actors.ex

defmodule KVStore do
  use GenServer
  
  # This executes within the supervisor or whoever starts the KVStore
  def start_link do
    # First arg is the callback module (this module)
    # The name arg registers the process globally as `KVStore`, essentially making it a singleton
    GenServer.start_link(KVStore, [], name: KVStore)  
  end
  
  ##
  # These next 3 functions execute within the calling process
  
  # callback called when the process starts up
  def init(_) do
    state = %{} # state is just an empty dictionary
    {:ok, state}
  end
  
  def handle_call({:add, key, value}, _from, state) do
    new_state = Map.put(state, key, value)
    {:reply, :ok, new_state}
  end
  
  def handle_call({:get, key}, _from, state) do
    # state doesn't change here, just give back the value
    {:reply, state[key], state}
  end
  
  ## These next two functions execute within the calling process, and are provided 
  # as an interface. They're not required, but it's idiomatic to provide "interface functions"
  # in your genserver/genevent/etc so that callers don't need to know the structure of the message
  # that is passed, they just need to know how to call a function
  
  def push(key, value) do
    # This sends the message. Anything sending a message accepts either a pid or a name associated with a pid. Since
    # the process registered itself as `KVStore` on startup, we can use KVStore here as the associated name
    GenServer.call(KVStore, {:push, key, value)
  end
  
  def get(key) do
    GenServer.call(KVStore, {:get, key})
  end
end

So then you could do

KVStore.put(:foo, 1)
KVStore.get(:foo)