diff --git a/Gemfile b/Gemfile
index 3b1b2cd2..5269fe34 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,8 +5,8 @@ gem 'bcrypt', '~> 3.1.7'
 gem 'bootstrap-will_paginate'
 gem 'carrierwave'
 gem 'coffee-rails', '~> 4.0.0'
-gem 'concurrent-ruby'
-gem 'concurrent-ruby-ext', platform: :ruby
+gem 'concurrent-ruby', '~> 1.0.0'
+gem 'concurrent-ruby-ext', '~> 1.0.0', platform: :ruby
 gem 'docker-api','~> 1.21.1',  require: 'docker'
 gem 'factory_girl_rails', '~> 4.0'
 gem 'forgery'
@@ -19,9 +19,9 @@ gem 'kramdown'
 gem 'newrelic_rpm'
 gem 'pg', platform: :ruby
 gem 'pry'
-gem 'puma'
+gem 'puma', '~> 2.15.3'
 gem 'pundit'
-gem 'rails', '~> 4.1.2'
+gem 'rails', '~> 4.1.13'
 gem 'rails-i18n', '~> 4.0.0'
 gem 'ransack'
 gem 'rubytree'
diff --git a/Gemfile.lock b/Gemfile.lock
index 0492a3e1..5972ea79 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -2,32 +2,32 @@ GEM
   remote: https://rubygems.org/
   specs:
     ZenTest (4.11.0)
-    actionmailer (4.1.10)
-      actionpack (= 4.1.10)
-      actionview (= 4.1.10)
+    actionmailer (4.1.14)
+      actionpack (= 4.1.14)
+      actionview (= 4.1.14)
       mail (~> 2.5, >= 2.5.4)
-    actionpack (4.1.10)
-      actionview (= 4.1.10)
-      activesupport (= 4.1.10)
+    actionpack (4.1.14)
+      actionview (= 4.1.14)
+      activesupport (= 4.1.14)
       rack (~> 1.5.2)
       rack-test (~> 0.6.2)
-    actionview (4.1.10)
-      activesupport (= 4.1.10)
+    actionview (4.1.14)
+      activesupport (= 4.1.14)
       builder (~> 3.1)
       erubis (~> 2.7.0)
-    activemodel (4.1.10)
-      activesupport (= 4.1.10)
+    activemodel (4.1.14)
+      activesupport (= 4.1.14)
       builder (~> 3.1)
-    activerecord (4.1.10)
-      activemodel (= 4.1.10)
-      activesupport (= 4.1.10)
+    activerecord (4.1.14)
+      activemodel (= 4.1.14)
+      activesupport (= 4.1.14)
       arel (~> 5.0.0)
     activerecord-jdbc-adapter (1.3.15)
       activerecord (>= 2.2)
     activerecord-jdbcpostgresql-adapter (1.3.15)
       activerecord-jdbc-adapter (~> 1.3.15)
       jdbc-postgres (>= 9.1)
-    activesupport (4.1.10)
+    activesupport (4.1.14)
       i18n (~> 0.6, >= 0.6.9)
       json (~> 1.7, >= 1.7.7)
       minitest (~> 5.1)
@@ -95,11 +95,10 @@ GEM
       execjs
     coffee-script-source (1.9.1)
     colorize (0.7.7)
-    concurrent-ruby (0.8.0)
-      ref (~> 1.0, >= 1.0.5)
-    concurrent-ruby (0.8.0-java)
-    concurrent-ruby-ext (0.8.0)
-      concurrent-ruby (~> 0.8.0)
+    concurrent-ruby (1.0.0)
+    concurrent-ruby (1.0.0-java)
+    concurrent-ruby-ext (1.0.0)
+      concurrent-ruby (~> 1.0.0)
     database_cleaner (1.4.1)
     debug_inspector (0.0.2)
     diff-lcs (1.2.5)
@@ -141,17 +140,17 @@ GEM
     jquery-turbolinks (2.1.0)
       railties (>= 3.1.0)
       turbolinks
-    json (1.8.2)
-    json (1.8.2-java)
+    json (1.8.3)
+    json (1.8.3-java)
