Gen_Servers in Erlang/Elixir: Easy as Jelly!

So, what is a GenServer? A GenServer facilitates communication between different Erlang/Elixir modules on the same or different nodes using a server-client relationship.

A GenServer creates its own separate process in an application. It maintains an internal state. Provides a mechanism for the callback function that can handle both synchronous and asynchronous requests. Both Erlang and Elixir provide a standard GenServer behavior that can be used or implemented by custom modules. There are differences between GenServer in Erlang or Elixir. However, let's discuss a general idea about it.

Let us define a module called Basic_Gen_Server. The Basic_Gen_Server module uses GenServer as its behavior. The GenServer wraps the entire module functionalities and starts them in a separate process. Thus it allows concurrency. It also maintains an internal state for the module. The BasicGen module must have certain standard functions and callback functions to behave like a GenServer.

The GenServer.start_link(module_name,[parameter list…],restart_strategy). It initiates our GenServer module called BasicGen. It performs all required internal operations for the initiation and finally calls the callback function init(). We can write our own code here to be executed as a part of the initiation.

# ELIXIR CODE:
defModule Basic_Gen_Server do
    use GenServer
# We implement the start_link function that returns {:ok,pid}# calling the GenServer.start_link method spawns a process for# our module Basic_Gen_Server and triggers init()def start do
        GenServer.start_link(__MODULE__,[])
    end
# We implement init triggered by start() methoddef init(initial_state) do
        {:ok, initial_state}
    end
end
{:ok,#PID<process_id>} = Basic_Gen_Server.start() # returns tuple

The following callback functions will help us perform different operations

# A Synchronous callback function
# matched by :identifying_atomdef handle_call(:identifying_atom, _from, [head | tail]) do {:reply, head, tail} end # An Asynchronous callback function # matched by :identifying_atom def handle_cast({:identifying_atom, someValue}, state) do {:noreply, [someValue | state]} end # An Asynchronous callback function # matched by :identifying_atom triggered by external requests def handle_info({:identifying_atom, someValue}, state) do {:noreply, [someValue | state]} end # Trigerred by: GenServer.call(process_id,{:identifying_atom, someValue}) 
GenServer.cast(process_id,{:identifying_atom, someValue})

We can also define functions that we can call from within the application that will trigger the handle_call, handle_cast, and handle_info callback functions.

# get_my_state() method used for
# calls within application using process_id
def get_my_state(process_id) do
    GenServer.call(process_id, {:get_state_atom, someValue}}
end
    
# set_my_state() method used for
# calls within application using process_id
def set_my_state(process_id, my_state) do
    GenServer.cast(process_id, {:set_state_atom, my_state}}
end

This will trigger the corresponding callback function.

def handle_call({:get_state_atom}, _from, my_state) do
    {:reply, my_state}
end
def handle_cast({:set_state_atom, someValue}, my_state) do
    {:noreply, my_state}
end

Thoughts and suggestions? shoot!

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