grpc是一个通用的rpc框架，用google实现，当然也有go语言的版本。在工作中主要用到这个库，所以看看源码加强自己对框架的了解。目前来说主要分析的都以go版本为主(并没有看其他语言版本).由于个人水平有限，代码中的有些思想也是个人揣测，难免有些错误，如果发现错误，还望帮忙指出。
2 源码目录浏览
grpc使用protobuf（google的序列化框架）作为通信协议，底层上使用http2作为其传输协议，grpc源码中自己实现了http2的服务端跟客户端，而并没有用net/http包。http2有很多特性能够高效的传输数据，具体特点可以看相关链接详细了解。grpc看名字大概能看出这些目录中代码是哪些关系，documentation目录是存放一些文档，benchmark是压测，credentials是验证，examples是例子，grpclb是负载均衡，grpclog是日志,health是服务健康检查,metadata是元数据(用户客户端给服务端传送一些特殊数据，具体可以看相关链接),naming目录是提供名字服务需要实现的接口(相当于一个dns),stats是统计信息,transport 传输层实现(主要是http2的客户端与服务端时实现, 不会详细说这个目录),还有其他一些比较无关紧要的目录就不一一介绍了。
3 客户端
在example目录中有两个比较简单的例子，就先从这里入手吧，
代码语言：javascript复制func main() {
// Set up a connection to the server.
//建立一个链接
conn, err := grpc.Dial(address, grpc.WithInsecure())
if err != nil {
log.Fatalf("did not connect: %v", err)
}
defer conn.Close()
c := pb.NewGreeterClient(conn)
// Contact the server and print out its response.
name := defaultName
if len(os.Args) > 1 {
name = os.Args[1]
}
//调用函数
r, err := c.SayHello(context.Background(), &pb.HelloRequest{Name: name})
if err != nil {
log.Fatalf("could not greet: %v", err)
}
log.Printf("Greeting: %s", r.Message)
}grcp.WithInsecure参数是在链接https服务端时不用检查服务端的证书(要是你相信服务端就不用检查).Dial函数对服务端建立一个连接， grpc.Dial函数:
代码语言：javascript复制func DialContext(ctx context.Context, target string, opts ...DialOption) (conn *ClientConn, err error) {
cc := &ClientConn{
target: target,
conns: make(map[Address]*addrConn),
}
cc.ctx, cc.cancel = context.WithCancel(context.Background())
defer func() {
select {
case <-ctx.Done():
conn, err = nil, ctx.Err()
default:
}
if err != nil {
cc.Close()
}
}()
//设置grpc的各种选项
for _, opt := range opts {
opt(&cc.dopts)
}
// Set defaults.
if cc.dopts.codec == nil {
//默认用protobuf编解码
cc.dopts.codec = protoCodec{}
}
if cc.dopts.bs == nil {
cc.dopts.bs = DefaultBackoffConfig
}
creds := cc.dopts.copts.TransportCredentials
//验证信息
if creds != nil && creds.Info().ServerName != "" {
cc.authority = creds.Info().ServerName
} else {
colonPos := strings.LastIndex(target, ":")
if colonPos == -1 {
colonPos = len(target)
}
cc.authority = target[:colonPos]
}
var ok bool
waitC := make(chan error, 1)
//启动一个goroutine启动名字服务器(类似dns)
go func() {
var addrs []Address
if cc.dopts.balancer == nil {
// Connect to target directly if balancer is nil.
// 如果没设置负载均衡器，则直接连接
addrs = append(addrs, Address{Addr: target})
} else {
var credsClone credentials.TransportCredentials
if creds != nil {
credsClone = creds.Clone()
}
config := BalancerConfig{
DialCreds: credsClone,
}
//启动负载均衡服务
if err := cc.dopts.balancer.Start(target, config); err != nil {
waitC <- err
return
}
ch := cc.dopts.balancer.Notify()
if ch == nil {
// There is no name resolver installed.
addrs = append(addrs, Address{Addr: target})
} else {
addrs, ok = <-ch
if !ok || len(addrs) == 0 {
waitC <- errNoAddr
return
}
}
}
for _, a := range addrs {
//给每个地址一个conn，连接池
if err := cc.resetAddrConn(a, false, nil); err != nil {
waitC <- err
return
}
}
close(waitC)
}()
var timeoutCh <-chan time.Time
if cc.dopts.timeout > 0 {
timeoutCh = time.After(cc.dopts.timeout)
}
select {
case <-ctx.Done():
return nil, ctx.Err()
case err := <-waitC:
if err != nil {
return nil, err
}
case <-timeoutCh:
return nil, ErrClientConnTimeout
}
// If balancer is nil or balancer.Notify() is nil, ok will be false here.
