说明

gRPC暴露了两个拦截器接口，分别是：

grpc.UnaryServerInterceptor服务端拦截器
grpc.UnaryClientInterceptor客户端拦截器

基于两个拦截器可以针对性的定制公共模块的封装代码，比如warden/logging.go是通用日志逻辑。

分析

服务端拦截器

让我们先看一下grpc.UnaryServerInterceptor的声明，官方代码位置：

// UnaryServerInfo consists of various information about a unary RPC on
// server side. All per-rpc information may be mutated by the interceptor.
type UnaryServerInfo struct {
    // Server is the service implementation the user provides. This is read-only.
    Server interface{}
    // FullMethod is the full RPC method string, i.e., /package.service/method.
    FullMethod string
}
// UnaryHandler defines the handler invoked by UnaryServerInterceptor to complete the normal
// execution of a unary RPC. If a UnaryHandler returns an error, it should be produced by the
// status package, or else gRPC will use codes.Unknown as the status code and err.Error() as
// the status message of the RPC.
type UnaryHandler func(ctx context.Context, req interface{}) (interface{}, error)
// UnaryServerInterceptor provides a hook to intercept the execution of a unary RPC on the server. info
// contains all the information of this RPC the interceptor can operate on. And handler is the wrapper
// of the service method implementation. It is the responsibility of the interceptor to invoke handler
// to complete the RPC.
type UnaryServerInterceptor func(ctx context.Context, req interface{}, info *UnaryServerInfo, handler UnaryHandler) (resp interface{}, err error)

看起来很简单包括：

一个UnaryServerInfo结构体用于Server和FullMethod字段传递，Server为gRPC server的对象实例，FullMethod为请求方法的全名
一个UnaryHandler方法用于传递Handler，就是基于proto文件service内声明而生成的方法
一个UnaryServerInterceptor用于拦截Handler方法，可在Handler执行前后插入拦截代码

为了更形象的说明拦截器的执行过程，请看基于proto生成的以下代码代码位置：

func _Demo_SayHello_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
    in := new(HelloReq)
    if err := dec(in); err != nil {
        return nil, err
    }
    if interceptor == nil {
        return srv.(DemoServer).SayHello(ctx, in)
    }
    info := &grpc.UnaryServerInfo{
        Server:     srv,
        FullMethod: "/demo.service.v1.Demo/SayHello",
    }
    handler := func(ctx context.Context, req interface{}) (interface{}, error) {
        return srv.(DemoServer).SayHello(ctx, req.(*HelloReq))
    }
    return interceptor(ctx, in, info, handler)
}

这个_Demo_SayHello_Handler方法是关键，该方法会被包装为grpc.ServiceDesc结构，被注册到gRPC内部，具体可在生成的pb.go代码内查找s.RegisterService(&_Demo_serviceDesc, srv)。

当gRPC server收到一次请求时，首先根据请求方法从注册到server内的grpc.ServiceDesc找到该方法对应的Handler如：_Demo_SayHello_Handler并执行
_Demo_SayHello_Handler执行过程请看上面具体代码，当interceptor不为nil时，会将SayHello包装为grpc.UnaryHandler结构传递给interceptor

这样就完成了UnaryServerInterceptor的执行过程。那么_Demo_SayHello_Handler内的interceptor是如何注入到gRPC server内，则看下面这段代码官方代码位置：

// UnaryInterceptor returns a ServerOption that sets the UnaryServerInterceptor for the
// server. Only one unary interceptor can be installed. The construction of multiple
// interceptors (e.g., chaining) can be implemented at the caller.
func UnaryInterceptor(i UnaryServerInterceptor) ServerOption {
    return func(o *options) {
        if o.unaryInt != nil {
            panic("The unary server interceptor was already set and may not be reset.")
        }
        o.unaryInt = i
    }
}

请一定注意这方法的注释！！！

Only one unary interceptor can be installed. The construction of multiple interceptors (e.g., chaining) can be implemented at the caller.

gRPC本身只支持一个interceptor，想要多interceptors需要自己实现~~所以warden基于grpc.UnaryClientInterceptor实现了interceptor chain，请看下面代码代码位置：

