Strictly speaking, you can fetch your timer list in the init/1 function of your IntervalServer, eg:

defmodule IntervalServer do
  use GenServer
  # ...
  def init(:ok) do
    timers = MyApp.Repo.all(MyApp.Timer)
    {:ok, timers}
  end
  # ...
end

…and you’ll have the timers in your state. The drawback of this approach is that your database access is holding up the application startup sequence, and the application will utterly fail to start if the database isn’t available.

A different approach is to have init/1 return {:ok, [], 0}, which means it initializes with an empty array of timers, but a timeout of 0 milliseconds. When the timeout expires (immediately after startup), handle_info(:timeout, state) will be called, and you can load your state in that callback instead, without hindering the application during startup… Eg:

defmodule IntervalServer do
  use GenServer
  # ...
  def init(:ok) do
    {:ok, [], 0}
  end

  def handle_info(:timeout, []) do
    timers = MyApp.Repo.all(MyApp.Timer)
    {:noreply, timers}
  end
  # ...
end

It depends on what your needs are. Doing it with timeouts, though, is the “friendliest” option – you can probably indicate in some other manner that the application isn’t ready yet, than having it fail to start at all. Perhaps some functionality is still useful, even without the database being (temporarily?) unavailable, etc.

NOTE: The IntervalServer can just be started in the top-level supervisor at this point, if you want to, eg. lib/my_app.ex in the start/2 function, as a worker.