     jwt (1.4.1)
     kramdown (1.6.0)
     mail (2.6.3)
       mime-types (>= 1.16, < 3)
     method_source (0.8.2)
-    mime-types (2.4.3)
+    mime-types (2.99)
     mini_portile (0.6.2)
-    minitest (5.5.1)
-    multi_json (1.11.0)
+    minitest (5.8.3)
+    multi_json (1.11.2)
     multi_xml (0.5.5)
     multipart-post (2.0.0)
     net-scp (1.2.1)
@@ -185,31 +184,29 @@ GEM
       method_source (~> 0.8.1)
       slop (~> 3.4)
       spoon (~> 0.0)
-    puma (2.11.1)
-      rack (>= 1.1, < 2.0)
-    puma (2.11.1-java)
-      rack (>= 1.1, < 2.0)
+    puma (2.15.3)
+    puma (2.15.3-java)
     pundit (0.3.0)
       activesupport (>= 3.0.0)
-    rack (1.5.2)
+    rack (1.5.5)
     rack-test (0.6.3)
       rack (>= 1.0)
-    rails (4.1.10)
-      actionmailer (= 4.1.10)
-      actionpack (= 4.1.10)
-      actionview (= 4.1.10)
-      activemodel (= 4.1.10)
-      activerecord (= 4.1.10)
-      activesupport (= 4.1.10)
+    rails (4.1.14)
+      actionmailer (= 4.1.14)
+      actionpack (= 4.1.14)
+      actionview (= 4.1.14)
+      activemodel (= 4.1.14)
+      activerecord (= 4.1.14)
+      activesupport (= 4.1.14)
       bundler (>= 1.3.0, < 2.0)
-      railties (= 4.1.10)
+      railties (= 4.1.14)
       sprockets-rails (~> 2.0)
     rails-i18n (4.0.4)
       i18n (~> 0.6)
       railties (~> 4.0)
-    railties (4.1.10)
-      actionpack (= 4.1.10)
-      activesupport (= 4.1.10)
+    railties (4.1.14)
+      actionpack (= 4.1.14)
+      activesupport (= 4.1.14)
       rake (>= 0.8.7)
       thor (>= 0.18.1, < 2.0)
     rainbow (2.0.0)
@@ -221,7 +218,6 @@ GEM
       i18n
       polyamorous (~> 1.2)
     rdoc (4.2.0)
-    ref (1.0.5)
     rspec (3.1.0)
       rspec-core (~> 3.1.0)
       rspec-expectations (~> 3.1.0)
@@ -286,12 +282,12 @@ GEM
     spoon (0.0.4)
       ffi
     spring (1.3.4)
-    sprockets (2.12.3)
+    sprockets (2.12.4)
       hike (~> 1.2)
       multi_json (~> 1.0)
       rack (~> 1.0)
       tilt (~> 1.1, != 1.3.0)
-    sprockets-rails (2.2.4)
+    sprockets-rails (2.3.3)
       actionpack (>= 3.0)
       activesupport (>= 3.0)
       sprockets (>= 2.8, < 4.0)
@@ -351,8 +347,8 @@ DEPENDENCIES
   carrierwave
   codeclimate-test-reporter
   coffee-rails (~> 4.0.0)
-  concurrent-ruby
-  concurrent-ruby-ext
+  concurrent-ruby (~> 1.0.0)
+  concurrent-ruby-ext (~> 1.0.0)
   database_cleaner
   docker-api (~> 1.21.1)
   factory_girl_rails (~> 4.0)
@@ -368,9 +364,9 @@ DEPENDENCIES
   nyan-cat-formatter
   pg
   pry
-  puma
+  puma (~> 2.15.3)
   pundit
-  rails (~> 4.1.2)
+  rails (~> 4.1.13)
   rails-i18n (~> 4.0.0)
   rake
   ransack
diff --git a/app/assets/javascripts/editor.js b/app/assets/javascripts/editor.js
index f67d1fb5..4c9b834e 100644
--- a/app/assets/javascripts/editor.js
+++ b/app/assets/javascripts/editor.js
@@ -100,12 +100,12 @@ $(function() {
     }
   };
 
