负载均衡
负载均衡算法：
- round-robin轮询法，所有server按顺序轮流分配
- minimum connection：first to choose minimize connection, whcih has minimum pressure. If the event takes a long time, use this approach
- hash
作用
1.解决单点故障：multi-server to keep healthy operation.
2.响应慢：增加负载均衡器和服务器，分摊访问压力，提高响应速度.
3.建立load balancer cluster to prevent load balancer fails.