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# This was our first attempt at improving the startup time. We introduce
# a handle_info/2 callback that will perform the data fetching. From init/1
# we simply send ourself a message to trigger this callback. This leads to a
# faster supervision tree startup, because each init/1 callback is now
# delegating work and not blocking for very long.
# As we will see in application_send_self_race.ex this is not a safe way to do
# things and can lead to race conditions.
defmodule HandleContinueBlogpost.ApplicationSendSelf do
use Application
defmodule MyServer do
use GenServer
require Logger
def start_link(type) do
GenServer.start_link(__MODULE__, type, name: type)
def init(type) do"#{type} - init start")
state = %{
type: type,
data: nil
# Send a message to ourself to load the data outside of the synchronous
# init block
self() |> send(:more_init)"#{type} - init end")
{:ok, state}
# Trigger the HTTP call here, instead of ini the init/1 callback
def handle_info(:more_init, state) do
data = get_data(state.type)
updated_state = Map.put(state, :data, data)
{:noreply, updated_state}
defp get_data(:users), do: http_get("")
defp get_data(:messages), do: http_get("")
defp get_data(:items), do: http_get("")
defp http_get(url) do"start #{url}")
# Pretend we are doing a network call here that returns some data
Enum.random(2000..4000) |> Process.sleep()
data = %{}"end #{url}")
def start(_type, _args) do
children = [
Supervisor.child_spec({MyServer, :users}, id: make_ref()),
Supervisor.child_spec({MyServer, :messages}, id: make_ref()),
Supervisor.child_spec({MyServer, :items}, id: make_ref())
opts = [strategy: :one_for_one, name: ContinueTest.Supervisor]
Supervisor.start_link(children, opts)