-  var confirmSubmission = function(event) {
-    event.preventDefault();
-    if (confirm($(this).data('message-confirm'))) {
-      submitCode();
-    }
-  };
+  //var confirmSubmission = function(event) {
+  //  event.preventDefault();
+  //  if (confirm($(this).data('message-confirm'))) {
+  //    submitCode();
+  //  }
+  //};
 
   var createSubmission = function(initiator, filter, callback) {
     showSpinner(initiator);
@@ -637,7 +637,8 @@ $(function() {
 
   var initializeWorkflowButtons = function() {
     $('#start').on('click', showWorkspaceTab);
-    $('#submit').on('click', confirmSubmission);
+    //$('#submit').on('click', confirmSubmission);
+    $('#submit').on('click', submitCode);
   };
 
   var initializeWorkspaceButtons = function() {
diff --git a/app/assets/javascripts/turtle.js b/app/assets/javascripts/turtle.js
index 8aa8a9fe..f2fa184b 100644
--- a/app/assets/javascripts/turtle.js
+++ b/app/assets/javascripts/turtle.js
@@ -33,6 +33,7 @@ function Turtle(pipe, canvas) {
             'x': xpos - dx,
             'y': ypos - dy
         }));
+        pipe.send('\n');
     });
 }
 
diff --git a/app/views/application/help.html.slim b/app/views/application/help.html.slim
index 50e72507..22c3f955 100644
--- a/app/views/application/help.html.slim
+++ b/app/views/application/help.html.slim
@@ -1,9 +1,6 @@
 - unless local_assigns[:modal]
   h1 = t('shared.help.headline')
 
-h2 = t('shared.help.general_help')
-== Forgery(:lorem_ipsum).paragraphs(10, html: true)
-
 - if local_assigns.has_key?(:execution_environment)
   h2 = t('shared.help.execution_environment_specific_help', execution_environment: execution_environment)
   = render_markdown(execution_environment.help)
diff --git a/app/views/exercises/implement.html.slim b/app/views/exercises/implement.html.slim
index a9ce3a54..6ede6d68 100644
--- a/app/views/exercises/implement.html.slim
+++ b/app/views/exercises/implement.html.slim
@@ -76,8 +76,7 @@
 
           br
           - if session[:lti_parameters].try(:has_key?, 'lis_outcome_service_url')
-            p.text-center = render('editor_button', classes: 'btn-lg btn-success', data: {:'data-message-confirm' => t('exercises.editor.confirm_submit'), :'data-url' => submit_exercise_path(@exercise)}, icon: 'fa fa-send', id: 'submit', label: t('exercises.editor.submit'))
-
+            p.text-center = render('editor_button', classes: 'btn-lg btn-success', data: {:'data-url' => submit_exercise_path(@exercise)}, icon: 'fa fa-send', id: 'submit', label: t('exercises.editor.submit'))
   - if qa_url
     #questions-column
       #questions-holder data-url="#{qa_url}/qa/index/#{@exercise.id}/#{@user_id}"
diff --git a/config/application.rb b/config/application.rb
index 744438a9..4b2a543a 100644
--- a/config/application.rb
+++ b/config/application.rb
@@ -33,6 +33,8 @@ module CodeOcean
     case (RUBY_ENGINE)
       when 'ruby'
         # ...
+        #this is enabled in prod for testing
+        config.middleware.use ActiveRecord::ConnectionAdapters::ConnectionManagement
       when 'jruby'
         # plattform specific
         java.lang.Class.for_name('javax.crypto.JceSecurity').get_declared_field('isRestricted').tap{|f| f.accessible = true; f.set nil, false}
diff --git a/config/docker.yml.erb b/config/docker.yml.erb
index 9f916acc..5ac309e8 100644
--- a/config/docker.yml.erb
+++ b/config/docker.yml.erb
@@ -18,8 +18,8 @@ production:
     active: true
     refill:
       async: false
-      batch_size: 32
-      interval: 30
+      batch_size: 8
+      interval: 15
       timeout: 60
   workspace_root: <%= Rails.root.join('tmp', 'files', Rails.env) %>
   ws_host: ws://localhost:4243
diff --git a/lib/docker_client.rb b/lib/docker_client.rb
index 654c6504..77bc40c6 100644
--- a/lib/docker_client.rb
+++ b/lib/docker_client.rb
@@ -88,6 +88,8 @@ class DockerClient
 
