分区时，会用到键。最简单的例子就是为键生成一个一致
性散列值，然后使用散列值对主题分区数进行取模，为消息选取分区。这样可以保证具有
相同键的消息总是被写到相同的分区上。第
3
章将详细介绍键的用法。
为了提高效率，消息被分批次写入
Kafka
。
批次
就是一组消息，这些消息属于同一个主题
和分区。如果每一个消息都单独穿行于网络，会导致大量的网络开销，把消息分成批次传
输可以减少网络开销。不过，这要在时间延迟和吞吐量之间作出权衡：批次越大，单位时
间内处理的消息就越多，单个消息的传输时间就越长。批次数据会被压缩，这样可以提升
数据的传输和存储能力，但要做更多的计算处理。 ...
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