浅谈RAC中的负载均衡
一次和同事聊RAC中的负载均衡，聊到服务器端的分发机制时大家都说不清，再此总结一下以备后用。
一次和同事聊RAC中的负载均衡，聊到服务器端的分发机制时大家都说不清，再此总结一下以备后用。
负载均衡Load Balance
客户端load balance
服务器端load balance
客户端load balance
客户端发起应用请求时，首先通过tnsnames.ora中随机选择一个vip地址，，然后通过该vip地址发起请求。这个vip地址实际上就是连接高可用的核心。当客户端访问的实例宕机了，甚至vip已经漂移到其它存活节点，这次连接就失败了。通知服务（FAN）会立刻通知客户端采用其它的vip重新连接，客户端选择tnsnames.ora中的其它vip继续尝试，直到连接成功为止。如果不采用vip方式连接，而直接使用IP地址连接不行么？答案是行，但是连接失败后需要30秒才能通知到客户端，30秒的代价太大了，完全失去了高可用的意义。
客户端的load balance不会判断当前服务器的负载，它只是随机分发。包括11gR2推出的SCAN也不会判断服务器端负载，它按照注册的3个vip地址轮询分发。服务器端的负载是如何判断的呢，我们继续往下看。
服务器端load balance
服务器端的负载均衡是监听器remote listener和local listener来合作完成的。监听器将当前负载信息在LBA（Load Balance Advisor）中注册，并且可以根据服务时间和吞吐量来设定负载类型。如果是Web服务可以配置服务时间作为负载均衡的条件，如果是查询密集型操作可以配置吞吐量类型。
总结一下上面的描述，客户端通过vip来实现连接高可用，服务器端通过remote listener，local listener和LBA的配置实现load balance。
在load balance中有两个总要的概念：
FAN：Fast Application Notification
快速通知，主动推送机制。既通知集群之间的节点，也通知客户端端到集群。
LBA：Load Balance Advisor
负载均衡建议器
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