Specifying `Resolve` callback used for name clashes of globally registered GenServer?

scottyscripts · September 4, 2019, 8:36pm

Background

I am running a distributed Elixir (Phoenix) application.

I want to run one instance of a globally registered GenServer for all my n nodes. (thinking singleton coming from OO). This process is under a supervision tree in my app.

The start_link function in my GenServer looks like:

 def start_link(_opts \\ []) do 
    GenServer.start_link(__MODULE__, [], name: {:global, __MODULE__})
  end

When I start my GenServer, I’m successfully seeing it registered globally and running.
If I try to run my GenServer on a connected node, I get an error as expected.

Problem

(assuming I’m running 2 nodes)
When the app is started, it looks like the GenServer process is being started on Node A and B. One of those nodes joins the cluster. Then when the other node joins the cluster, I am getting an error as expected based on the erlang :global docs

from Erlang -- global -

When new nodes are added to the network, they are informed of the globally registered names that already exist. The network is also informed of any global names in newly connected nodes. If any name clashes are discovered, function Resolve is called. Its purpose is to decide which pid is correct. If the function crashes, or returns anything other than one of the pids, the name is unregistered. This function is called once for each name clash.

What I Want To Do

I would like to handle this error.
I have tried trapping exits in my GenServer’s init function by adding Process.flag(:trap_exit, true)

But realized that the default Resolve function to handle these name clashes sends a kill message to a process (which I don’t think I can trap)

I noticed that one of the other Resolve functions would allow me to handle this error

Erlang -- global

When registering the GenServer globally in start_link, is there any way to specify the Resolve callback used in case of a name clash?

scottyscripts · September 4, 2019, 9:35pm

So I don’t think what I’m trying to do is possible (or via the way I’m trying to do it)

Options specified in GenServer’s start_link function, are eventually passed to erlang’s :gen.start

github.com

elixir-lang/elixir/blob/main/lib/elixir/lib/gen_server.ex#L922


      
                end
              end
          
              @doc false
              def terminate(_reason, _state) do
                :ok
              end
          
              @doc false
              def code_change(_old, state, _extra) do
                {:ok, state}
              end
          
              defoverridable code_change: 3, terminate: 2, handle_info: 2, handle_cast: 2, handle_call: 3
            end
          end
          
          defmacro __before_compile__(env) do
            unless Module.defines?(env.module, {:init, 1}) do
              message = """
              function init/1 required by behaviour GenServer is not implemented \

according to Erlang -- gen_server,

If ServerName={global,GlobalName}, the gen_server process id registered globally as GlobalName using global:register_name/2 If no name is provided, the gen_server process is not registered.

global:register_name/2 does not provide the option to specify the Resolve callback as global:register_name/3 does (Erlang -- global)