Next, we'll need two utility functions to abstract away some of the GenServer logic. We'll start by creating the function responsible for writing new data to our ETS Presence cache:

  def write(topic, username, status) do
    row = %{
      topic: topic,
      username: username,
      status: status,
      timestamp: DateTime.utc_now()
    }
    GenServer.cast(__MODULE__, {:write, row})
  end

This function isn't doing nearly as much as it looks like it's doing: It takes in the topic, the user's username, and the status we want to record, builds a map from these, which includes the current timestamp, and then uses a GenServer message cast (an asynchronous call) to itself to write that row. We'll deal with this message later!

lookup is an even easier function! This time, we'll just want synchronous GenServer message call to get the data back out:

  def lookup do
    GenServer.call(__MODULE__, {:lookup})
  end