defmodule Money.ExchangeRates.Retriever do
@moduledoc """
Implements a `GenServer` to retrieve exchange rates from
a configured retrieveal module on a periodic or on demand basis.
By default exchange rates are retrieved from [Open Exchange Rates](http://openexchangerates.org).
The default period of execution is 5 minutes (300_000 milliseconds). The
period of retrieval is configured in `config.exs` or the appropriate
environment configuration. For example:
config :ex_money,
retrieve_every: 300_000
"""
use GenServer
require Logger
@etag_cache :etag_cache
@doc """
Starts the exchange rates retrieval service
"""
def start(config \\ Money.ExchangeRates.config()) do
Money.ExchangeRates.Supervisor.start_retriever(config)
end
@doc """
Stop the exchange rates retrieval service.
The service can be restarted with `restart/0`.
"""
def stop do
Money.ExchangeRates.Supervisor.stop_retriever()
end
@doc """
Restart the exchange rates retrieval service
"""
def restart do
Money.ExchangeRates.Supervisor.restart_retriever()
end
@doc """
Delete the exchange rates retrieval service
The service can be started again with `start/1`
"""
def delete do
Money.ExchangeRates.Supervisor.delete_retriever()
end
@doc false
def start_link(name, config \\ Money.ExchangeRates.config()) do
GenServer.start_link(__MODULE__, config, name: name)
end
@doc """
Forces retrieval of the latest exchange rates
Sends a message ot the exchange rate retrieval worker to request
current rates be retrieved and stored.
Returns:
* `{:ok, rates}` if exchange rates request is successfully sent.
* `{:error, reason}` if the request cannot be send.
This function does not return exchange rates, for that see
`Money.ExchangeRates.latest_rates/0` or
`Money.ExchangeRates.historic_rates/1`.
"""
def latest_rates() do
case Process.whereis(__MODULE__) do
nil -> {:error, exchange_rate_service_error()}
_pid -> GenServer.call(__MODULE__, :latest_rates)
end
end
@doc """
Forces retrieval of historic exchange rates for a single date
* `date` is a date returned by `Date.new/3` or any struct with the
elements `:year`, `:month` and `:day` or
* a `Date.Range.t` created by `Date.range/2` that specifies a
range of dates to retrieve
Returns:
* `{:ok, rates}` if exchange rates request is successfully sent.
* `{:error, reason}` if the request cannot be send.
Sends a message ot the exchange rate retrieval worker to request
historic rates for a specified date or range be retrieved and
stored.
This function does not return exchange rates, for that see
`Money.ExchangeRates.latest_rates/0` or
`Money.ExchangeRates.historic_rates/1`.
"""
def historic_rates(%Date{calendar: Calendar.ISO} = date) do
case Process.whereis(__MODULE__) do
nil -> {:error, exchange_rate_service_error()}
_pid -> GenServer.call(__MODULE__, {:historic_rates, date})
end
end
def historic_rates(%{year: year, month: month, day: day}) do
case Date.new(year, month, day) do
{:ok, date} -> historic_rates(date)
error -> error
end
end
def historic_rates(%Date.Range{first: from, last: to}) do
historic_rates(from, to)
end
@doc """
Forces retrieval of historic exchange rates for a range of dates
* `from` is a date returned by `Date.new/3` or any struct with the
elements `:year`, `:month` and `:day`.
* `to` is a date returned by `Date.new/3` or any struct with the
elements `:year`, `:month` and `:day`.
Returns:
* `{:ok, rates}` if exchange rates request is successfully sent.
* `{:error, reason}` if the request cannot be send.
Sends a message to the exchange rate retrieval process for each
date in the range `from`..`to` to request historic rates be
retrieved.
