January 2018
Beginner to intermediate
234 pages
7h 8m
Chinese
Content preview from Kafka权威指南
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90
｜
第
6
章
倍的磁盘空间。我们一般会在可用性和存储硬件之间作出权衡。
那么该如何确定一个主题需要几个副本呢？这要看主题的重要程度，以及你愿意付出多少
成本来换取可用性。有时候这与你的偏执程度也有点关系。
如果因
broker
重启导致的主题不可用是可接受的（这在集群里是很正常的行为），那么把
复制系数设为
1
就可以了。在作出这个权衡的时候，要确保这样不会对你的组织和用户造
成影响，因为你在节省了硬件成本的同时也降低了可用性。复制系数为
2
意味着可以容忍
1
个
broker
发生失效，看起来已经足够了。不过要记住，有时候
1
个
broker
发生失效会导
致集群不稳定（通常是旧版的
Kafka
），迫使你重启另一个
broker
——集群控制器。也就是
说，如果将复制系数设为
2
，就有可能因为重启等问题导致集群不可用。所以这是一个两
难的选择。
基于以上几点原因，我们建议在要求可用性的场景里把复制系数设为
3
。在大多数情况下，
这已经足够安全了——不过我们也见过有些银行使用
5
个副本，以防不测。
副本的分布也很重要。默认情况下，
Kafka
会确保分区的每个副本被放在不同的
broker
上。
不过，有时候这样仍然不够安全。如果这些
broker
处于同一个机架上，一旦机架的交换
机发生故障，分区就会不可用，这时候把复制系数设为多少都不管用。为了避免机架级别
的故障，我们建议把
broker
分布在多个不同的机架上，并使用 broker.rack 参数来为每个
broker
配置所在机架的名字。如果配置了机架名字，
Kafka
会保证分区的副本被分布在多
个机架上，从而获得更高的可用性。我们已经在第 ...
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