   def self.create_container(execution_environment)
     tries ||= 0
+    Rails.logger.info "docker_client: self.create_container with creation options:"
+    Rails.logger.info(container_creation_options(execution_environment))
     container = Docker::Container.create(container_creation_options(execution_environment))
     local_workspace_path = generate_local_workspace_path
     # container.start always creates the passed local_workspace_path on disk. Seems like we have to live with that, therefore we can also just create the empty folder ourselves.
@@ -97,7 +99,7 @@ class DockerClient
     container.status = :created
     container
   rescue Docker::Error::NotFoundError => error
-    Rails.logger.info('create_container: Got Docker::Error::NotFoundError: ' + error)
+    Rails.logger.info('create_container: Got Docker::Error::NotFoundError: ' + error.to_s)
     destroy_container(container)
     #(tries += 1) <= RETRY_COUNT ? retry : raise(error)
   end
@@ -113,6 +115,8 @@ class DockerClient
         create_workspace_file(container: container, file: file)
       end
     end
+  rescue Docker::Error::NotFoundError => error
+    Rails.logger.info('create_workspace_files: Rescued from Docker::Error::NotFoundError: ' + error.to_s)
   end
   private :create_workspace_files
 
@@ -131,6 +135,9 @@ class DockerClient
     if(container)
       container.delete(force: true, v: true)
     end
+  rescue Docker::Error::NotFoundError => error
+    Rails.logger.error('destroy_container: Rescued from Docker::Error::NotFoundError: ' + error.to_s)
+    Rails.logger.error('No further actions are done concerning that.')
   end
 
   def execute_arbitrary_command(command, &block)
@@ -297,7 +304,12 @@ class DockerClient
 
   def self.return_container(container, execution_environment)
     Rails.logger.debug('returning container ' + container.to_s)
-    clean_container_workspace(container)
+    begin
+      clean_container_workspace(container)
+    rescue Docker::Error::NotFoundError => error
+      Rails.logger.info('return_container: Rescued from Docker::Error::NotFoundError: ' + error.to_s)
+      Rails.logger.info('Nothing is done here additionally. The container will be exchanged upon its next retrieval.')
+    end
     DockerContainerPool.return_container(container, execution_environment)
     container.status = :returned
   end
diff --git a/lib/docker_container_pool.rb b/lib/docker_container_pool.rb
index d027076b..1d02876f 100644
--- a/lib/docker_container_pool.rb
+++ b/lib/docker_container_pool.rb
@@ -1,6 +1,6 @@
 require 'concurrent/future'
 require 'concurrent/timer_task'
-require 'concurrent/utilities'
+
 
 class DockerContainerPool
 
@@ -18,10 +18,19 @@ class DockerContainerPool
     @config ||= CodeOcean::Config.new(:docker).read(erb: true)[:pool]
   end
 
+  def self.containers
+    @containers
+  end
+
+  def self.all_containers
+    @all_containers
+  end
+
   def self.remove_from_all_containers(container, execution_environment)
     @all_containers[execution_environment.id]-=[container]
     if(@containers[execution_environment.id].include?(container))
       @containers[execution_environment.id]-=[container]
+      Rails.logger.debug('Removed container ' + container.to_s + ' from all_pool for execution environment ' + execution_environment.to_s + '. Remaining containers in all_pool ' + @all_containers[execution_environment.id].size.to_s)
     end
   end
 