// The lbWatcher goroutine will not be created.
if ok {
go cc.lbWatcher()
}
return cc, nil
}通过dial这个函数，grpc已经建立了到服务端的连接，启动了自定义负载平衡（如果有的话）. pb.NewGreeterClient这行代码是通过protoc工具自动生成的，它包一个grpc连接包裹在一个struct内方便调用生成的客户端grpc调用代码。接下来grpc客户端调用SayHello向服务器发送rpc请求。
代码语言：javascript复制func (c *greeterClient) SayHello(ctx context.Context, in *HelloRequest, opts ...grpc.CallOption) (*HelloReply, error) {
out := new(HelloReply)
//调用实际的发送请求函数
err := grpc.Invoke(ctx, "/helloworld.Greeter/SayHello", in, out, c.cc, opts...)
if err != nil {
return nil, err
}
return out, nil
}
//最后主要是invoke函数
func invoke(ctx context.Context, method string, args, reply interface{}, cc *ClientConn, opts ...CallOption) (e error) {
c := defaultCallInfo
for _, o := range opts {
//调用之前的hook
if err := o.before(&c); err != nil {
return toRPCErr(err)
}
}
defer func() {
for _, o := range opts {
//执行完后的hook
o.after(&c)
}
}()
//trace相关代码
if EnableTracing {
c.traceInfo.tr = trace.New("grpc.Sent." methodFamily(method), method)
defer c.traceInfo.tr.Finish()
c.traceInfo.firstLine.client = true
if deadline, ok := ctx.Deadline(); ok {
c.traceInfo.firstLine.deadline = deadline.Sub(time.Now())
}
c.traceInfo.tr.LazyLog(&c.traceInfo.firstLine, false)
// TODO(dsymonds): Arrange for c.traceInfo.firstLine.remoteAddr to be set.
defer func() {
if e != nil {
c.traceInfo.tr.LazyLog(&fmtStringer{"%v", []interface{}{e}}, true)
c.traceInfo.tr.SetError()
}
}()
}
//统计相关代码
if stats.On() {
ctx = stats.TagRPC(ctx, &stats.RPCTagInfo{FullMethodName: method})
begin := &stats.Begin{
Client: true,
BeginTime: time.Now(),
FailFast: c.failFast,
}
stats.HandleRPC(ctx, begin)
}
defer func() {
//结束后的统计相关代码
if stats.On() {
end := &stats.End{
Client: true,
EndTime: time.Now(),
Error: e,
}
stats.HandleRPC(ctx, end)
}
}()
topts := &transport.Options{
Last: true,
Delay: false,
}
for {
var (
err error
t transport.ClientTransport
stream *transport.Stream
// Record the put handler from Balancer.Get(...). It is called once the
// RPC has completed or failed.
put func()
)
// TODO(zhaoq): Need a formal spec of fail-fast.
//传输层的配置
callHdr := &transport.CallHdr{
Host: cc.authority,
Method: method,
}
if cc.dopts.cp != nil {
callHdr.SendCompress = cc.dopts.cp.Type()
}
gopts := BalancerGetOptions{
BlockingWait: !c.failFast,
}
//得到传输成连接，在http2中一个传输单位是一个流。
t, put, err = cc.getTransport(ctx, gopts)
if err != nil {
// TODO(zhaoq): Probably revisit the error handling.
if _, ok := err.(*rpcError); ok {
return err
}
if err == errConnClosing || err == errConnUnavailable {
if c.failFast {
return Errorf(codes.Unavailable, "%v", err)
}
continue
}
// All the other errors are treated as Internal errors.
return Errorf(codes.Internal, "%v", err)
}
if c.traceInfo.tr != nil {
c.traceInfo.tr.LazyLog(&payload{sent: true, msg: args}, true)
}
// 发送请求
stream, err = sendRequest(ctx, cc.dopts.codec, cc.dopts.cp, callHdr, t, args, topts)
if err != nil {
if put != nil {
put()
put = nil
}
// Retry a non-failfast RPC when
// i) there is a connection error; or
// ii) the server started to drain before this RPC was initiated.
