Implement Synth as an exchange rate provider (#574)

* Implement Synth as an exchange rate provider

* Add assertions to provider interface test

* Assert the correct provider error is raised

* Remove unnecessary parens

Gemfile

@@ -54,4 +54,7 @@ end
 group :test do
   gem "capybara"
   gem "selenium-webdriver"
+  gem "mocha"
+  gem "vcr"
+  gem "webmock"
 end

Gemfile.lock

@@ -134,6 +134,9 @@ GEM
       xpath (~> 3.2)
     concurrent-ruby (1.2.3)
     connection_pool (2.4.1)
+    crack (1.0.0)
+      bigdecimal
+      rexml
     crass (1.0.6)
     date (3.3.4)
     debug (1.9.1)
@@ -164,6 +167,7 @@ GEM
       fugit (>= 1.1)
       railties (>= 6.0.0)
       thor (>= 0.14.1)
+    hashdiff (1.1.0)
     highline (3.0.1)
     hotwire-livereload (1.3.1)
       actioncable (>= 6.0.0)
@@ -214,6 +218,8 @@ GEM
     matrix (0.4.2)
     mini_mime (1.1.5)
     minitest (5.21.2)
+    mocha (2.1.0)
+      ruby2_keywords (>= 0.0.5)
     msgpack (1.7.2)
     net-http (0.4.1)
       uri
@@ -335,6 +341,7 @@ GEM
       ruby-lsp (>= 0.14.2, < 0.15.0)
       sorbet-runtime (>= 0.5.9897)
     ruby-progressbar (1.13.0)
+    ruby2_keywords (0.0.5)
     rubyzip (2.3.2)
     selenium-webdriver (4.18.1)
       base64 (~> 0.2)
@@ -371,11 +378,16 @@ GEM
     unicode-display_width (2.5.0)
     uri (0.13.0)
     useragent (0.16.10)
+    vcr (6.2.0)
     web-console (4.2.1)
       actionview (>= 6.0.0)
       activemodel (>= 6.0.0)
       bindex (>= 0.4.0)
       railties (>= 6.0.0)
+    webmock (3.23.0)
+      addressable (>= 2.8.0)
+      crack (>= 0.3.2)
+      hashdiff (>= 0.4.0, < 2.0.0)
     webrick (1.8.1)
     websocket (1.2.10)
     websocket-driver (0.7.6)
@@ -408,6 +420,7 @@ DEPENDENCIES
   inline_svg
   letter_opener
   lucide-rails!
+  mocha
   pagy
   pg (~> 1.5)
   propshaft
@@ -422,7 +435,9 @@ DEPENDENCIES
   tailwindcss-rails
   turbo-rails
   tzinfo-data
+  vcr
   web-console
+  webmock
 
 RUBY VERSION
    ruby 3.3.0p0

app/models/account/syncable.rb

@@ -62,7 +62,7 @@ module Account::Syncable
             next if existing_rates_set.include?([ rc_from, rc_to, rc_date.to_s ])
 
             logger.info "Fetching exchange rate from provider for account #{self.name}: #{self.id} (#{rc_from} to #{rc_to} on #{rc_date})"
-            rate = ExchangeRate.fetch_rate_from_provider(rc_from, rc_to, rc_date)
+            rate = ExchangeRate.find_rate_or_fetch from: rc_from, to: rc_to, date: rc_date
             ExchangeRate.create! base_currency: rc_from, converted_currency: rc_to, date: rc_date, rate: rate if rate
         end
 

app/models/concerns/providable.rb

@@ -0,0 +1,13 @@
+# `Providable` serves as an extension point for integrating multiple providers.
+# For an example of a multi-provider, multi-concept implementation,
+# see: https://github.com/maybe-finance/maybe/pull/561
+
+module Providable
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def exchange_rates_provider
+      Provider::Synth.new
+    end
+  end
+end

app/models/exchange_rate.rb

@@ -1,42 +1,22 @@
 class ExchangeRate < ApplicationRecord
+  include Provided
+
   validates :base_currency, :converted_currency, presence: true
 
   class << self
-    def convert(from, to, amount)
+    def find_rate(from:, to:, date:)
-      rate = ExchangeRate.find_by(base_currency: from, converted_currency: to, date: Date.current)
+      find_by \
-      return nil if rate.nil?
+        base_currency: Money::Currency.new(from).iso_code,
-      amount * rate.rate
+        converted_currency: Money::Currency.new(to).iso_code,
+        date: date
     end
 