// Use attachs a global inteceptor to the server.
// For example, this is the right place for a rate limiter or error management inteceptor.
func (s *Server) Use(handlers ...grpc.UnaryServerInterceptor) *Server {
    finalSize := len(s.handlers) + len(handlers)
    if finalSize >= int(_abortIndex) {
        panic("warden: server use too many handlers")
    }
    mergedHandlers := make([]grpc.UnaryServerInterceptor, finalSize)
    copy(mergedHandlers, s.handlers)
    copy(mergedHandlers[len(s.handlers):], handlers)
    s.handlers = mergedHandlers
    return s
}
// interceptor is a single interceptor out of a chain of many interceptors.
// Execution is done in left-to-right order, including passing of context.
// For example ChainUnaryServer(one, two, three) will execute one before two before three, and three
// will see context changes of one and two.
func (s *Server) interceptor(ctx context.Context, req interface{}, args *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
    var (
        i     int
        chain grpc.UnaryHandler
    )
    n := len(s.handlers)
    if n == 0 {
        return handler(ctx, req)
    }
    chain = func(ic context.Context, ir interface{}) (interface{}, error) {
        if i == n-1 {
            return handler(ic, ir)
        }
        i++
        return s.handlers[i](ic, ir, args, chain)
    }
    return s.handlers[0](ctx, req, args, chain)
}

很简单的逻辑：

warden server使用Use方法进行grpc.UnaryServerInterceptor的注入，而func (s *Server) interceptor本身就实现了grpc.UnaryServerInterceptor
func (s *Server) interceptor可以根据注册的grpc.UnaryServerInterceptor顺序从前到后依次执行

而warden在初始化的时候将该方法本身注册到了gRPC server，在NewServer方法内可以看到下面代码：

opt = append(opt, keepParam, grpc.UnaryInterceptor(s.interceptor))
s.server = grpc.NewServer(opt...)

如此完整的服务端拦截器逻辑就串联完成。

客户端拦截器

让我们先看一下grpc.UnaryClientInterceptor的声明，官方代码位置：

// UnaryInvoker is called by UnaryClientInterceptor to complete RPCs.
type UnaryInvoker func(ctx context.Context, method string, req, reply interface{}, cc *ClientConn, opts ...CallOption) error
// UnaryClientInterceptor intercepts the execution of a unary RPC on the client. invoker is the handler to complete the RPC
// and it is the responsibility of the interceptor to call it.
// This is an EXPERIMENTAL API.
type UnaryClientInterceptor func(ctx context.Context, method string, req, reply interface{}, cc *ClientConn, invoker UnaryInvoker, opts ...CallOption) error

看起来和服务端拦截器并没有什么太大的区别，比较简单包括：

一个UnaryInvoker表示客户端具体要发出的执行方法
一个UnaryClientInterceptor用于拦截Invoker方法，可在Invoker执行前后插入拦截代码

具体执行过程，请看基于proto生成的下面代码代码位置：

func (c *demoClient) SayHello(ctx context.Context, in *HelloReq, opts ...grpc.CallOption) (*google_protobuf1.Empty, error) {
    out := new(google_protobuf1.Empty)
    err := grpc.Invoke(ctx, "/demo.service.v1.Demo/SayHello", in, out, c.cc, opts...)
    if err != nil {
        return nil, err
    }
    return out, nil
}

当客户端调用SayHello时可以看到执行了grpc.Invoke方法，并且将fullMethod和其他参数传入，最终会执行下面代码官方代码位置：

// Invoke sends the RPC request on the wire and returns after response is
// received.  This is typically called by generated code.
//
// All errors returned by Invoke are compatible with the status package.
func (cc *ClientConn) Invoke(ctx context.Context, method string, args, reply interface{}, opts ...CallOption) error {
    // allow interceptor to see all applicable call options, which means those
    // configured as defaults from dial option as well as per-call options
    opts = combine(cc.dopts.callOptions, opts)
    if cc.dopts.unaryInt != nil {
        return cc.dopts.unaryInt(ctx, method, args, reply, cc, invoke, opts...)
    }
    return invoke(ctx, method, args, reply, cc, opts...)
}

其中的unaryInt即为客户端连接创建时注册的拦截器，使用下面代码注册官方代码位置：

// WithUnaryInterceptor returns a DialOption that specifies the interceptor for
// unary RPCs.
func WithUnaryInterceptor(f UnaryClientInterceptor) DialOption {
    return newFuncDialOption(func(o *dialOptions) {
        o.unaryInt = f
    })
}

需要注意的是客户端的拦截器在官方gRPC内也只能支持注册一个，与服务端拦截器interceptor chain逻辑类似warden在客户端拦截器也做了相同处理，并且在客户端连接时进行注册，请看下面代码代码位置：

