Sending messages from an external script to a running Elixir app

Questions / Help
1

Hey everyone.

I would like to send messages from an external script to a running Elixir app. The script should be short-lived and terminate immediately after forwarding the message. It could of course be written in Elixir too and they’ll run on the same server.

Thanks!

2

I think you can do this in multiple ways. Define a port to receive messages or use the rpc to send messages directly in elixir.

/path/to/_build/prod/rel/my_app/bin/my_app rpc "Genserver.cast(...)"

1 Like
3

Yep, I use something like this to update my blog when I have added a new post or modified an existing one:

sudo -u www-data /var/www/blog/bin/mebe rpc "GenServer.call(Mebe2.Engine.Worker, :refresh, 30_000)"

Mix releases include the “rpc” script builtin.

3 Likes
4

Really cool solution!

ps: Security Notes:

  • I hope your document root is not pointed at /var/www
  • You have directory listing and all the other nice stuff disabled with the default apache or another unix handler
  • For the future please create a specific user for your projects, applications, scripts and put them into respective directories and run from that location with that user

ie: blog

user: blog
homedir: /home/blog

$ > ps
... blog ...... /home/blog/bin/mebe ......
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5

The rpc solutions looks easy, but that’s only for release? I’d like to run this in dev (maybe even test?) as well.

6

Hi and thanks for the concern. My document root is indeed not /var/www, and directory listing is not enabled. The blog could sure run with a different user, but there is nothing interesting on the server that the user could access, so I’m not too worried about it. But should leave a note to do it the next time I do something on the box.

But this is not at all related to the matter at hand.

To do this in development, you can start your application with a name (--name or --sname) and then use IEx’s --rpc-eval flag (and I think IEx also needs to be given a different name).

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7

rpc is good; however fairly slow because it need to start up a separate erlang node just to talk to the existing running system. If you need to interact with the running system every second, you can make a simple server listening on a UNIX domain socket, that respond to a minimal text based API. This is fairly simple with gen_tcp. Another benefit (at least in my view) is you get to limit what you respond to. My client program is just a shell script that invokes socat.

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8

beam_notify might be a good library to look into: GitHub - nerves-networking/beam_notify: Send a message from a shell script to the BEAM

5 Likes
9

Excellent, looks like just what I wanted.

For reference, this is what I got working:

defmodule Notify do
  use GenServer

  def start_link(args) do
    GenServer.start_link(__MODULE__, args, name: __MODULE__)
  end

  @impl true
  def init(_opts) do
    BEAMNotify.start_link(
      path: "/tmp/notify_socket",
      dispatcher: &send(__MODULE__, {:notify, &1, &2})
    )
  end

  @impl true
  def handle_info({:notify, msg, _}, state) do
    IO.inspect(msg)
    {:noreply, state}
  end
end

And the script in priv:

#!/bin/sh

BEAM_NOTIFY=$(ls $(dirname $0)/../_build/dev/lib/beam_notify/priv/beam_notify)
$BEAM_NOTIFY -p /tmp/notify_socket -- $1

I’ll have to modify the beam_notify path in production, but other than that I’m good to go.

1 Like