"""
def historic_rates(%Date{calendar: Calendar.ISO} = from, %Date{calendar: Calendar.ISO} = to) do
case Process.whereis(__MODULE__) do
nil ->
{:error, exchange_rate_service_error()}
_pid ->
for date <- Date.range(from, to) do
historic_rates(date)
end
end
end
def historic_rates(%{year: y1, month: m1, day: d1}, %{year: y2, month: m2, day: d2}) do
with {:ok, from} <- Date.new(y1, m1, d1),
{:ok, to} <- Date.new(y2, m2, d2) do
historic_rates(from, to)
end
end
@doc """
Updated the configuration for the Exchange Rate
Service
"""
def reconfigure(%Money.ExchangeRates.Config{} = config) do
GenServer.call(__MODULE__, {:reconfigure, config})
end
@doc """
Return the current configuration of the Exchange Rates
Retrieval service
"""
def config do
GenServer.call(__MODULE__, :config)
end
@doc """
Retrieve exchange rates from an external HTTP
service.
This function is primarily intended for use by
an exchange rates api module.
"""
def retrieve_rates(url, config) when is_binary(url) do
url
|> String.to_charlist()
|> retrieve_rates(config)
end
def retrieve_rates(url, config) when is_list(url) do
url = List.to_string(url)
headers = if_none_match_header(url)
{url, headers}
|> Cldr.Http.get_with_headers(verify_peer: Map.get(config, :verify_peer, true))
|> process_response(url, config)
end
defp process_response({:ok, headers, body}, url, config) do
rates = config.api_module.decode_rates(body)
cache_etag(headers, url)
{:ok, rates}
end
defp process_response({:not_modified, headers}, url, _config) do
cache_etag(headers, url)
{:ok, :not_modified}
end
defp process_response({:error, reason}, _url, _config) do
{:error, {Money.ExchangeRateError, "#{inspect reason}"}}
end
defp if_none_match_header(url) do
case get_etag(url) do
{etag, date} ->
[
{'If-None-Match', etag},
{'If-Modified-Since', date}
]
_ ->
[]
end
end
defp cache_etag(headers, url) do
etag = :proplists.get_value('etag', headers)
date = :proplists.get_value('date', headers)
if etag?(etag, date) do
:ets.insert(@etag_cache, {url, {etag, date}})
else
:ets.delete(@etag_cache, url)
end
end
defp get_etag(url) do
case :ets.lookup(@etag_cache, url) do
[{^url, cached_value}] -> cached_value
[] -> nil
end
end
defp etag?(etag, date) do
etag != :undefined && date != :undefined
end
#
# Server implementation
#
@doc false
def init(config) do
:erlang.process_flag(:trap_exit, true)
config.cache_module.init()
if is_integer(config.retrieve_every) do
log(config, :info, log_init_message(config.retrieve_every))
schedule_work(0)
end
if config.preload_historic_rates do
log(config, :info, "Preloading historic rates for #{inspect(config.preload_historic_rates)}")
schedule_work(config.preload_historic_rates, config.cache_module)
end
if :ets.info(@etag_cache) == :undefined do
:ets.new(@etag_cache, [:named_table, :public])
end
{:ok, config}
end
@doc false
def terminate(:normal, config) do
config.cache_module.terminate()
end
@doc false
def terminate(:shutdown, config) do
config.cache_module.terminate()
end
@doc false
def terminate(other, _config) do
Logger.error("[ExchangeRates.Retriever] Terminate called with unhandled #{inspect(other)}")
end
@doc false
def handle_call(:latest_rates, _from, config) do
{:reply, retrieve_latest_rates(config), config}
end
@doc false
def handle_call({:historic_rates, date}, _from, config) do
{:reply, retrieve_historic_rates(date, config), config}
end
@doc false
def handle_call({:reconfigure, new_configuration}, _from, config) do
config.cache_module.terminate()
{:ok, new_config} = init(new_configuration)
{:reply, new_config, new_config}
end
@doc false
def handle_call(:config, _from, config) do
{:reply, config, config}
end
@doc false
def handle_call(:stop, _from, config) do
{:stop, :normal, :ok, config}
end
@doc false
def handle_call({:stop, reason}, _from, config) do
{:stop, reason, :ok, config}
end
@doc false
def handle_info(:latest_rates, config) do
retrieve_latest_rates(config)
schedule_work(config.retrieve_every)
{:noreply, config}
end
@doc false
def handle_info({:historic_rates, %Date{calendar: Calendar.ISO} = date}, config) do
retrieve_historic_rates(date, config)
{:noreply, config}
end
@doc false
def handle_info(:stop, config) do
{:stop, :normal, config}
end
@doc false
def handle_info({:stop, reason}, config) do
{:stop, reason, config}
end
@doc false
def handle_info(message, config) do
Logger.error("Invalid message for ExchangeRates.Retriever: #{inspect(message)}")
{:noreply, config}
end
defp retrieve_latest_rates(%{callback_module: callback_module} = config) do
case config.api_module.get_latest_rates(config) do
{:ok, :not_modified} ->
log(config, :success, "Retrieved latest exchange rates successfully. Rates unchanged.")
