Golang 1.4 net/rpc client源码解析
Golang 1.4 net/rpc client源码解析
net/rpc是golang标准库提供的rpc框架,下面我们重点看下net/rpc是如何实现的。 我本机源码安装路径在/usr/local/go,这net/rpc(golang 1.4版本)涉及到的相关代码主要有:
client.go
server.go
首先我们先从client.go,客户端入手看:
type ClientCodecinterface{// WriteRequest must be safe for concurrent use by multiple goroutines. WriteRequest(*Request,interface{}) error
ReadResponseHeader(*Response) error
ReadResponseBody(interface{}) error
Close() error
}
type Callstruct{ServiceMethodstring// The name of the service and method to call. Argsinterface{}// The argument to the function (*struct). Replyinterface{}// The reply from the function (*struct). Error error // After completion, the error status. Done chan *Call// Strobes when call is complete. Tracer*Trace// tracer }
type Clientstruct{
codec ClientCodec
reqMutex sync.Mutex// protects following
request Request
mutex sync.Mutex// protects following
seq uint64
pending map[uint64]*Call
closing bool// user has called Close
shutdown bool// server has told us to stop }
func (client *Client) send(call *Call){// client要想复用,保证线程安全,加上请求锁reqMutex是必须的。
client.reqMutex.Lock()
defer client.reqMutex.Unlock()// 这其实是针对map的另外一把锁,这样可以更细粒度的操作
client.mutex.Lock()// client如果外部调用关闭,那么call也是结束状态,之后我们再分析call.done() if client.shutdown || client.closing {
call.Error=ErrShutdown
client.mutex.Unlock()
call.done()return}// 重点来了!seq序号自增在把call请求暂存在pennding的map中,锁释放
seq := client.seq
client.seq++
client.pending[seq]= call
client.mutex.Unlock()// 这一块代码属于编码请求了,因为rpc涉及到调用具体是谁,所以需要把method传给rpc server// 这里的Seq是用于当server response的时候,seq从client->server,再从server->client,然后反查map,定位call对象使用的。
client.request.Seq= seq
client.request.ServiceMethod= call.ServiceMethod// inject tracer,这个请忽视。。。
client.request.Tracer= call.Tracer
err := client.codec.WriteRequest(&client.request, call.Args)if err !=nil{
client.mutex.Lock()
call = client.pending[seq]delete(client.pending, seq)
client.mutex.Unlock()if call !=nil{
call.Error= err
call.done()}}}
我们使用rpc的时候,都知道client是线程安全的,client其实是基于单个socket连接来,依赖channel来实现复用连接以及并行的。而临时的调用对象Call都是保存在Client的map中的,至于每个call怎么查找,也是根据seq序列号在请求server时候转发过去,之后response的时候,client根据返回的seq再反查结果的。不难看出,实现了ClientCodec之后就可以自定义rpc协议请求头和内容了。那么send���数中的Call对象是从哪里来的?
// 我们rpc请求的时候,调用就是这个方法,传入方法名,参数,获取返回等
func (client *Client)Call(serviceMethod string, args interface{}, reply interface{}) error {// Call里面调用了client.Go,然后返回一个chan,之后阻塞等待,这是基本的同步调用
call :=<-client.Go(serviceMethod, args, reply, make(chan *Call,1)).Donereturn call.Error}
func (client *Client)Go(serviceMethod string, args interface{}, reply interface{},done chan *Call)*Call{// 构建call对象
call :=new(Call)
call.ServiceMethod= serviceMethod
call.Args= args
call.Reply= reply
// 如果非外部传入call,自己构建 ifdone==nil{done= make(chan *Call,10)// buffered. }else{// If caller passes done != nil, it must arrange that // done has enough buffer for the number of simultaneous // RPCs that will be using that channel. If the channel // is totally unbuffered, it's best not to run at all. if cap(done)==0{
log.Panic("rpc: done channel is unbuffered")}}
call.Done=done// 发送请求
client.send(call)return call
}
在初始化client的时候,我们会指定ip,port等
// Dial connects to an RPC server at the specified network address.
