Elixir GenServer投射和start_link

Question

These are my first 2 hours of Elixir so it might be a noobish question but I have some issues with spawning up a GenServer and casting it. I'm using this module which watches for changes in a file in my system and, each time a new entry is added in that file, I want to cast the log to a gen server which analyzes it and does whatever is supposed to do based on it's findings.

Here's the module that watches the changes:

defmodule Test.Watcher do
    use ExFSWatch, dirs: ["/var/log/"]

    import Test.Receiver, only: [analyze_log: 2]

    def callback(file_path, actions) do
        if file_path == "/var/log/syslog" do
            if :modified in actions do
                #~~~~ WHERE DO I GET THIS pid FROM?
                analyze_log pid, get_log
            end
        end
    end

    def get_log do
        {log, _} = System.cmd("tail", ["-n", "1", "/var/log/syslog"])
        log
    end
end

The watcher works very well and it receives the new logs but I'm having a problem with Test.Receiver which uses GenServer .

My first question is...where do I start this gen server? ExFSWatch has it's own start method and I can't override that. Do I call start_link each time a new log passes in (I doubt that but I had to ask)?

Ny all the examples I read I'm supposed to start it somewhere else, grab it's pid and pass it as a param to the analyze_log method so, if I'm right, my only remaining problem is finding out a good place to start the Test.Receiver Genserver and grabbing it's pid so I can use it inside the callback method.

defmodule Test.Receiver do
    use GenServer

    def start_link do
        GenServer.start_link(__MODULE__, :ok, [])
    end

    def analyze_log(pid, log) do
        GenServer.cast(pid, {:analyze_log, log})
    end

    def init(:ok) do
        {:ok, %{}}
    end

    def handle_cast({:analyze_log, log}, state) do
        IO.puts log
        {:noreply, state}
    end
end

Answer 1

Ok, to expand on @Dogbert's answer and clear it out for others who are starting with Elixir I will provide the solution that worked out for me. It may not be the most elegant one as I lack the knowledge yet but works.

As Dogbert said, you need to register your GenServer with a name and call it by that name. So I went to my main file and spawned a supervisor which keeps these listeners up and active like so:

defmodule Test do
    use Application

    def start(_type, _args) do
        import Supervisor.Spec, warn: false

        Fail2ban.Watcher.start

        children = [
            # ... I have more here but let's keep it simple
            supervisor(Test.Receiver, [Test.Receiver]),
        ]

        opts = [strategy: :one_for_one, name: Test.Supervisor]
        Supervisor.start_link(children, opts)
    end
end

As you can see, the second parameter passed onto the supervisor call is the name under which I want to register this module. It may be anything you like I presume since I'm not very sure on that. This second parameter goes into my receiver's start_link and registers under that name:

def start_link(name \\ nil) do
    GenServer.start_link(__MODULE__, nil, [name: name])
end

With this in place now remember this method on the receiver that I had problems regarding the pid which I didn't know where to get from:

defmodule Test.Watcher do
    use ExFSWatch, dirs: ["/var/log/"]

    import Test.Receiver, only: [analyze_log: 2]

    def callback(file_path, actions) do
        if file_path == "/var/log/syslog" do
            if :modified in actions do
                # Instead of the pid, pass the name under which it was registered
                analyze_log Test.Receiver, get_log
            end
        end
    end

    def get_log do
        {log, _} = System.cmd("tail", ["-n", "1", "/var/log/syslog"])
        log
    end
end

Works for me, open to discussion and improvements. It's my 2nd day now of Elixir and I started to grasp it's ways of ticking. Coming from Python, Ruby, PHP myself it's a little hard different grabbing and passing information as parameters and state all the time but..I see the sun now.