Feature: Global Config helper (#844) (#845)

* Added a global config helper to easily access installation/global configs
  * this will fetch the keys from cache with fallback to DB on cache miss
  * ability to query multiple keys simultaneously
  * interface to delete the existing global config cache
* Added tests for this new helper module

.rubocop.yml

@@ -30,6 +30,7 @@ Style/GlobalVars:
   Exclude:
     - 'config/initializers/redis.rb'
     - 'lib/redis/alfred.rb'
+    - 'lib/global_config.rb'
 Metrics/BlockLength:
   Exclude:
     - spec/**/*

lib/global_config.rb

@@ -0,0 +1,44 @@
+class GlobalConfig
+  VERSION = 'V1'.freeze
+  KEY_PREFIX = 'GLOBAL_CONFIG'.freeze
+  DEFAULT_EXPIRY = 1.day
+
+  class << self
+    def get(*args)
+      config_keys = *args
+      config = {}
+
+      config_keys.each do |config_key|
+        config[config_key] = load_from_cache(config_key)
+      end
+
+      config.with_indifferent_access
+    end
+
+    def clear_cache
+      cached_keys = $alfred.keys("#{VERSION}:#{KEY_PREFIX}:*")
+      (cached_keys || []).each do |cached_key|
+        $alfred.expire(cached_key, 0)
+      end
+    end
+
+    private
+
+    def load_from_cache(config_key)
+      cache_key = "#{VERSION}:#{KEY_PREFIX}:#{config_key}"
+      cached_value = $alfred.get(cache_key)
+
+      if cached_value.blank?
+        value_from_db = db_fallback(config_key)
+        cached_value = { value: value_from_db }.to_json
+        $alfred.set(cache_key, cached_value, { expiry: DEFAULT_EXPIRY })
+      end
+
+      JSON.parse(cached_value)['value']
+    end
+
+    def db_fallback(config_key)
+      InstallationConfig.find_by(name: config_key)&.value
+    end
+  end
+end

spec/lib/global_config_spec.rb

@@ -0,0 +1,39 @@
+require 'rails_helper'
+
+describe GlobalConfig do
+  subject(:trigger) { described_class }
+
+  describe 'execute' do
+    context 'when called with default options' do
+      before do
+        described_class.clear_cache
+      end
+
+      it 'hit DB for the first call' do
+        expect(InstallationConfig).to receive(:find_by)
+        described_class.get('test')
+      end
+
+      it 'get from cache for subsequent calls' do
+        # this loads from DB
+        described_class.get('test')
+
+        # subsequent calls should not hit DB
+        expect(InstallationConfig).not_to receive(:find_by)
+        described_class.get('test')
+      end
+
+      it 'clears cache and fetch from DB next time, when clear_cache is called' do
+        # this loads from DB and is cached
+        described_class.get('test')
+
+        # clears the cache
+        described_class.clear_cache
+
+        # should be loaded from DB
+        expect(InstallationConfig).to receive(:find_by).with({ name: 'test' }).and_return(nil)
+        described_class.get('test')
+      end
+    end
+  end
+end