January 2018
Beginner to intermediate
234 pages
7h 8m
Chinese
Content preview from Kafka权威指南
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84
｜
第
5
章
修改，而且
broker
要知道如何处理包含了两种消息格式的文件。
Kafka
附带了一个叫
DumpLogSe�ent
的工具，可以用它查看片段的内容。它可以显示每
个消息的偏移量、校验和、魔术数字节、消息大小和压缩算法。运行该工具的方法如下：
bin/kafka-run-class.sh kafka.tools.DumpLogSegments
如果使用了 --deep-iteration 参数，可以显示被压缩到包装消息里的消息。
5.5.4
索引
消费者可以从
Kafka
的任意可用偏移量位置开始读取消息。假设消费者要读取从偏移量
100
开始的
1MB
消息，那么
broker
必须立即定位到偏移量
100
（可能是在分区的任意一个片段
里），然后开始从这个位置读取消息。为了帮助
broker
更快地定位到指定的偏移量，
Kafka
为每个分区维护了一个索引。索引把偏移量映射到片段文件和偏移量在文件里的位置。
索引也被分成片段，所以在删除消息时，也可以删除相应的索引。
Kafka
不维护索引的
校验和。如果索引出现损坏，
Kafka
会通过重新读取消息并录制偏移量和位置来重新生
成索引。如果有必要，管理员可以删除索引，这样做是绝对安全的，
Kafka
会自动重新
生成这些索引。
5.5.5
清理
一般情况下，
Kafka
会根据设置的时间保留数据，把超过时效的旧数据删除掉。不过，试
想一下这样的场景，如果你使用
Kafka
保存客户的收货地址，那么保存客户的最新地址比
保存客户上周甚至去年的地址要有意义得多，这样你就不用担心会用错旧地址，而且短时 ...
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