// 在这两种情况下重试，1 链接错误 2 在rpc初始化之前服务端已经开始服务
if _, ok := err.(transport.ConnectionError); ok || err == transport.ErrStreamDrain {
if c.failFast {
return toRPCErr(err)
}
continue
}
return toRPCErr(err)
}
//收消息
err = recvResponse(ctx, cc.dopts, t, &c, stream, reply)
if err != nil {
if put != nil {
put()
put = nil
}
if _, ok := err.(transport.ConnectionError); ok || err == transport.ErrStreamDrain {
if c.failFast {
return toRPCErr(err)
}
continue
}
return toRPCErr(err)
}
if c.traceInfo.tr != nil {
c.traceInfo.tr.LazyLog(&payload{sent: false, msg: reply}, true)
}
//关闭一个http2流
t.CloseStream(stream, nil)
if put != nil {
put()
put = nil
}
//Errorf会判断返回十分ok
return Errorf(stream.StatusCode(), "%s", stream.StatusDesc())
}
}在这个函数最主要是两个函数，一个是sendRequest,一个是recvResponse,首先看看sendRequest函数：
代码语言：javascript复制func sendRequest(ctx context.Context, codec Codec, compressor Compressor, callHdr *transport.CallHdr, t transport.ClientTransport, args interface{}, opts *transport.Options) (_ *transport.Stream, err error) {
// 创建一个http2流
stream, err := t.NewStream(ctx, callHdr)
if err != nil {
return nil, err
}
defer func() {
if err != nil {
// If err is connection error, t will be closed, no need to close stream here.
if _, ok := err.(transport.ConnectionError); !ok {
t.CloseStream(stream, err)
}
}
}()
var (
cbuf *bytes.Buffer
outPayload *stats.OutPayload
)
//压缩不为空
if compressor != nil {
cbuf = new(bytes.Buffer)
}
//统计
if stats.On() {
outPayload = &stats.OutPayload{
Client: true,
}
}
//编码并压缩数据
outBuf, err := encode(codec, args, compressor, cbuf, outPayload)
if err != nil {
return nil, Errorf(codes.Internal, "grpc: %v", err)
}
//写入流
err = t.Write(stream, outBuf, opts)
if err == nil && outPayload != nil {
outPayload.SentTime = time.Now()
stats.HandleRPC(ctx, outPayload)
}
// t.NewStream(...) could lead to an early rejection of the RPC (e.g., the service/method
// does not exist.) so that t.Write could get io.EOF from wait(...). Leave the following
// recvResponse to get the final status.
if err != nil && err != io.EOF {
return nil, err
}
// Sent successfully.
return stream, nil
}可以看到这个函数相当简单，做了两件事情，编码压缩数据并发送.再来看看recvResponse函数：
代码语言：javascript复制func recvResponse(ctx context.Context, dopts dialOptions, t transport.ClientTransport, c *callInfo, stream *transport.Stream, reply interface{}) (err error) {
// Try to acquire header metadata from the server if there is any.
defer func() {
if err != nil {
if _, ok := err.(transport.ConnectionError); !ok {
t.CloseStream(stream, err)
}
}
}()
c.headerMD, err = stream.Header()
if err != nil {
return
}
p := &parser{r: stream}
var inPayload *stats.InPayload
if stats.On() {
inPayload = &stats.InPayload{
Client: true,
}
}
for {
//一直读到流关闭
if err = recv(p, dopts.codec, stream, dopts.dc, reply, math.MaxInt32, inPayload); err != nil {
if err == io.EOF {
break
}
return
}
}
if inPayload != nil && err == io.EOF && stream.StatusCode() == codes.OK {
// TODO in the current implementation, inTrailer may be handled before inPayload in some cases.
// Fix the order if necessary.
stats.HandleRPC(ctx, inPayload)
}
c.trailerMD = stream.Trailer()
return nil
}
func recv(p *parser, c Codec, s *transport.Stream, dc Decompressor, m interface{}, maxMsgSize int, inPayload *stats.InPayload) error {
//接受数据
pf, d, err := p.recvMsg(maxMsgSize)
if err != nil {
return err
}
if inPayload != nil {
inPayload.WireLength = len(d)
}
if err := checkRecvPayload(pf, s.RecvCompress(), dc); err != nil {
return err
}
if pf == compressionMade {
//解压
d, err = dc.Do(bytes.NewReader(d))
if err != nil {
return Errorf(codes.Internal, "grpc: failed to decompress the received message %v", err)
}
}
if len(d) > maxMsgSize {
// TODO: Revisit the error code. Currently keep it consistent with java
// implementation.
return Errorf(codes.Internal, "grpc: received a message of %d bytes exceeding %d limit", len(d), maxMsgSize)
}
//数据解码
if err := c.Unmarshal(d, m); err != nil {
return Errorf(codes.Internal, "grpc: failed to unmarshal the received message %v", err)
}
if inPayload != nil {
inPayload.RecvTime = time.Now()
inPayload.Payload = m
// TODO truncate large payload.
inPayload.Data = d
inPayload.Length = len(d)
}
return nil
}这里可以看到一个recvRespon可能会处理多个返回，但是确实在同一个for循环中处理的，有点奇怪。客户端代码大概就是这个流程。代码来说不算太复杂。（主要不钻进http2的实现,刚开始我就去看http2，一头雾水） 其中还有重要的地方就是负载均衡，通过它我们可以根据算法自动选择要连接的ip跟地址，还有验证的使用