-    def get_rate(from, to, date)
+    def find_rate_or_fetch(from:, to:, date:)
-      _from = Money::Currency.new(from)
+      find_rate(from:, to:, date:) || fetch_rate_from_provider(from:, to:, date:).tap(&:save!)
-      _to = Money::Currency.new(to)
-      find_by! base_currency: _from.iso_code, converted_currency: _to.iso_code, date: date
-    rescue
-      logger.warn "Exchange rate not found for #{_from.iso_code} to #{_to.iso_code} on #{date}"
-      nil
     end
 
     def get_rate_series(from, to, date_range)
       where(base_currency: from, converted_currency: to, date: date_range).order(:date)
     end
-
-    # TODO: Replace with generic provider
-    # See https://github.com/maybe-finance/maybe/pull/556
-    def fetch_rate_from_provider(from, to, date)
-      response = Faraday.get("https://api.synthfinance.com/rates/historical") do |req|
-        req.headers["Authorization"] = "Bearer #{ENV["SYNTH_API_KEY"]}"
-        req.params["date"] = date.to_s
-        req.params["from"] = from
-        req.params["to"] = to
-      end
-
-      if response.success?
-        rates = JSON.parse(response.body)
-        rates.dig("data", "rates", to)
-      else
-        nil
-      end
-    end
   end
 end

app/models/exchange_rate/provided.rb

@@ -0,0 +1,24 @@
+module ExchangeRate::Provided
+  extend ActiveSupport::Concern
+  include Providable
+
+  class_methods do
+    private
+      def fetch_rate_from_provider(from:, to:, date:)
+        response = exchange_rates_provider.fetch_exchange_rate \
+          from: Money::Currency.new(from).iso_code,
+          to: Money::Currency.new(to).iso_code,
+          date: date
+
+        if response.success?
+          ExchangeRate.new \
+            base_currency: from,
+            converted_currency: to,
+            rate: response.rate,
+            date: date
+        else
+          raise response.error
+        end
+      end
+  end
+end

app/models/provider/base.rb

@@ -0,0 +1,18 @@
+
+class Provider::Base
+  ProviderError = Class.new(StandardError)
+
+  TRANSIENT_NETWORK_ERRORS = [
+    Faraday::TimeoutError,
+    Faraday::ConnectionFailed,
+    Faraday::SSLError,
+    Faraday::ClientError,
+    Faraday::ServerError
+  ]
+
+  class << self
+    def known_transient_errors
+      TRANSIENT_NETWORK_ERRORS + [ ProviderError ]
+    end
+  end
+end

app/models/provider/synth.rb

@@ -0,0 +1,51 @@
+class Provider::Synth
+  include Retryable
+
+  def initialize(api_key = ENV["SYNTH_API_KEY"])
+    @api_key = api_key || ENV["SYNTH_API_KEY"]
+  end
+
+  def fetch_exchange_rate(from:, to:, date:)
+    retrying Provider::Base.known_transient_errors do |on_last_attempt|
+      response = Faraday.get("#{base_url}/rates/historical") do |req|
+        req.headers["Authorization"] = "Bearer #{api_key}"
+        req.params["date"] = date.to_s
+        req.params["from"] = from
+        req.params["to"] = to
+      end
+
+      if response.success?
+        ExchangeRateResponse.new \
+          rate: JSON.parse(response.body).dig("data", "rates", to),
+          success?: true,
+          raw_response: response
+      else
+        if on_last_attempt
+          ExchangeRateResponse.new \
+            success?: false,
+            error: build_error(response),
+            raw_response: response
+        else
+          raise build_error(response)
+        end
+      end
+    end
+  end
+
+  private
+    attr_reader :api_key
+
+    ExchangeRateResponse = Struct.new :rate, :success?, :error, :raw_response, keyword_init: true
+
+    def base_url
+      "https://api.synthfinance.com"
+    end
+
+    def build_error(response)
+      Provider::Base::ProviderError.new(<<~ERROR)
+        Failed to fetch exchange rate from #{self.class}
+          Status: #{response.status}
+          Body: #{response.body.inspect}
+      ERROR
+    end
+end

