Go Academy · Lesson

gRPC Interceptors and Metadata

Auth, logging middleware and gRPC metadata

Lesson 4 of 413 steps

gRPC Interceptors and Metadata is a free Go Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Go Academy learning path, and your progress syncs across the web and the CoddyKit app. The Go Academy course includes 4 lessons in total.

What are interceptors?

gRPC interceptors (middleware) run before or after RPC handlers. They add cross-cutting concerns: logging, auth token validation, metrics, panic recovery, and tracing.

Unary server interceptor

A unary server interceptor wraps the handler with additional logic:

func LoggingInterceptor(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
    log.Printf("RPC: %s", info.FullMethod)
    resp, err := handler(ctx, req)
    log.Printf("done: %v", err)
    return resp, err
}

grpcSrv := grpc.NewServer(grpc.UnaryInterceptor(LoggingInterceptor))

Stream server interceptor

Similar to unary but wraps the stream handler:

func StreamLogging(srv interface{}, ss grpc.ServerStream, info *grpc.StreamServerInfo, handler grpc.StreamHandler) error {
    log.Printf("stream: %s", info.FullMethod)
    return handler(srv, ss)
}

Chaining interceptors

Use grpc.ChainUnaryInterceptor (Go gRPC v1.38+) to apply multiple interceptors in order:

grpcSrv := grpc.NewServer(
    grpc.ChainUnaryInterceptor(LoggingInterceptor, AuthInterceptor, MetricsInterceptor),
)

Client interceptors

Client-side unary interceptors wrap outgoing calls — useful for injecting auth tokens, retries, or tracing headers:

conn, _ := grpc.NewClient(addr,
    grpc.WithUnaryInterceptor(ClientAuthInterceptor),
    grpc.WithTransportCredentials(insecure.NewCredentials()),
)

gRPC metadata

Metadata is key-value pairs sent alongside an RPC, analogous to HTTP headers. Use metadata.New and metadata.NewOutgoingContext on the client.

md := metadata.New(map[string]string{"authorization": "Bearer " + token})
ctx := metadata.NewOutgoingContext(context.Background(), md)

Reading metadata on server

Extract incoming metadata with metadata.FromIncomingContext:

md, ok := metadata.FromIncomingContext(ctx)
if !ok { return nil, status.Error(codes.Unauthenticated, "no metadata") }
token := md["authorization"]

Sending metadata from server

Send response headers and trailers from the server with grpc.SetHeader and grpc.SetTrailer:

grpc.SetHeader(ctx, metadata.Pairs("x-request-id", requestID))

go-grpc-middleware

The github.com/grpc-ecosystem/go-grpc-middleware/v2 library provides ready-made interceptors: recovery, logging (zap/logrus), auth, and more.

Auth interceptor pattern

Extract and validate a token in an interceptor; attach the user to the context for downstream handlers:

func AuthInterceptor(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
    md, _ := metadata.FromIncomingContext(ctx)
    if !validateToken(md["authorization"]) {
        return nil, status.Error(codes.Unauthenticated, "invalid token")
    }
    return handler(ctx, req)
}

Panic recovery interceptor

Wrap handlers to recover from panics and return an Internal error instead of crashing:

defer func() {
    if r := recover(); r != nil {
        err = status.Errorf(codes.Internal, "panic: %v", r)
    }
}()

Quick Check

How do you pass authentication tokens between a gRPC client and server?

Answer it in the interactive lesson

Recap: gRPC Interceptors and Metadata

Key points:

Unary interceptor: func(ctx, req, info, handler) (resp, error)
grpc.ChainUnaryInterceptor for multiple interceptors
Metadata is key-value pairs; analogous to HTTP headers
Auth interceptor: extract metadata → validate → add user to context

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Frequently Asked Questions

Is the “gRPC Interceptors and Metadata” lesson free?

Yes — any 3 lessons of the Go Academy learning path, including “gRPC Interceptors and Metadata”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Go Academy course includes 4 lessons in total.

What will I learn in “gRPC Interceptors and Metadata”?

Auth, logging middleware and gRPC metadata You practise Go Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Go Academy?

No prior experience is required. Go Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “gRPC Interceptors and Metadata” lesson take?

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