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ex_money
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Money functions for operations on and localization of a money data type with support for ISO 4217 currencies and ISO 24165 digial tokens (crypto currencies).
Current section
Files
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Current section
Files
lib/money/exchange_rates/http.ex
defmodule Money.ExchangeRates.HTTP do
@moduledoc false
@doc """
Performs an HTTP GET request, automatically attaching conditional request headers
(`if-none-match`, `if-modified-since`) from a node-wide ETag cache keyed by URL.
Persists response headers back into the cache after each request.
Returns:
* `{:ok, body}` — the request succeeded and `body` contains the response body
* `{:ok, :not_modified}` — the server returned 304; the cached response is still valid
* `{:error, {Money.ExchangeRateError, reason}}` — the request failed
"""
@spec get(url :: binary, opts :: keyword) ::
{:ok, term} | {:ok, :not_modified} | {:error, {Money.ExchangeRateError, binary}}
def get(url, opts \\ []) do
cache = init_cache()
http_client = Money.get_env(:exchange_rates_http_client, Localize.Utils.Http, :module)
case http_client.get_with_headers({url, build_headers(cache, url)}, opts) do
{:ok, headers, body} ->
save_headers(cache, url, headers)
{:ok, body}
{:not_modified, headers} ->
save_headers(cache, url, headers)
{:ok, :not_modified}
{:error, reason} ->
{:error, {Money.ExchangeRateError, "#{inspect(reason)}"}}
end
end
# The ETag cache is a node-wide named table shared by all retrievers. Because
# multiple named retrievers may call `get/2` concurrently, creation must be
# race-safe: two processes can both observe `:undefined` from `:ets.whereis/1`
# and then race on `:ets.new/2`, where the loser would raise `ArgumentError`
# for a duplicate named table. Rescue that case and return the existing table.
defp init_cache do
case :ets.whereis(:etag_cache) do
:undefined ->
try do
:ets.new(:etag_cache, [:named_table, :public])
rescue
ArgumentError -> :etag_cache
end
_table ->
:etag_cache
end
end
defp build_headers(cache, url) do
etag = :ets.lookup_element(cache, url, 2, nil)
last_modified = :ets.lookup_element(cache, url, 3, nil)
Enum.reject(
[{~c"if-none-match", etag}, {~c"if-modified-since", last_modified}],
&is_nil(elem(&1, 1))
)
end
defp save_headers(cache, url, headers) do
etag = :proplists.get_value(~c"etag", headers, nil)
last_modified = :proplists.get_value(~c"last-modified", headers, nil)
:ets.insert(cache, {url, etag, last_modified})
end
end