lib/money.rb

@@ -26,7 +26,7 @@ class Money
     # TODO: Replace with injected rate store
     def exchange_to(other_currency, date = Date.current)
         return self if @currency == Money::Currency.new(other_currency)
-        rate = ExchangeRate.get_rate(@currency, other_currency, date)
+        rate = ExchangeRate.find_rate(from: @currency, to: other_currency, date: date)
         return nil if rate.nil?
         Money.new(@amount * rate.rate, other_currency)
     end

lib/retryable.rb

@@ -0,0 +1,19 @@
+module Retryable
+  def retrying(retryable_errors = [], max_retries: 3)
+    attempts = 0
+
+    begin
+      on_last_attempt = attempts == max_retries - 1
+
+      yield on_last_attempt
+    rescue *retryable_errors => e
+      attempts += 1
+
+      if attempts < max_retries
+        retry
+      else
+        raise e
+      end
+    end
+  end
+end

test/interfaces/exchange_rate_provider_interface_test.rb

@@ -0,0 +1,27 @@
+require "test_helper"
+
+module ExchangeRateProviderInterfaceTest
+  extend ActiveSupport::Testing::Declarative
+
+  test "exchange rate provider interface" do
+    assert_respond_to @subject, :fetch_exchange_rate
+  end
+
+  test "exchange rate provider response contract" do
+    accounting_for_http_calls do
+      response = @subject.fetch_exchange_rate from: "USD", to: "MXN", date: Date.current
+
+      assert_respond_to response, :rate
+      assert_respond_to response, :success?
+      assert_respond_to response, :error
+      assert_respond_to response, :raw_response
+    end
+  end
+
+  private
+    def accounting_for_http_calls
+      VCR.use_cassette "synth_exchange_rate" do
+        yield
+      end
+    end
+end

test/models/exchange_rate_test.rb

@@ -1,7 +1,42 @@
 require "test_helper"
 
 class ExchangeRateTest < ActiveSupport::TestCase
-  # test "the truth" do
+  test "find rate in db" do
-  #   assert true
+    assert_equal exchange_rates(:day_29_ago_eur_to_usd),
-  # end
+      ExchangeRate.find_rate_or_fetch(from: "EUR", to: "USD", date: 29.days.ago.to_date)
+  end
+
+  test "fetch rate from provider when it's not found in db" do
+    ExchangeRate
+      .expects(:fetch_rate_from_provider)
+      .returns(ExchangeRate.new(base_currency: "USD", converted_currency: "MXN", rate: 1.0, date: Date.current))
+
+    ExchangeRate.find_rate_or_fetch from: "USD", to: "MXN", date: Date.current
+  end
+
+  test "provided rates are saved to the db" do
+    VCR.use_cassette "synth_exchange_rate" do
+      assert_difference "ExchangeRate.count", 1 do
+        ExchangeRate.find_rate_or_fetch from: "USD", to: "MXN", date: Date.current
+      end
+    end
+  end
+
+  test "retrying, then raising on provider error" do
+    Faraday.expects(:get).returns(OpenStruct.new(success?: false)).times(3)
+
+    error = assert_raises Provider::Base::ProviderError do
+      ExchangeRate.find_rate_or_fetch from: "USD", to: "MXN", date: Date.current
+    end
+
+    assert_match "Failed to fetch exchange rate from Provider::Synth", error.message
+  end
+
+  test "retrying, then raising on network error" do
+    Faraday.expects(:get).raises(Faraday::TimeoutError).times(3)
+
+    assert_raises Faraday::TimeoutError do
+      ExchangeRate.find_rate_or_fetch from: "USD", to: "MXN", date: Date.current
+    end
+  end
 end

