Summary - gRPC

Combine server and client to write a gRPC service.

gRPC summary

The flow: write the contract in a .proto → generate Go code with protoc → on the server implement the interface, Register and Serve → on the client make calls with timeouts using NewClient + the generated client. Errors travel with status and codes.

Never change field numbers; mark removed ones reserved.

Interceptors

The gRPC counterpart of middleware is the interceptor. It is added to the server with the grpc.UnaryInterceptor(...) option and runs before/after every unary call:

func logla(ctx context.Context, req any, info *grpc.UnaryServerInfo, h grpc.UnaryHandler) (any, error) {
    bas := time.Now()
    resp, err := h(ctx, req)
    log.Println(info.FullMethod, time.Since(bas))
    return resp, err
}

Exercise Interceptor simulation

Define type Handler func(string) string. Write intercept(h Handler) Handler: print ">" before the call and "<" after, then return the result. Print the result of intercept(func(s string) string { return s + "!" })("go").

Show example solution

package main

import "fmt"

type Handler func(string) string

func intercept(h Handler) Handler {
    return func(s string) string {
        fmt.Println(">")
        r := h(s)
        fmt.Println("<")
        return r
    }
}

func main() {
    fmt.Println(intercept(func(s string) string { return s + "!" })("go"))
}

Test yourself

How does a client recognize a "resource not found" case?
- When the response is empty
- status.Code(err) == codes.NotFound
- err == nil
- From an HTTP 404 header
Show answer

status.Code(err) == codes.NotFound — gRPC error codes are read through the status package; NotFound is the code the server returns with status.Error(codes.NotFound, ...).

Error code

return nil, status.Error(codes.____, "ürün yok")

Show answer

return nil, status.Error(codes.NotFound, "ürün yok")

Group summary

.proto

Contract
protoc

Code generation
Interceptor

gRPC middleware
codes.DeadlineExceeded

Time ran out

Next lesson: Service skeleton

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