January 2018
Beginner to intermediate
234 pages
7h 8m
Chinese
Content preview from Kafka权威指南
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86
｜
第
5
章
5.5.7
被删除的事件
如果只为每个键保留最近的一个消息，那么当需要删除某个特定键所对应的所有消息时，
我们该怎么办？这种情况是有可能发生的，比如一个用户不再使用我们的服务，那么完全
可以把与这个用户相关的所有信息从系统中删除。
为了彻底把一个键从系统里删除，应用程序必须发送一个包含该键且值为 null 的消息。清
理线程发现该消息时，会先进行常规的清理，只保留值为 null 的消息。该消息（被称为
墓
碑消息
）会被保留一段时间，时间长短是可配置的。在这期间，消费者可以看到这个墓碑
消息，并且发现它的值已经被删除。于是，如果消费者往数据库里复制
Kafka
的数据，当
它看到这个墓碑消息时，就知道应该要把相关的用户信息从数据库里删除。在这个时间段
过后，清理线程会移除这个墓碑消息，这个键也将从
Kafka
分区里消失。重要的是，要留
给消费者足够多的时间，让他看到墓碑消息，因为如果消费者离线几个小时并错过了墓碑
消息，就看不到这个键，也就不知道它已经从
Kafka
里删除，从而也就不会去删除数据库
里的相关数据了。
5.5.8
何时会清理主题
就像
delete
策略不会删除当前活跃的片段一样，
compact
策略也不会对当前片段进行清理。
只有旧片段里的消息才会被清理。
在
0.10.0
和更早的版本里，
Kafka
会在包含脏记录的主题数量达到
50%
时进行清理。这样
做的目的是避免太过频繁的清理（因为清理会影响主题的读写性能），同时也避免存在太
多脏记录（因为它们会占用磁盘空间）。浪费
50%
的磁盘空间给主题存放脏记录，然后进 ...
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