@@ -29,6 +38,7 @@ class DockerContainerPool
     @all_containers[execution_environment.id]+=[container]
     if(!@containers[execution_environment.id].include?(container))
       @containers[execution_environment.id]+=[container]
+      Rails.logger.debug('Added container ' + container.to_s + ' to all_pool for execution environment ' + execution_environment.to_s + '. Containers in all_pool: ' + @all_containers[execution_environment.id].size.to_s)
     else
       Rails.logger.info('failed trying to add existing container ' + container.to_s + ' to execution_environment ' + execution_environment.to_s)
     end
@@ -57,9 +67,28 @@ class DockerContainerPool
       Rails.logger.debug('get_container fetched container  ' + container.to_s + ' for execution environment ' + execution_environment.to_s)
       # just access and the following if we got a container. Otherwise, the execution_environment might be just created and not fully exist yet.
       if(container)
-        Rails.logger.debug('get_container remaining avail. containers:  ' + @containers[execution_environment.id].size.to_s)
-        Rails.logger.debug('get_container all container count: ' + @all_containers[execution_environment.id].size.to_s)
+        begin
+          # check whether the container is running. exited containers go to the else part.
+          # Dead containers raise a NotFOundError on the container.json call. This is handled in the rescue block.
+          if(container.json['State']['Running'])
+            Rails.logger.debug('get_container remaining avail. containers:  ' + @containers[execution_environment.id].size.to_s)
+            Rails.logger.debug('get_container all container count: ' + @all_containers[execution_environment.id].size.to_s)
+          else
+            Rails.logger.error('docker_container_pool.get_container retrieved a container not running. Container will be removed from list:  ' +  container.to_s)
+            remove_from_all_containers(container, execution_environment)
+            Rails.logger.error('Creating a new container and returning that.')
+            container = create_container(execution_environment)
+            DockerContainerPool.add_to_all_containers(container, execution_environment)
+          end
+        rescue Docker::Error::NotFoundError => error
+          Rails.logger.error('docker_container_pool.get_container rescued from Docker::Error::NotFoundError. Most likely, the container is not there any longer. Removing faulty entry from list: ' +  container.to_s)
+          remove_from_all_containers(container, execution_environment)
+          Rails.logger.error('Creating a new container and returning that.')
+          container = create_container(execution_environment)
+          DockerContainerPool.add_to_all_containers(container, execution_environment)
+        end
       end
+      # returning nil is no problem. then the pool is just depleted.
       container
     else
       create_container(execution_environment)
@@ -71,7 +100,7 @@ class DockerContainerPool
   end
 
   def self.refill
-    ExecutionEnvironment.where('pool_size > 0').each do |execution_environment|
+    ExecutionEnvironment.where('pool_size > 0').order(pool_size: :desc).each do |execution_environment|
       if config[:refill][:async]
         Concurrent::Future.execute { refill_for_execution_environment(execution_environment) }
       else
@@ -85,18 +114,19 @@ class DockerContainerPool
     if refill_count > 0
       Rails.logger.info('Adding ' + refill_count.to_s + ' containers for execution_environment ' +  execution_environment.name )
       c = refill_count.times.map { create_container(execution_environment) }
-      Rails.logger.debug('Created containers: ' + c.to_s )
+      Rails.logger.info('Created containers: ' + c.to_s )
+      #c.each { |container| return_container(container, execution_environment) }
       @containers[execution_environment.id] += c
       @all_containers[execution_environment.id] += c
-      Rails.logger.debug('@containers ' + @containers.object_id.to_s + ' has:'+ @containers[execution_environment.id].to_s)
-      Rails.logger.debug('@all_containers '  + @containers.object_id.to_s + ' has:'+ @all_containers[execution_environment.id].to_s)
+      Rails.logger.debug('@containers  for ' + execution_environment.name.to_s + ' (' + @containers.object_id.to_s + ') has the following content: '+ @containers[execution_environment.id].to_s)
+      Rails.logger.debug('@all_containers for '  + execution_environment.name.to_s + ' (' + @all_containers.object_id.to_s + ') has the following content: ' + @all_containers[execution_environment.id].to_s)
       #refill_count.times.map { create_container(execution_environment) }
     end
 
   end
 
   def self.start_refill_task
-    @refill_task = Concurrent::TimerTask.new(execution_interval: config[:refill][:interval], run_now: false, timeout_interval: config[:refill][:timeout]) { refill }
+    @refill_task = Concurrent::TimerTask.new(execution_interval: config[:refill][:interval], run_now: true, timeout_interval: config[:refill][:timeout]) { refill }
     @refill_task.execute
   end
 end