{:ok, config.cache_module.latest_rates()}
{:ok, rates} ->
retrieved_at = DateTime.utc_now()
config.cache_module.store_latest_rates(rates, retrieved_at)
apply(callback_module, :latest_rates_retrieved, [rates, retrieved_at])
log(config, :success, "Retrieved latest exchange rates successfully")
{:ok, rates}
{:error, reason} ->
log(config, :failure, "Could not retrieve latest exchange rates: #{inspect(reason)}")
{:error, reason}
end
end
defp retrieve_historic_rates(date, %{callback_module: callback_module} = config) do
case config.api_module.get_historic_rates(date, config) do
{:ok, :not_modified} ->
log(config, :success, "Historic exchange rates for #{Date.to_string(date)} are unchanged")
{:ok, config.cache_module.historic_rates(date)}
{:ok, rates} ->
config.cache_module.store_historic_rates(rates, date)
apply(callback_module, :historic_rates_retrieved, [rates, date])
log(
config,
:success,
"Retrieved historic exchange rates for #{Date.to_string(date)} successfully"
)
{:ok, rates}
{:error, reason} ->
log(
config,
:failure,
"Could not retrieve historic exchange rates " <>
"for #{Date.to_string(date)}: #{inspect(reason)}"
)
{:error, reason}
end
end
defp schedule_work(delay_ms) when is_integer(delay_ms) do
Process.send_after(self(), :latest_rates, delay_ms)
end
defp schedule_work(%Date.Range{} = date_range, cache_module) do
for date <- date_range do
schedule_work(date, cache_module)
end
end
# Don't retrieve historic rates if they are
# already cached. Note that this is only
# called at retriever initialization, not
# through the public api.
#
# This depends on:
# 1. The cache is persistent, like Cache.Dets
# 2. The assumption that historic rates don't change
# A persistent cache will reduce the number of
# external API calls and it means the cache
# will survive restarts both intentional and
# unintentional
defp schedule_work(%Date{calendar: Calendar.ISO} = date, cache_module) do
case cache_module.historic_rates(date) do
{:ok, _rates} ->
:ok
{:error, _} ->
Process.send(self(), {:historic_rates, date}, [])
end
end
defp schedule_work({%Date{} = from, %Date{} = to}, cache_module) do
schedule_work(Date.range(from, to), cache_module)
end
defp schedule_work(date_string, cache_module) when is_binary(date_string) do
parts = String.split(date_string, "..")
case parts do
[date] -> schedule_work(Date.from_iso8601(date), cache_module)
[from, to] -> schedule_work({Date.from_iso8601(from), Date.from_iso8601(to)}, cache_module)
end
end
# Any non-numeric value, or non-date value means
# we don't schedule work - ie there is no periodic
# retrieval
defp schedule_work(_, _cache_module) do
:ok
end
@doc false
def log(%{log_levels: log_levels}, key, message) do
case Map.get(log_levels, key) do
nil ->
nil
log_level ->
Logger.log(log_level, message)
end
end
defp log_init_message(every) do
{every, plural_every} = seconds(every)
"Exchange Rates will be retrieved now and then every #{every} #{plural_every}."
end
defp seconds(milliseconds) do
seconds = div(milliseconds, 1000)
plural = if seconds == 1, do: "second", else: "seconds"
{seconds, plural}
end
defp exchange_rate_service_error do
{Money.ExchangeRateError, "Exchange rate service does not appear to be running"}
end
end
