diff --git a/app/controllers/concerns/submission_scoring.rb b/app/controllers/concerns/submission_scoring.rb
index 997ebae5..85da5106 100644
--- a/app/controllers/concerns/submission_scoring.rb
+++ b/app/controllers/concerns/submission_scoring.rb
@@ -49,14 +49,15 @@ module SubmissionScoring
 
   private :execute_test_file
 
-  def feedback_message(file, output)
-    # set_locale
+  def feedback_message(_file, output)
+    # TODO: why did we comment out set_locale and render_markdown?
+    set_locale
     if output[:score] == Assessor::MAXIMUM_SCORE && output[:file_role] == 'teacher_defined_test'
       I18n.t('exercises.implement.default_test_feedback')
     elsif output[:score] == Assessor::MAXIMUM_SCORE && output[:file_role] == 'teacher_defined_linter'
       I18n.t('exercises.implement.default_linter_feedback')
     else
-      # render_markdown(file.feedback_message)
+      render_markdown(file.feedback_message)
     end
   end
 
diff --git a/app/controllers/submissions_controller.rb b/app/controllers/submissions_controller.rb
index fe281b29..77cafbb8 100644
--- a/app/controllers/submissions_controller.rb
+++ b/app/controllers/submissions_controller.rb
@@ -9,7 +9,7 @@ class SubmissionsController < ApplicationController
   include Tubesock::Hijack
 
   before_action :set_submission,
-    only: %i[download download_file render_file run score extract_errors show statistics test]
+    only: %i[download download_file render_file run score extract_errors show statistics]
   before_action :set_files, only: %i[download download_file render_file show run]
   before_action :set_file, only: %i[download_file render_file run]
   before_action :set_mime_type, only: %i[download_file render_file]
@@ -134,7 +134,7 @@ class SubmissionsController < ApplicationController
 
   def handle_websockets(tubesock, runner, socket)
     tubesock.send_data JSON.dump({'cmd' => 'status', 'status' => :container_running})
-    @output = String.new
+    @output = +''
 
     socket.on :output do |data|
       Rails.logger.info("#{Time.zone.now.getutc}: Container sending: #{data}")
@@ -162,14 +162,14 @@ class SubmissionsController < ApplicationController
     tubesock.onmessage do |event|
       event = JSON.parse(event).deep_symbolize_keys
       case event[:cmd].to_sym
-      when :client_kill
-        EventMachine.stop_event_loop
-        kill_socket(tubesock)
-        Rails.logger.debug('Client exited container.')
-      when :result
-        socket.send event[:data]
-      else
-        Rails.logger.info("Unknown command from client: #{event[:cmd]}")
+        when :client_kill
+          EventMachine.stop_event_loop
+          kill_socket(tubesock)
+          Rails.logger.debug('Client exited container.')
+        when :result
+          socket.send event[:data]
+        else
+          Rails.logger.info("Unknown command from client: #{event[:cmd]}")
       end
     rescue JSON::ParserError
       Rails.logger.debug { "Data received from client is not valid json: #{data}" }
@@ -239,9 +239,8 @@ class SubmissionsController < ApplicationController
   def score
     Thread.new do
       hijack do |tubesock|
-        if @embed_options[:disable_run]
-          return kill_socket(tubesock)
-        end
+        return kill_socket(tubesock) if @embed_options[:disable_run]
+
         tubesock.send_data(@submission.calculate_score)
         # To enable hints when scoring a submission, uncomment the next line:
         # send_hints(tubesock, StructuredError.where(submission: @submission))
@@ -268,7 +267,7 @@ class SubmissionsController < ApplicationController
   # private :set_docker_client
 
   def set_file
-    @file = @files.detect { |file| file.name_with_extension == sanitize_filename }
+    @file = @files.detect {|file| file.name_with_extension == sanitize_filename }
     head :not_found unless @file
   end
   private :set_file
diff --git a/app/models/runner.rb b/app/models/runner.rb
index cb566661..7c812307 100644
--- a/app/models/runner.rb
+++ b/app/models/runner.rb
@@ -22,18 +22,18 @@ class Runner < ApplicationRecord
 