test/models/provider/synth_test.rb

@@ -0,0 +1,9 @@
+require "test_helper"
+
+class Provider::SynthTest < ActiveSupport::TestCase
+  include ExchangeRateProviderInterfaceTest
+
+  setup do
+    @subject = Provider::Synth.new
+  end
+end

test/test_helper.rb

@@ -1,6 +1,17 @@
 ENV["RAILS_ENV"] ||= "test"
 require_relative "../config/environment"
 require "rails/test_help"
+require "minitest/mock"
+require "minitest/autorun"
+require "mocha/minitest"
+
+VCR.configure do |config|
+  config.cassette_library_dir = "test/vcr_cassettes"
+  config.hook_into :webmock
+  config.ignore_localhost = true
+  config.default_cassette_options = { erb: true }
+  config.filter_sensitive_data("<SYNTH_API_KEY>") { ENV["SYNTH_API_KEY"] }
+end
 
 module ActiveSupport
   class TestCase
@@ -16,3 +27,5 @@ module ActiveSupport
     end
   end
 end
+
+Dir[Rails.root.join("test", "interfaces", "**", "*.rb")].each { |f| require f }

test/vcr_cassettes/synth_exchange_rate.yml

@@ -0,0 +1,69 @@
+---
+http_interactions:
+- request:
+    method: get
+    uri: https://api.synthfinance.com/rates/historical?date=<%= Date.current.to_s %>&from=USD&to=MXN
+    body:
+      encoding: US-ASCII
+      string: ''
+    headers:
+      User-Agent:
+      - Faraday v2.9.0
+      Authorization:
+      - Bearer <SYNTH_API_KEY>
+      Accept-Encoding:
+      - gzip;q=1.0,deflate;q=0.6,identity;q=0.3
+      Accept:
+      - "*/*"
+  response:
+    status:
+      code: 200
+      message: OK
+    headers:
+      Date:
+      - Wed, 27 Mar 2024 02:54:11 GMT
+      Content-Type:
+      - application/json; charset=utf-8
+      Content-Length:
+      - '138'
+      Connection:
+      - keep-alive
+      Cf-Ray:
+      - 86ac182ad9ec7ce5-LAX
+      Cf-Cache-Status:
+      - DYNAMIC
+      Cache-Control:
+      - max-age=0, private, must-revalidate
+      Etag:
+      - W/"46780d3f34043bb3bc799b1efae62418"
+      Strict-Transport-Security:
+      - max-age=63072000; includeSubDomains
+      Vary:
+      - Accept-Encoding
+      Referrer-Policy:
+      - strict-origin-when-cross-origin
+      Rndr-Id:
+      - 3ca97b82-f963-43a3
+      X-Content-Type-Options:
+      - nosniff
+      X-Frame-Options:
+      - SAMEORIGIN
+      X-Permitted-Cross-Domain-Policies:
+      - none
+      X-Render-Origin-Server:
+      - Render
+      X-Request-Id:
+      - 64731a8c-4cad-4e42-81c9-60b0d3634a0f
+      X-Runtime:
+      - '0.021432'
+      X-Xss-Protection:
+      - '0'
+      Server:
+      - cloudflare
+      Alt-Svc:
+      - h3=":443"; ma=86400
+    body:
+      encoding: ASCII-8BIT
+      string: '{"data":{"date":"<%= Date.current.to_s %>","source":"USD","rates":{"MXN":16.64663}},"meta":{"total_records":1,"credits_used":1,"credits_remaining":976}}'
+  recorded_at: Wed, 27 Mar 2024 02:54:11 GMT
+recorded_with: VCR 6.2.0