// Server represents an RPC Server.
type Server struct {
	serviceMap sync.Map   // map[string]*service
	reqLock    sync.Mutex // protects freeReq
	freeReq    *Request   // 相当于 Request 对象池
	respLock   sync.Mutex // protects freeResp
	freeResp   *Response  // 相当于 Response 对象池
}

type Request struct {
	ServiceMethod string   // format: "Service.Method"
	Seq           uint64   // sequence number chosen by client
	next          *Request // for free list in Server
}

type Response struct {
	ServiceMethod string    // echoes that of the Request
	Seq           uint64    // echoes that of the request
	Error         string    // error, if any.
	next          *Response // for free list in Server
}

type methodType struct {
	sync.Mutex // protects counters
	method     reflect.Method
	ArgType    reflect.Type
	ReplyType  reflect.Type
	numCalls   uint
}

type service struct {
	name   string                 // name of service
	rcvr   reflect.Value          // receiver of methods for the service
	typ    reflect.Type           // type of the receiver
	method map[string]*methodType // registered methods
}

// ServerCodec为RPC会话的服务器端实现了RPC请求的读取和RPC响应的写入。 
// 服务器成对调用ReadRequestHeader和ReadRequestBody来读取来自连接的请求，并调用writerresponse来写回响应。 
// 服务器在完成连接后调用Close。 
// 可以使用nil参数调用ReadRequestBody，以强制读取并丢弃请求体。 有关并发访问的信息，请参阅NewClient的注释。 
type ServerCodec interface {
	ReadRequestHeader(*Request) error
	ReadRequestBody(any) error
	WriteResponse(*Response, any) error

	// Close can be called multiple times and must be idempotent.
	Close() error
}

// Client represents an RPC Client.
// There may be multiple outstanding Calls associated
// with a single Client, and a Client may be used by
// multiple goroutines simultaneously.
type Client struct {
	codec ClientCodec

	reqMutex sync.Mutex         // protects following
	request  Request            // Request pool

	mutex    sync.Mutex         // protects following
	seq      uint64
	pending  map[uint64]*Call   // 存储多个调用
	closing  bool               // user has called Close
	shutdown bool               // server has told us to stop
}

type Request struct {
	ServiceMethod string   // format: "Service.Method"
	Seq           uint64   // sequence number chosen by client
	next          *Request // for free list in Server
}

//一个ClientCodec实现RPC请求的写入和为RPC会话的客户端读取RPC响应。  
//客户端调用WriteRequest向连接写入请求  
//并成对调用ReadResponseHeader和ReadResponseBody  
//读取响应。方法完成时，客户机调用Close连接。
//ReadResponseBody可以用nil来调用参数强制读取响应体，然后丢弃。  
type ClientCodec interface {
	WriteRequest(*Request, any) error
	ReadResponseHeader(*Response) error
	ReadResponseBody(any) error

	Close() error
}