This document discusses handling external APIs with Elixir. It describes using OTP behaviors and agents to build a robust system that can handle errors from external APIs. Key points covered include: 1. Using agents to store messages in a queue when an API call fails or hits rate limits. 2. Implementing a TelegramMessageQueue agent to push and pull messages from the queue. 3. Having a process pull from the queue on a timer and retry sending messages if API calls fail, hitting retry thresholds to avoid overloading the external API.

1. HANDLING EXTERNAL
APIS WITH ELIXIR
ALEX ROZUMII

2. ТЕКСТ
WHY?
▸ Because:
▸ External API fail (50x)
▸ Network fails
▸ Have rate limits

5. TelegramInterface.pull_messages(Keyword.get(state, :offset, 0))
Enum.each(items, fn(update) ->
case update do
%Nadia.Model.Update{message: %Nadia.Model.Message{date: the_date}} ->
#IT WORKS
_ ->
Rollbax.report()
end
end
end

6. case Dota.live_league_games do
{:ok, curr_state} ->
# REAL LOGIC
{:noreply, tick(prev_state, curr_state)}
{:error, %Nadia.Model.Error{reason: :connect_timeout}} ->
Process.send_after(self, :timer_event, @wait_ms_before_next_polling_error)
{:noreply, tick(prev_state, prev_state)}
{:error, {:error, %HTTPoison.Error{id: nil, reason: _reason}}} ->
Process.send_after(self, :timer_event, @wait_ms_before_next_empty_response)
{:noreply, tick(prev_state, prev_state)}
{:error, {:ok, %HTTPoison.Response{status_code: 500}}} ->
Process.send_after(self, :timer_event, @wait_ms_before_next_500_error)
{:noreply, tick(prev_state, prev_state)}
end

8. ТЕКСТ
HOW?
▸ OTP
▸ Agents
▸ Behaviours

9. defmodule Telegram.MessageQueue do
def start_link do
Agent.start_link(fn -> [] end, name: __MODULE__)
end
# @doc Flushes all existing records
def flush! do
Agent.cast(__MODULE__, fn(_) -> [] end)
end
# @doc Pushes message for given receiver to queue
def push_message(tlg_id, message, opts) do
item = {tlg_id, message, opts}
# Agent.get(GosuObs.Telegram.MessageQueue, fn set -> set end)
Agent.update(__MODULE__, &( [item | &1 ]))
end
# @doc Gets a top message, grouped if possible
def pull_top_message do
Agent.get_and_update(__MODULE__, fn queue ->
case List.last(queue) do
nil ->
{nil, []}
msg ->
end
end
end
end

10. def handle_info(:timer_event, state) do
case Telegram.MessageQueue.pull_top_message do
{tlg_id, message, opts} ->
case send_message(tlg_id, message, opts) do
:ok ->
Process.send_after(self, :timer_event, @wait_ms_before_next_message_send)
nil
:error ->
Process.send_after(self, :timer_event, @wait_ms_before_next_message_send)
nil
:wait ->
Process.send_after(self, :timer_event, @wait_ms_before_next_message_send_limit)
nil
:retry ->
Process.send_after(self, :timer_event, @wait_ms_before_next_message)
GosuObs.Telegram.MessageQueue.push_message(tlg_id, message, opts)
end
{:noreply, state}
nil ->
Process.send_after(self, :timer_event, @wait_ms_before_next_message_pull)
{:noreply, state}
end
end

11. defp send_message(t, message, opts) do
case Nadia.send_message(t, message, opts) do
{:ok, _result} ->
:ok
{:error, %Nadia.Model.Error{reason: "Please wait»}} ->
:wait
result ->
Rollbax.report()
:retry
end
end

13. THANKS!