api_test.exs

defmodule TokenHolder do
  require Logger

  def start_link(user,passwd) do
    Agent.start_link(fn ->
      tok_time = get_token user, passwd
      {user,passwd,tok_time}
    end, name: __MODULE__)
  end

  # refresh the token if older that three seconds
  @max_age 3000000

  def token do
    Agent.get_and_update(__MODULE__, fn state={user,passwd,{token,retrieved}} ->
      Logger.info "[PID=#{inspect self}=TokenHolder] Retrieving token"
      now = :os.timestamp
      age = :timer.now_diff(now, retrieved)
      if(age < @max_age) do
        Logger.info "Token age is #{age} - we can still use it"
        # return old token and old state
        {token,state}
      else
        Logger.info "Token age is #{age} - we must get a new one"
        # retrieve new token, return it and return changed state
        tok_time = {token,_} = get_token user, passwd
        {token,{user,passwd,tok_time}}
      end
    end)
  end

  defp get_token(_user,_passwd) do
    token = :random.uniform 100
    {token,:os.timestamp}
  end

end

defmodule MyAPI do
  @moduledoc "Simulates API access"
  require Logger

  @doc "Simulates accessing one API endpoint"
  def users do
    token = TokenHolder.token
    Logger.info "[PID=#{inspect self}] We are accessing API using token #{token}"
    :timer.sleep 1000
    Logger.info "[PID=#{inspect self}] We got answer."
    ["John","George","Martin"]
  end

end

defmodule Main do
  require Logger

  def main do
    # start logger
    :ok = Application.ensure_started :logger
    # start TokenHolder
    {:ok,_} = TokenHolder.start_link "user", "secret"

    # these two requests will be done in parallel
    master = self
    spawn_link(fn ->
      users = MyAPI.users
      Logger.info "users = #{inspect users}"
      send master, :done
    end)
    spawn_link(fn ->
      users = MyAPI.users
      Logger.info "users = #{inspect users}"
      send master, :done
    end)

    # wait for the requests to be finished
    receive do :done -> :ok end
    receive do :done -> :ok end

    Logger.info "We are waiting now to simulate token expiration..."
    :timer.sleep 5000

    # these two requests will be done in parallel with a new token
    spawn_link(fn ->
      users = MyAPI.users
      Logger.info "users = #{inspect users}"
    end)
    spawn_link(fn ->
      users = MyAPI.users
      Logger.info "users = #{inspect users}"
    end)
  end

end

Main.main