I want to run different instances of the same app (not copies of the same app) on the same server (clustering) using different ports, so I used libcluster library and did the following (in application.ex):

def start(_type, _args) do
    import Supervisor.Spec
    # Define Topologies for clustering
    topologies = [
      example: [
        strategy: Cluster.Strategy.Gossip
      ]
    ]
    # List all child processes to be supervised
    children = [
      # For Clustering
      {Cluster.Supervisor, [topologies, [name: MyApp.ClusterSupervisor]]},
      # Start the endpoint when the application starts
      supervisor(MyAppWeb.Endpoint, []),
      {ConCache, [name: :my_cache, ttl_check_interval: false]}
    ]

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    result = Supervisor.start_link(children, opts)
end

I even tried this:

topologies = [
      example: [
        strategy: Cluster.Strategy.Gossip,
        config: [hosts: [:"a@127.0.0.1", :"b@127.0.0.1",:"b@127.0.0.1",:"d@127.0.0.1"]],
      ]
    ]

but when I tried the following on the server (Ubuntu 18):

 PORT=3000 elixir --sname a -S mix phx.server

then

 PORT=3001 elixir --sname b -S mix phx.server

I get the following error

** (Mix) Could not start application my_app: MyApp.Application.start(:normal, []) returned an error: shutdown: failed to start child: MyAppWeb.Endpoint
    ** (EXIT) shutdown: failed to start child: {:ranch_listener_sup, MyAppWeb.Endpoint.HTTP}
        ** (EXIT) shutdown: failed to start child: :ranch_acceptors_sup
            ** (EXIT) {:listen_error, MyAppWeb.Endpoint.HTTP, :eaddrinuse}

what should I do to make it work.