In my case, sessions are in the database for analytics and persistence purposes, but from the point of view of the front-end, the only place that gets queried is in cache. It would be possible to have a fallback mechanism that would fall-back to a database lookup on cache miss, but I want to avoid doing that because doing that would open up some vulnerabilities to overloading the database.

Perhaps something like this would have the desired effect:

defmodule MyApp.Application do
  alias MyApp.Contexts.MyContext
  use Application

  def start(_type, _args) do
    import Supervisor.Spec, warn: false

    :ok = MyContext.warm_up_cache()
    
    children = [
      supervisor(MyApp.Repo, []),
    ]
    
    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
  end
end

Application.start_phase looks like another option.

Docs are a little light but the Erlang docs go into greater detail.