     return runner if runner.save
 
-    raise(RunnerNotAvailableError, 'No runner available')
+    raise RunnerNotAvailableError.new('No runner available')
   end
 
   def copy_files(files)
     url = "#{runner_url}/files"
-    body = {files: files.map { |filename, content| {filepath: filename, content: content} }}
+    body = {files: files.map {|filename, content| {filepath: filename, content: content} }}
     response = Faraday.patch(url, body.to_json, HEADERS)
     return unless response.status == 404
 
     # runner has disappeared for some reason
     destroy
-    raise(RunnerNotAvailableError, 'Runner unavailable')
+    raise RunnerNotAvailableError.new('Runner unavailable')
   end
 
   def execute_command(command)
@@ -42,7 +42,7 @@ class Runner < ApplicationRecord
     if response.status == 404
       # runner has disappeared for some reason
       destroy
-      raise(RunnerNotAvailableError, 'Runner unavailable')
+      raise RunnerNotAvailableError.new('Runner unavailable')
     end
     parse response
   end
diff --git a/app/models/submission.rb b/app/models/submission.rb
index 1a529630..249fdc3f 100644
--- a/app/models/submission.rb
+++ b/app/models/submission.rb
@@ -72,7 +72,7 @@ class Submission < ApplicationRecord
     # expects the full file path incl. file extension
     # Caution: There must be no unnecessary path prefix included.
     # Use `file.ext` rather than `./file.ext`
-    collect_files.detect { |file| file.filepath == file_path }
+    collect_files.detect {|file| file.filepath == file_path }
   end
 
   def normalized_score
@@ -165,7 +165,7 @@ class Submission < ApplicationRecord
           container_execution_time: execution_time,
           status: exit_code.zero? ? :ok : :failed,
           stdout: stdout,
-          stderr: stderr
+          stderr: stderr,
         }
         test_result(output, file)
       end
@@ -202,7 +202,7 @@ class Submission < ApplicationRecord
   end
 
   def command_for(template, file)
-    filepath = collect_files.find { |f| f.name_with_extension == file }.filepath
+    filepath = collect_files.find {|f| f.name_with_extension == file }.filepath
     template % command_substitutions(filepath)
   end
 
@@ -210,7 +210,7 @@ class Submission < ApplicationRecord
     {
       class_name: File.basename(filename, File.extname(filename)).camelize,
       filename: filename,
-      module_name: File.basename(filename, File.extname(filename)).underscore
+      module_name: File.basename(filename, File.extname(filename)).underscore,
     }
   end
 end
diff --git a/db/migrate/20210519134938_create_runners.rb b/db/migrate/20210519134938_create_runners.rb
index e747ed4a..91ea4814 100644
--- a/db/migrate/20210519134938_create_runners.rb
+++ b/db/migrate/20210519134938_create_runners.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class CreateRunners < ActiveRecord::Migration[6.1]
   def change
     create_table :runners do |t|
diff --git a/lib/runner/runner_connection.rb b/lib/runner/runner_connection.rb
index 4f692edd..2aae3b45 100644
--- a/lib/runner/runner_connection.rb
+++ b/lib/runner/runner_connection.rb
@@ -11,10 +11,10 @@ class RunnerConnection
     @socket = Faye::WebSocket::Client.new(url, [], ping: 5)
 
     %i[open message error close].each do |event_type|
-      @socket.on(event_type) { |event| __send__(:"on_#{event_type}", event) }
+      @socket.on(event_type) {|event| __send__(:"on_#{event_type}", event) }
     end
 
-    EVENTS.each { |event_type| instance_variable_set(:"@#{event_type}_callback", ->(e) {}) }
+    EVENTS.each {|event_type| instance_variable_set(:"@#{event_type}_callback", ->(e) {}) }
     @start_callback = -> {}
     @exit_code = 0
   end