Bringing child nodes up on network

Questions / Help
,
1

Hi!
I’ve been able to setup child nodes, but I’m trying to get them online. The intention is to make the nodes act as separate computers, such that I can broadcast messages to them via udp.

Code I’ve been using for parent node:

Node.start(:master@localhost, :shortnames)

(When running :longnames, I get a name error when initializing child nodes)

Code for initializing child nodes:

defmodule Child_Nodes do

  def init child_names do
    :os.cmd('epmd -daemon')
    for child_name <- child_names do
      {:ok, node} = start_child child_name
      load_code node
    end
  end

  def project_module_path do
    Path.absname('_build/dev/ebin') |>
    to_charlist()
  end

  def start_child name do
    :slave.start(:localhost, name)
  end

  def load_code node do
    :rpc.block_call(node, :code, :add_paths, [:code.get_path()])
    :rpc.block_call(node, :code, :add_paths, [project_module_path()])
  end
end

This allows me to run my projects code on the child_nodes and access them using Node.ping(). However, each node are going to find eachother over udp using this module, with different ports:

defmodule Broadcasting do

  @broadcast_timeout 1000

  @localhost {10, 0, 0, 68}

  #Node-name format: OrderHandler_<port>@<address>

  def init port do

    {:ok, socket} = :gen_udp.open(port, [active: true, broadcast: true])

    Task.async fn -> broadcast socket, port end

    IO.puts "Node at port #{port} initialized"

    listen socket, port

  end

  def broadcast socket, port do

    :gen_udp.send(socket, {255,255,255,255}, port, to_string(port))

    :timer.sleep(@broadcast_timeout)

    broadcast socket, port

  end

  def listen socket, port do

    local_msg = to_string(port)

    receive do

      {:udp, _socket, _address, port, ^local_msg} ->

        IO.inspect port

        IO.puts "localmsg"

        listen socket, port

      {:udp, _socket, address, remote_port, _msg} ->

        string_address = ip_to_string(address)

        IO.puts "new node added at port: #{port}"

        #"OrderHandler_" <> to_string(remote_port) <> "@" <> string_address |> String.to_atom |> Node.ping

        #localhost-version

        "OrderHandler_" <> to_string(remote_port) <> "@localhost" |> String.to_atom |> Node.ping

      end

  end

  def ip_to_string ip do

    :inet.ntoa(ip) |> to_string()

  end

end

Is it possible to bring all child nodes online?

2

What is going to call this broadcasting module? A task? I think this is a place you should use gen_servers.

Also, the nodes have to be on the same layer-2 network that supports udp broadcasting and most major cloud providers don’t support this.

You might need to give beam.smp special setuid permissions (CAP_NET_RAW iirc) for it to be able to broadcast.

Finally you may run out of udp connections if you’re using nf_conntrack on Ubuntu 18.04.

3

The plan was to simply spawn the broadcasting procedure in each child node:

for node <- Node.list() do
Node.spawn(node, fn -> Broadcasting.init('uniqe ports') end)
end

The thing is that they aren’t able to find each other once the broadcasting starts.
This isn’t going to be a long-time robust solution, it is only going to be a replacement for the lack of computers atm

4

What’s the result of gen_udp.send?

5 
iex(2)> Node.start(:master@localhost, :shortnames)
{:ok, #PID<0.275.0>}
iex(master@localhost)3> Child_Nodes.init(['1', '2', '3'])
[:ok, :ok, :ok]
iex(master@localhost)4> Node.spawn(:'1@localhost', fn -> Broadcasting.init(3456) end) 
#PID<17862.95.0>
iex(master@localhost)5> Node at port 3456 initialized
iex(master@localhost)5> new node added at port: 3456
iex(master@localhost)6> Node.spawn(:'3@localhost', fn -> Broadcasting.init(3455) end) 
#PID<17960.95.0>
iex(master@localhost)7> Node at port 3455 initialized
iex(master@localhost)7> new node added at port: 3455

They are able to detect themselves, but not each other

6

What is the result of :gen_udp.send? Can you IO.inspect it?

7

Sorry, for the late reply, inspecting the :gen_udp.send was a good idea:

iex(master@localhost)4> Node at port 3456 initialized
iex(master@localhost)4> new node added at port: 3456
iex(master@localhost)4> :ok
iex(master@localhost)4> {:error, :closed}