// Use attachs a global inteceptor to the Client.
// For example, this is the right place for a circuit breaker or error management inteceptor.
func (c *Client) Use(handlers ...grpc.UnaryClientInterceptor) *Client {
    finalSize := len(c.handlers) + len(handlers)
    if finalSize >= int(_abortIndex) {
        panic("warden: client use too many handlers")
    }
    mergedHandlers := make([]grpc.UnaryClientInterceptor, finalSize)
    copy(mergedHandlers, c.handlers)
    copy(mergedHandlers[len(c.handlers):], handlers)
    c.handlers = mergedHandlers
    return c
}
// chainUnaryClient creates a single interceptor out of a chain of many interceptors.
//
// Execution is done in left-to-right order, including passing of context.
// For example ChainUnaryClient(one, two, three) will execute one before two before three.
func (c *Client) chainUnaryClient() grpc.UnaryClientInterceptor {
    n := len(c.handlers)
    if n == 0 {
        return func(ctx context.Context, method string, req, reply interface{},
            cc *grpc.ClientConn, invoker grpc.UnaryInvoker, opts ...grpc.CallOption) error {
            return invoker(ctx, method, req, reply, cc, opts...)
        }
    }
    return func(ctx context.Context, method string, req, reply interface{},
        cc *grpc.ClientConn, invoker grpc.UnaryInvoker, opts ...grpc.CallOption) error {
        var (
            i            int
            chainHandler grpc.UnaryInvoker
        )
        chainHandler = func(ictx context.Context, imethod string, ireq, ireply interface{}, ic *grpc.ClientConn, iopts ...grpc.CallOption) error {
            if i == n-1 {
                return invoker(ictx, imethod, ireq, ireply, ic, iopts...)
            }
            i++
            return c.handlers[i](ictx, imethod, ireq, ireply, ic, chainHandler, iopts...)
        }
        return c.handlers[0](ctx, method, req, reply, cc, chainHandler, opts...)
    }
}

如此完整的客户端拦截器逻辑就串联完成。

实现自己的拦截器

以服务端拦截器logging为例：

// serverLogging warden grpc logging
func serverLogging() grpc.UnaryServerInterceptor {
    return func(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
        // NOTE: handler执行之前的拦截代码：主要获取一些关键参数，如耗时计时、ip等
        // 如果自定义的拦截器只需要在handler执行后，那么可以直接执行handler
        startTime := time.Now()
        caller := metadata.String(ctx, metadata.Caller)
        if caller == "" {
            caller = "no_user"
        }
        var remoteIP string
        if peerInfo, ok := peer.FromContext(ctx); ok {
            remoteIP = peerInfo.Addr.String()
        }
        var quota float64
        if deadline, ok := ctx.Deadline(); ok {
            quota = time.Until(deadline).Seconds()
        }
        // call server handler
        resp, err := handler(ctx, req) // NOTE: 以具体执行的handler为分界线！！！
        // NOTE: handler执行之后的拦截代码：主要进行耗时计算、日志记录
        // 如果自定义的拦截器在handler执行后不需要逻辑，这可直接返回
        // after server response
        code := ecode.Cause(err).Code()
        duration := time.Since(startTime)
        // monitor
        statsServer.Timing(caller, int64(duration/time.Millisecond), info.FullMethod)
        statsServer.Incr(caller, info.FullMethod, strconv.Itoa(code))
        logFields := []log.D{
            log.KVString("user", caller),
            log.KVString("ip", remoteIP),
            log.KVString("path", info.FullMethod),
            log.KVInt("ret", code),
            // TODO: it will panic if someone remove String method from protobuf message struct that auto generate from protoc.
            log.KVString("args", req.(fmt.Stringer).String()),
            log.KVFloat64("ts", duration.Seconds()),
            log.KVFloat64("timeout_quota", quota),
            log.KVString("source", "grpc-access-log"),
        }
        if err != nil {
            logFields = append(logFields, log.KV("error", err.Error()), log.KV("stack", fmt.Sprintf("%+v", err)))
        }
        logFn(code, duration)(ctx, logFields...)
        return resp, err
    }
}

内置拦截器

自适应限流拦截器

更多关于自适应限流的信息，请参考：kratos 自适应限流

package grpc
import (
    pb "kratos-demo/api"
    "kratos-demo/internal/service"
    "github.com/go-kratos/kratos/pkg/conf/paladin"
    "github.com/go-kratos/kratos/pkg/net/rpc/warden"
    "github.com/go-kratos/kratos/pkg/net/rpc/warden/ratelimiter"
)
// New new a grpc server.
func New(svc *service.Service) *warden.Server {
    var rc struct {
        Server *warden.ServerConfig
    }
    if err := paladin.Get("grpc.toml").UnmarshalTOML(&rc); err != nil {
        if err != paladin.ErrNotExist {
            panic(err)
        }
    }
    ws := warden.NewServer(rc.Server)
    // 挂载自适应限流拦截器到 warden server，使用默认配置
    limiter := ratelimiter.New(nil)
    ws.Use(limiter.Limit())
    // 注意替换这里：
    // RegisterDemoServer方法是在"api"目录下代码生成的
    // 对应proto文件内自定义的service名字，请使用正确方法名替换
    pb.RegisterDemoServer(ws.Server(), svc)
    ws, err := ws.Start()
    if err != nil {
        panic(err)
    }
    return ws
}

扩展阅读

warden快速开始
 warden基于pb生成
 warden负载均衡
 warden服务发现

warden interceptor