Lifecycle hooks enable you to perform custom actions by pausing instances as an Auto Scaling group launches or terminates them. For example, while your newly launched instance is paused, you could install or configure software on it.
AS在拉起一个新的实例时候，ELB如何判断何时可以承接业务流量处理？根据AMI创建实例完成，但有时候还需要有一些客户化的应用进行安装和配置，这个时候就需要AS的生命周期Hook了。
