gee-rpc/doc/geerpc-day2.md
本文是7天用Go从零实现RPC框架GeeRPC的第二篇。
对 net/rpc 而言，一个函数需要能够被远程调用，需要满足如下五个条件：
更直观一些：
func (t *T) MethodName(argType T1, replyType *T2) error
根据上述要求，首先我们封装了结构体 Call 来承载一次 RPC 调用所需要的信息。
// Call represents an active RPC.
type Call struct {
Seq uint64
ServiceMethod string // format "<service>.<method>"
Args interface{} // arguments to the function
Reply interface{} // reply from the function
Error error // if error occurs, it will be set
Done chan *Call // Strobes when call is complete.
}
func (call *Call) done() {
call.Done <- call
}
为了支持异步调用，Call 结构体中添加了一个字段 Done，Done 的类型是 chan *Call，当调用结束时，会调用 call.done() 通知调用方。
接下来，我们将实现 GeeRPC 客户端最核心的部分 Client。
// 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 {
cc codec.Codec
opt *Option
sending sync.Mutex // protect following
header codec.Header
mu sync.Mutex // protect following
seq uint64
pending map[uint64]*Call
closing bool // user has called Close
shutdown bool // server has told us to stop
}
var _ io.Closer = (*Client)(nil)
var ErrShutdown = errors.New("connection is shut down")
// Close the connection
func (client *Client) Close() error {
client.mu.Lock()
defer client.mu.Unlock()
if client.closing {
return ErrShutdown
}
client.closing = true
return client.cc.Close()
}
// IsAvailable return true if the client does work
func (client *Client) IsAvailable() bool {
client.mu.Lock()
defer client.mu.Unlock()
return !client.shutdown && !client.closing
}
Client 的字段比较复杂：
Close 方法，而 shutdown 置为 true 一般是有错误发生。紧接着，实现和 Call 相关的三个方法。
func (client *Client) registerCall(call *Call) (uint64, error) {
client.mu.Lock()
defer client.mu.Unlock()
if client.closing || client.shutdown {
return 0, ErrShutdown
}
call.Seq = client.seq
client.pending[call.Seq] = call
client.seq++
return call.Seq, nil
}
func (client *Client) removeCall(seq uint64) *Call {
client.mu.Lock()
defer client.mu.Unlock()
call := client.pending[seq]
delete(client.pending, seq)
return call
}
func (client *Client) terminateCalls(err error) {
client.sending.Lock()
defer client.sending.Unlock()
client.mu.Lock()
defer client.mu.Unlock()
client.shutdown = true
for _, call := range client.pending {
call.Error = err
call.done()
}
}
对一个客户端端来说，接收响应、发送请求是最重要的 2 个功能。那么首先实现接收功能，接收到的响应有三种情况：
func (client *Client) receive() {
var err error
for err == nil {
var h codec.Header
if err = client.cc.ReadHeader(&h); err != nil {
break
}
call := client.removeCall(h.Seq)
switch {
case call == nil:
// it usually means that Write partially failed
// and call was already removed.
err = client.cc.ReadBody(nil)
case h.Error != "":
call.Error = fmt.Errorf(h.Error)
err = client.cc.ReadBody(nil)
call.done()
default:
err = client.cc.ReadBody(call.Reply)
if err != nil {
call.Error = errors.New("reading body " + err.Error())
}
call.done()
}
}
// error occurs, so terminateCalls pending calls
client.terminateCalls(err)
}
创建 Client 实例时，首先需要完成一开始的协议交换，即发送 Option 信息给服务端。协商好消息的编解码方式之后，再创建一个子协程调用 receive() 接收响应。
func NewClient(conn net.Conn, opt *Option) (*Client, error) {
f := codec.NewCodecFuncMap[opt.CodecType]
if f == nil {
err := fmt.Errorf("invalid codec type %s", opt.CodecType)
log.Println("rpc client: codec error:", err)
return nil, err
}
// send options with server
if err := json.NewEncoder(conn).Encode(opt); err != nil {
log.Println("rpc client: options error: ", err)
_ = conn.Close()
return nil, err
}
return newClientCodec(f(conn), opt), nil
}
func newClientCodec(cc codec.Codec, opt *Option) *Client {
client := &Client{
seq: 1, // seq starts with 1, 0 means invalid call
cc: cc,
opt: opt,
pending: make(map[uint64]*Call),
}
go client.receive()
return client
}
还需要实现 Dial 函数，便于用户传入服务端地址，创建 Client 实例。为了简化用户调用，通过 ...*Option 将 Option 实现为可选参数。
func parseOptions(opts ...*Option) (*Option, error) {
// if opts is nil or pass nil as parameter
if len(opts) == 0 || opts[0] == nil {
return DefaultOption, nil
}
if len(opts) != 1 {
return nil, errors.New("number of options is more than 1")
}
opt := opts[0]
opt.MagicNumber = DefaultOption.MagicNumber
if opt.CodecType == "" {
opt.CodecType = DefaultOption.CodecType
}
return opt, nil
}
// Dial connects to an RPC server at the specified network address
func Dial(network, address string, opts ...*Option) (client *Client, err error) {
opt, err := parseOptions(opts...)
