go-micro selector 选择器

　　selecor是一种基于registry的选择服务节点和标记它们状态的的机制，有Random、RoundRobin两种策略，代码如下：

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// Selector builds on the registry as a mechanism to pick nodes // and mark their status. This allows host pools and other things // to be built using various algorithms. type Selector interface {     Init(opts ...Option) error     Options() Options     // Select returns a function which should return the next node     Select(service string, opts ...SelectOption) (Next, error)     // Mark sets the success/error against a node     Mark(service string, node *registry.Node, err error)     // Reset returns state back to zero for a service     Reset(service string)     // Close renders the selector unusable     Close() error     // Name of the selector     String() string }   // Next is a function that returns the next node // based on the selector's strategy type Next func() (*registry.Node, error)   // Filter is used to filter a service during the selection process type Filter func([]*registry.Service) []*registry.Service   // Strategy is a selection strategy e.g random, round robin type Strategy func([]*registry.Service) Next

　　

　　cacheSelector：

　　1、以本地缓存作为服务节点信息的存储，可以快速提供节点信息

　　2、字段 ttl 作为缓存时间，默认是1分钟；过期了则重新去registry获取