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In this example we are going to transfer an image file to the server using a gRPC client. We will be using client-side streams technique, so file will be delivered as in small chunks. Once all the chunks are delivered to server, it will be saved. Server will return its unique name back to client.


I have hard-coded some variables in the code but you should use environment variables. Also it is open for improvements. I tried to keep it as short as possible!


Protocol Buffer


Run protoc --go_out=plugins=grpc:. --go_opt=paths=source_relative pkg/proto/*.proto command to generate compiled file.


// pkg/proto/upload.proto

syntax = "proto3";

package proto;

option go_package = ".;uploadpb";

service UploadService {
rpc Upload(stream UploadRequest) returns (UploadResponse) {}
}

message UploadRequest {
string mime = 1;
bytes chunk = 2;
}

message UploadResponse {
string name = 1;
}

Client


main.go


package main

import (
"context"
"flag"
"log"

"github.com/you/transfer/internal/upload"

"google.golang.org/grpc"
)

func main() {
// Catch user input.
flag.Parse()
if flag.NArg() == 0 {
log.Fatalln("Missing file path")
}

// Initialise gRPC connection.
conn, err := grpc.Dial(":50051", grpc.WithInsecure())
if err != nil {
log.Fatalln(err)
}
defer conn.Close()

// Start uploading the file. Error if failed, otherwise echo download URL.
client := upload.NewClient(conn)
name, err := client.Upload(context.Background(), flag.Arg(0))
if err != nil {
log.Fatalln(err)
}
log.Println(name)
}

upload.go


package upload

import (
"context"
"io"
"os"
"time"

"google.golang.org/grpc"

uploadpb "github.com/you/transfer/pkg/proto"
)

type Client struct {
client uploadpb.UploadServiceClient
}

func NewClient(conn grpc.ClientConnInterface) Client {
return Client{
client: uploadpb.NewUploadServiceClient(conn),
}
}

func (c Client) Upload(ctx context.Context, file string) (string, error) {
ctx, cancel := context.WithDeadline(ctx, time.Now().Add(10*time.Second))
defer cancel()

stream, err := c.client.Upload(ctx)
if err != nil {
return "", err
}

fil, err := os.Open(file)
if err != nil {
return "", err
}

// Maximum 1KB size per stream.
buf := make([]byte, 1024)

for {
num, err := fil.Read(buf)
if err == io.EOF {
break
}
if err != nil {
return "", err
}

if err := stream.Send(&uploadpb.UploadRequest{Chunk: buf[:num]}); err != nil {
return "", err
}
}

res, err := stream.CloseAndRecv()
if err != nil {
return "", err
}

return res.GetName(), nil
}

Server


main.go


package main

import (
"log"
"net"

"github.com/you/transfer/internal/storage"
"github.com/you/transfer/internal/upload"

"google.golang.org/grpc"

uploadpb "github.com/you/transfer/pkg/proto"
)

func main() {
// Initialise TCP listener.
lis, err := net.Listen("tcp", ":50051")
if err != nil {
log.Fatal(err)
}
defer lis.Close()

// Bootstrap upload server.
uplSrv := upload.NewServer(storage.New("tmp/"))

// Bootstrap gRPC server.
rpcSrv := grpc.NewServer()

// Register and start gRPC server.
uploadpb.RegisterUploadServiceServer(rpcSrv, uplSrv)
log.Fatal(rpcSrv.Serve(lis))
}

upload.go


package upload

import (
"io"

"github.com/you/transfer/internal/storage"

"google.golang.org/grpc/codes"
"google.golang.org/grpc/status"

uploadpb "github.com/you/transfer/pkg/proto"
)

type Server struct {
storage storage.Manager
}

func NewServer(storage storage.Manager) Server {
return Server{
storage: storage,
}
}

func (s Server) Upload(stream uploadpb.UploadService_UploadServer) error {
name := "some-unique-name.png"
file := storage.NewFile(name)

for {
req, err := stream.Recv()
if err == io.EOF {
if err := s.storage.Store(file); err != nil {
return status.Error(codes.Internal, err.Error())
}

return stream.SendAndClose(&uploadpb.UploadResponse{Name: name})
}
if err != nil {
return status.Error(codes.Internal, err.Error())
}

if err := file.Write(req.GetChunk()); err != nil {
return status.Error(codes.Internal, err.Error())
}
}
}

file.go


package storage

import (
"bytes"
)

type File struct {
name string
buffer *bytes.Buffer
}

func NewFile(name string) *File {
return &File{
name: name,
buffer: &bytes.Buffer{},
}
}

func (f *File) Write(chunk []byte) error {
_, err := f.buffer.Write(chunk)

return err
}

storage.go


package storage

import (
"io/ioutil"
)

type Manager interface {
Store(file *File) error
}

var _ Manager = &Storage{}

type Storage struct {
dir string
}

func New(dir string) Storage {
return Storage{
dir: dir,
}
}

func (s Storage) Store(file *File) error {
if err := ioutil.WriteFile(s.dir+file.name, file.buffer.Bytes(), 0644); err != nil {
return err
}

return nil
}

Test


// Run server first
$ go run -race cmd/server/main.go

// Upload file
$ go run -race cmd/client/main.go ~/Desktop/test.png
2021/04/13 11:30:03 some-unique-name.png

If you check tmp folder, the some-unique-name.png file should be there now.