if err != nil {
return nil, err
}
conn, err := net.Dial(network, address)
if err != nil {
return nil, err
}
// close the connection if client is nil
defer func() {
if client == nil {
_ = conn.Close()
}
}()
return NewClient(conn, opt)
}
此时，GeeRPC 客户端已经具备了完整的创建连接和接收响应的能力了，最后还需要实现发送请求的能力。
func (client *Client) send(call *Call) {
// make sure that the client will send a complete request
client.sending.Lock()
defer client.sending.Unlock()
// register this call.
seq, err := client.registerCall(call)
if err != nil {
call.Error = err
call.done()
return
}
// prepare request header
client.header.ServiceMethod = call.ServiceMethod
client.header.Seq = seq
client.header.Error = ""
// encode and send the request
if err := client.cc.Write(&client.header, call.Args); err != nil {
call := client.removeCall(seq)
// call may be nil, it usually means that Write partially failed,
// client has received the response and handled
if call != nil {
call.Error = err
call.done()
}
}
}
// Go invokes the function asynchronously.
// It returns the Call structure representing the invocation.
func (client *Client) Go(serviceMethod string, args, reply interface{}, done chan *Call) *Call {
if done == nil {
done = make(chan *Call, 10)
} else if cap(done) == 0 {
log.Panic("rpc client: done channel is unbuffered")
}
call := &Call{
ServiceMethod: serviceMethod,
Args: args,
Reply: reply,
Done: done,
}
client.send(call)
return call
}
// Call invokes the named function, waits for it to complete,
// and returns its error status.
func (client *Client) Call(serviceMethod string, args, reply interface{}) error {
call := <-client.Go(serviceMethod, args, reply, make(chan *Call, 1)).Done
return call.Error
}
Go 和 Call 是客户端暴露给用户的两个 RPC 服务调用接口，Go 是一个异步接口，返回 call 实例。Call 是对 Go 的封装，阻塞 call.Done，等待响应返回，是一个同步接口。至此，一个支持异步和并发的 GeeRPC 客户端已经完成。
第一天 GeeRPC 只实现了服务端，因此我们在 main 函数中手动模拟了整个通信过程，今天我们就将 main 函数中通信部分替换为今天的客户端吧。
startServer 没有发生变化。
func startServer(addr chan string) {
// pick a free port
l, err := net.Listen("tcp", ":0")
if err != nil {
log.Fatal("network error:", err)
}
log.Println("start rpc server on", l.Addr())
addr <- l.Addr().String()
geerpc.Accept(l)
}
在 main 函数中使用了 client.Call 并发了 5 个 RPC 同步调用，参数和返回值的类型均为 string。
func main() {
log.SetFlags(0)
addr := make(chan string)
go startServer(addr)
client, _ := geerpc.Dial("tcp", <-addr)
defer func() { _ = client.Close() }()
time.Sleep(time.Second)
// send request & receive response
var wg sync.WaitGroup
for i := 0; i < 5; i++ {
wg.Add(1)
go func(i int) {
defer wg.Done()
args := fmt.Sprintf("geerpc req %d", i)
var reply string
if err := client.Call("Foo.Sum", args, &reply); err != nil {
log.Fatal("call Foo.Sum error:", err)
}
log.Println("reply:", reply)
}(i)
}
wg.Wait()
}
运行结果如下：
start rpc server on [::]:50658
&{Foo.Sum 5 } geerpc req 3
&{Foo.Sum 1 } geerpc req 0
&{Foo.Sum 3 } geerpc req 1
&{Foo.Sum 2 } geerpc req 4
&{Foo.Sum 4 } geerpc req 2
reply: geerpc resp 1
reply: geerpc resp 5
reply: geerpc resp 3
reply: geerpc resp 2
reply: geerpc resp 4