How can I serve files while using GRPC - go

Question

Is there any way how to serve files in Go with GRPC, like in gin-gonic's variant:
router.Static("/static", "/var/www")

Answer 1

You can't do it exactly like that.
But you can use the proto bytes type and put the file bytes in that field.
Also (as pointed out in the comments) with large files you should use streaming instead of a unary call. (most GRPC implementation have a limit of 4MB per message).
Proto example:
syntax = "proto3";
message Response {
bytes fileChunk = 1;
}
message Request {
string fileName = 1;
}
service TestService {
rpc Download(Request) returns (stream Response);
}
Server implementation example:
func (srv *Server) Download(req *pbgo.Request, responseStream pbgo.TestService_DownloadServer) error {
bufferSize := 64 *1024 //64KiB, tweak this as desired
file, err := os.Open(req.GetFileName())
if err != nil {
fmt.Println(err)
return err
}
defer file.Close()
buff := make([]byte, bufferSize)
for {
bytesRead, err := file.Read(buff)
if err != nil {
if err != io.EOF {
fmt.Println(err)
}
break
}
resp := &pbgo.Response{
FileChunk: buff[:bytesRead],
}
err = responseStream.Send(resp)
if err != nil {
log.Println("error while sending chunk:", err)
return err
}
}
return nil
}
Client would call it like this:
conn, err := grpc.Dial("localhost:9090", grpc.WithInsecure())
if err != nil {
log.Fatal("client could connect to grpc service:", err)
}
c := pbgo.NewTestServiceClient(conn)
fileStreamResponse, err := c.Download(context.TODO(), &pbgo.Request{
FileName: "test.txt",
})
if err != nil {
log.Println("error downloading:", err)
return
}
for {
chunkResponse, err := fileStreamResponse.Recv()
if err == io.EOF {
log.Println("received all chunks")
break
}
if err != nil {
log.Println("err receiving chunk:", err)
break
}
log.Printf("got new chunk with data: %s \n", chunkResponse.FileChunk)
}
If you need to be able to serve arbitrary files, you would need to handle which files you allow serving (say someone requests the file /etc/passwd or something).
Not sure what exactly is the use case here.