func Dial(network, address string)(*Client, error){
conn, err := net.Dial(network, address)if err !=nil{returnnil, err
}returnNewClient(conn),nil}// 我们看到其实NewClient内部使用的默认的gob编码,gobClientCodes实现了Codec的接口
func NewClient(conn io.ReadWriteCloser)*Client{
encBuf := bufio.NewWriter(conn)
client :=&gobClientCodec{conn, gob.NewDecoder(conn), gob.NewEncoder(encBuf), encBuf}returnNewClientWithCodec(client)}// 当然也提供自定义的codec,你可以使用thrift协议、messagepack等来扩展 // codec to encode requests and decode responses.
func NewClientWithCodec(codec ClientCodec)*Client{
client :=&Client{
codec: codec,
pending: make(map[uint64]*Call),}
go client.input()return client
}
type gobClientCodec struct{
rwc io.ReadWriteCloser
dec *gob.Decoder
enc *gob.Encoder
encBuf *bufio.Writer}
最后,NewClient会后台开启一枚goroutine,就是接受server返回然后转发具体调用者了。
func (client *Client) input(){var err error
var response Responsefor err ==nil{// 二话不说先获取Response的头
response =Response{}
err = client.codec.ReadResponseHeader(&response)if err !=nil{break}// 头部中包含了序列号,用于定位pending map使用的
seq := response.Seq// 小粒度锁删除map,获取call对象
client.mutex.Lock()
call := client.pending[seq]delete(client.pending, seq)
client.mutex.Unlock()switch{// 如果pending找不到,那么肯定是异常了 case call ==nil:// We've got no pending call. That usually means that // WriteRequest partially failed, and call was already // removed; response is a server telling us about an // error reading request body. We should still attempt // to read error body, but there's no one to give it to.
err = client.codec.ReadResponseBody(nil)if err !=nil{
err = errors.New("reading error body: "+ err.Error())}// rpc 报错了,解不开什么的都有可能 case response.Error!="":// We've got an error response. Give this to the request; // any subsequent requests will get the ReadResponseBody // error if there is one.
call.Error=ServerError(response.Error)
err = client.codec.ReadResponseBody(nil)if err !=nil{
err = errors.New("reading error body: "+ err.Error())}
call.done()default:// 默认还是正常的处理,获取Body给Reply,让调用者可见
err = client.codec.ReadResponseBody(call.Reply)if err !=nil{
call.Error= errors.New("reading body "+ err.Error())}
call.done()}}// 如果有啥不可逆的异常,那么只能shutdown client了。全部退出吧 // Terminate pending calls.
client.reqMutex.Lock()
client.mutex.Lock()
client.shutdown =true
closing := client.closing
if err == io.EOF {if closing {
err =ErrShutdown}else{
err = io.ErrUnexpectedEOF}}// 之前pending的也一个个结束吧,避免调用者都等待 for _, call := range client.pending {
call.Error= err
call.done()}
client.mutex.Unlock()
client.reqMutex.Unlock()if debugLog && err != io.EOF &&!closing {
log.Println("rpc: client protocol error:", err)}}
最后call.done做了什么了,相比你也猜到:
// 把call对象传递给调用者,主要是获取内部的Error
func (call *Call)done(){select{case call.Done<- call:// ok default:// We don't want to block here. It is the caller's responsibility to make// sure the channel has enough buffer space. See comment in Go(). if debugLog {
log.Println("rpc: discarding Call reply due to insufficient Done chan capacity")}}}
大致的分析就结束了,但是完整的rpc框架,还应该包括,服务发现,服务降级,服务追踪,服务容错等, 服务发现:可以使用zk,以及配合client定制的方式实现
服务降级:可以在zk中指定服务质量,以及根据回馈系统来drop request
服务追踪:最近我在看Twitter的Zipkin和Google的Dapper,对核心rpc库修改的方式避免大量植入代码,但是golang要做到这点有点困难,一是AOP不好支持,所以现在只能考虑用侵入代码,有更好思路的可以联系我!
服务容错:因为input本身单连接请求获取server,有可能<-call一直不返回,导致业务大量hang,这个可以考虑加上一些channel的timeout特性来实现,只不过浪费了一些内存。
总体来说net/rpc还是一个不错的框架,但是几个地方需要考虑,一个是全局大锁reqMutex,另外是call对象会大量创建(可否考虑call pool等)
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