Only one RPC server allowed per process?

[andrew...@gmail.com]

I've been prototyping an RPC application using goroutines instead of separate processes. While multiple objects can be registered with RPC then connected to individually, all remote procedure calls are routed to the first object registered. Here is simple code which reproduces the behaviour (with error checking removed - no errors occur):

package main

import (

    "fmt"

    "net"

    "net/rpc"

)

type Server struct {

    id int

}

func (this *Server) Receive(i int, reply *int) error {

    fmt.Println("Server", this.id, "received", i)

    *reply = this.id

    return nil

}

func (this *Server) run(address string) {

    rpc.Register(this)

    ln, _ := net.Listen("tcp", address)

    for {

        cxn, _ := ln.Accept()

        go rpc.ServeConn(cxn)

    }

}

func main() {

    addresses := map[int]string {

        1: "127.0.0.1:10000",

        2: "127.0.0.1:10001",

        3: "127.0.0.1:10002",

    }

    for id, address := range addresses {

        server := Server{id}

        go server.run(address)

    }

    

    servers := make(map[int]*rpc.Client)

    for id, address := range addresses {

        server, _ := rpc.Dial("tcp", address)

        servers[id] = server

    }

    for id, server := range servers {

        var reply int

        server.Call("Server.Receive", id, &reply)

        fmt.Println("Returned", reply)

    }

}

Program output:

Server 1 received 1

Returned 1

Server 1 received 2

Returned 1

Server 1 received 3

Returned 1

All remote procedure calls to Server.Receive are being routed only to the server object with ID 1. Is this expected behaviour? I am running Go 1.2 on Windows 8.1.

Thanks!

Andrew Helwer

[Kyle Lemons]

You're using the default RPC server when you call rpc.Register, so no, you can't have more than one.

If you want more than one, make separate RPC servers:

http://play.golang.org/p/g7VHJ03E-j

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[Andrew Helwer]

Great, thanks! Serves me right for not checking my errors - the second and third calls to rpc.Register return the error "service already defined".

Andrew Helwer

[Ross Salas]

Also note that I received a panic with the posted code.   There is no guarantee that all the servers are "running" before continuing on the the second for loop where they are Dialed.  Therefore, the first server is being Dialed before it finished Registering.  If I put time.Sleep(2 * time.Second) between the first two for loops, that gives enough time for all servers to fire up and be ready.  You may want to look into better ways of coordinating these actions.  Also,  for your production code, you may want to make sure that all servers are running before continuing on (make sure they didn't error while starting up).

[Kyle Lemons]

Check out my response:

http://play.golang.org/p/g7VHJ03E-j

It ensures that the listen has completed before the dial.  (bonus points because it also works in the playground when you do this right)