5 files changed, 69 insertions, 28 deletions
diff --git a/lib/active_storage/engine.rb b/lib/active_storage/engine.rb
index d35d3c16db..d066a689eb 100644
--- a/lib/active_storage/engine.rb
+++ b/lib/active_storage/engine.rb
@@ -25,31 +25,28 @@ module ActiveStorage
     end
 
     config.after_initialize do |app|
-      config_choice = app.config.active_storage.service
-      config_file   = Pathname.new(Rails.root.join("config/storage_services.yml"))
-
-      if config_choice
+      if config_choice = app.config.active_storage.service
+        config_file = Pathname.new(Rails.root.join("config/storage_services.yml"))
         raise("Couldn't find Active Storage configuration in #{config_file}") unless config_file.exist?
 
-        begin
-          require "yaml"
-          require "erb"
-          configs = YAML.load(ERB.new(config_file.read).result) || {}
+        require "yaml"
+        require "erb"
 
-          if service_configuration = configs[config_choice.to_s].symbolize_keys
-            service_name = service_configuration.delete(:service)
+        configs =
+          begin
+            YAML.load(ERB.new(config_file.read).result) || {}
+          rescue Psych::SyntaxError => e
+            raise "YAML syntax error occurred while parsing #{config_file}. " \
+                  "Please note that YAML must be consistently indented using spaces. Tabs are not allowed. " \
+                  "Error: #{e.message}"
+          end
 
-            ActiveStorage::Blob.service = ActiveStorage::Service.configure(service_name, service_configuration)
-          else
-            raise "Couldn't configure Active Storage as #{config_choice} was not found in #{config_file}"
+        ActiveStorage::Blob.service =
+          begin
+            ActiveStorage::Service.configure config_choice, configs
+          rescue => e
+            raise e, "Cannot load `Rails.config.active_storage.service`:\n#{e.message}", e.backtrace
           end
-        rescue Psych::SyntaxError => e
-          raise "YAML syntax error occurred while parsing #{config_file}. " \
-                "Please note that YAML must be consistently indented using spaces. Tabs are not allowed. " \
-                "Error: #{e.message}"
-        rescue => e
-          raise e, "Cannot load `Rails.config.active_storage.service`:\n#{e.message}", e.backtrace
-        end
       end
     end
   end
diff --git a/lib/active_storage/service.rb b/lib/active_storage/service.rb
index f15958fda9..1a6a55739f 100644
--- a/lib/active_storage/service.rb
+++ b/lib/active_storage/service.rb
@@ -32,13 +32,15 @@
 class ActiveStorage::Service
   class ActiveStorage::IntegrityError < StandardError; end
 
-  def self.configure(service, **options)
-    begin
-      require "active_storage/service/#{service.to_s.downcase}_service"
-      ActiveStorage::Service.const_get(:"#{service}Service").new(**options)
-    rescue LoadError => e
-      puts "Couldn't configure service: #{service} (#{e.message})"
-    end
+  def self.configure(service_name, configurations)
+    require 'active_storage/service/configurator'
+    Configurator.new(service_name, configurations).build
+  end
+
+  # Override in subclasses that stitch together multiple services and hence
+  # need to do additional lookups from configurations. See MirrorService.
+  def self.build(config, configurations) #:nodoc:
+    new(config)
   end
 
   def upload(key, io, checksum: nil)
diff --git a/lib/active_storage/service/configurator.rb b/lib/active_storage/service/configurator.rb
new file mode 100644
index 0000000000..5054e07ec7
--- /dev/null
+++ b/lib/active_storage/service/configurator.rb
@@ -0,0 +1,31 @@
+class ActiveStorage::Service::Configurator #:nodoc:
+  def initialize(service_name, configurations)
+    @service_name, @configurations = service_name.to_sym, configurations.symbolize_keys
+  end
+
+  def build
+    service_class.build(service_config.except(:service), @configurations)
+  end
+
+  private
+    def service_class
+      resolve service_class_name
+    end
+
+    def service_class_name
+      service_config.fetch :service do
+        raise "Missing Active Storage `service: …` configuration for #{service_config.inspect}"
+      end
+    end
+
+    def service_config
+      @configurations.fetch @service_name do
+        raise "Missing configuration for the #{@service_name.inspect} Active Storage service. Configurations available for #{@configurations.keys.inspect}"
+      end
+    end
+
+    def resolve(service_class_name)
+      require "active_storage/service/#{service_class_name.to_s.downcase}_service"
+      ActiveStorage::Service.const_get(:"#{service_class_name}Service")
+    end
+end
diff --git a/lib/active_storage/service/mirror_service.rb b/lib/active_storage/service/mirror_service.rb
index 7ec166aace..eec1f2af65 100644
--- a/lib/active_storage/service/mirror_service.rb
+++ b/lib/active_storage/service/mirror_service.rb
@@ -5,6 +5,17 @@ class ActiveStorage::Service::MirrorService < ActiveStorage::Service
 
   delegate :download, :exist?, :url, to: :primary
 
+  # Stitch together from named configuration.
+  def self.build(mirror_config, all_configurations) #:nodoc:
+    primary = ActiveStorage::Service.configure(mirror_config.fetch(:primary), all_configurations)
+
+    mirrors = mirror_config.fetch(:mirrors).collect do |service_name|
+      ActiveStorage::Service.configure(service_name.to_sym, all_configurations)
+    end
+
+    new primary: primary, mirrors: mirrors
+  end
+
   def initialize(primary:, mirrors:)
     @primary, @mirrors = primary, mirrors
   end
diff --git a/lib/active_storage/storage_services.yml b/lib/active_storage/storage_services.yml
index d3f001a27b..a93304d88f 100644
--- a/lib/active_storage/storage_services.yml
+++ b/lib/active_storage/storage_services.yml
@@ -24,4 +24,4 @@ google:
 mirror:
   service: Mirror
   primary: local
-  secondaries: [ amazon, google ]
+  mirrors: [ amazon, google ]