diff --git a/app/assets/javascripts/editor/editor.js.erb b/app/assets/javascripts/editor/editor.js.erb
index 0be56058..0b446c7c 100644
--- a/app/assets/javascripts/editor/editor.js.erb
+++ b/app/assets/javascripts/editor/editor.js.erb
@@ -323,6 +323,9 @@ configureEditors: function () {
     var button = $('#requestComments');
     button.prop('disabled', true);
     button.on('click', function () {
+      if ($('#editor').data('show-interventions') == true){
+        $('#rfc_intervention_text').hide()
+      }
       $('#comment-modal').modal('show');
     });
 
@@ -571,6 +574,80 @@ configureEditors: function () {
 
   },
 
+    /**
+     * interventions
+     * */
+    initializeInterventionTimer: function() {
+        $.ajax({
+            data: {
+                exercise_id: $('#editor').data('exercise-id'),
+                user_id: $('#editor').data('user-id')
+            },
+            dataType: 'json',
+            method: 'GET',
+            // get working times for this exercise
+            url: $('#editor').data('working-times-url'),
+            success: function (data) {
+                var percentile75 = data['working_time_75_percentile'];
+                var accumulatedWorkTimeUser = data['working_time_accumulated'];
+
+                var timeUntilBreak = 20 * 60 * 1000;
+                var minTimeUntilAskQuestion = 15 * 60 * 1000;
+
+                if ((accumulatedWorkTimeUser - percentile75) > 0) {
+                    // working time is already over 75 percentile
+                    var timeUntilAskQuestion = minTimeUntilAskQuestion;
+                } else {
+                    // working time is less than 75 percentile
+                    // ensure we give user at least 10 minutes before we bother the user
+                    var timeUntilAskForRFC = (percentile75 - accumulatedWorkTimeUser) > minTimeUntilAskQuestion ? (percentile75 - accumulatedWorkTimeUser) : minTimeUntilAskQuestion;
+                }
+
+                // if notifications are too close to each other, ensure some time differences between them
+                if (Math.abs(timeUntilAskForRFC - timeUntilBreak) < 5 * 1000 * 60){
+                    timeUntilBreak = timeUntilBreak * 2;
+                }
+
+                setTimeout(function() {
+                    $('#break-intervention-modal').modal('show');
+                    $.ajax({
+                        data: {
+                            intervention_type: 'BreakIntervention'
+                        },
+                        dataType: 'json',
+                        type: 'POST',
+                        url: $('#editor').data('intervention-save-url')});
+                }, timeUntilBreak);
+
+
+                setTimeout(function() {
+                    var button = $('#requestComments');
+                    if (!button.prop('disabled')){
+                        $('#rfc_intervention_text').show();
+                        $('#comment-modal').modal('show');
+                        $.ajax({
+                            data: {
+                                intervention_type: 'QuestionIntervention'
+                            },
+                            dataType: 'json',
+                            type: 'POST',
+                            url: $('#editor').data('intervention-save-url')});
+                    };
+                }, timeUntilAskForRFC);
+            }
+        });
+    },
+
+    initializeSearchButton: function(){
+        $('#btn-search-col').button().click(function(){
+            var search = $('#search-col').val();
+            var course_token = $('#sidebar-collapsed').data('course_token')
+            window.open(`https://open.hpi.de/courses/${course_token}/pinboard?query=${search}`, '_blank');
+        })
+
+        $('#sidebar-search-collapsed').on('click',this.handleSideBarToggle.bind(this));
+    },
+
 
   initializeEverything: function() {
     this.initializeRegexes();
@@ -585,6 +662,10 @@ configureEditors: function () {
     this.initializeDescriptionToggle();
     this.initializeSideBarTooltips();
     this.initializeTooltips();
+    if ($('#editor').data('show-interventions') == true){
+        this.initializeInterventionTimer();
+    }
+    this.initializeSearchButton();
     this.initPrompt();
     this.renderScore();
     this.showFirstFile();
diff --git a/app/controllers/concerns/lti.rb b/app/controllers/concerns/lti.rb
index 7c168ec6..7483327d 100644
--- a/app/controllers/concerns/lti.rb
+++ b/app/controllers/concerns/lti.rb
@@ -74,7 +74,12 @@ module Lti
   private :require_valid_consumer_key
 
   def require_valid_exercise_token
-    @exercise = Exercise.find_by(token: params[:custom_token])
+    proxy_exercise = ProxyExercise.find_by(token: params[:custom_token])
+    unless proxy_exercise.nil?
+      @exercise = proxy_exercise.get_matching_exercise(@current_user)
+    else
+      @exercise = Exercise.find_by(token: params[:custom_token])
+    end
     refuse_lti_launch(message: t('sessions.oauth.invalid_exercise_token')) unless @exercise
   end
   private :require_valid_exercise_token
@@ -129,19 +134,16 @@ module Lti
   private :set_current_user
 
   def store_lti_session_data(options = {})
-    exercise = Exercise.where(token: options[:parameters][:custom_token]).first
-    exercise_id = exercise.id unless exercise.nil?
-
-    current_user = ExternalUser.find_or_create_by(consumer_id: options[:consumer].id, external_id: options[:parameters][:user_id].to_s)
     lti_parameters = LtiParameter.find_or_create_by(consumers_id: options[:consumer].id,
-                                                    external_users_id: current_user.id,
-                                                    exercises_id: exercise_id)
+                                                    external_users_id: @current_user.id,
+                                                    exercises_id: @exercise.id)
 
     lti_parameters.lti_parameters = options[:parameters].slice(*SESSION_PARAMETERS).to_json
     lti_parameters.save!
+    @lti_parameters = lti_parameters
 
     session[:consumer_id] = options[:consumer].id
-    session[:external_user_id] = current_user.id
+    session[:external_user_id] = @current_user.id
   end
   private :store_lti_session_data
 
diff --git a/app/controllers/exercises_controller.rb b/app/controllers/exercises_controller.rb
index 79314208..62799e27 100644
--- a/app/controllers/exercises_controller.rb
+++ b/app/controllers/exercises_controller.rb
@@ -6,9 +6,10 @@ class ExercisesController < ApplicationController
 
   before_action :handle_file_uploads, only: [:create, :update]
   before_action :set_execution_environments, only: [:create, :edit, :new, :update]
-  before_action :set_exercise, only: MEMBER_ACTIONS + [:clone, :implement, :run, :statistics, :submit, :reload]
+  before_action :set_exercise, only: MEMBER_ACTIONS + [:clone, :implement, :working_times, :intervention, :run, :statistics, :submit, :reload]
   before_action :set_external_user, only: [:statistics]
   before_action :set_file_types, only: [:create, :edit, :new, :update]
+  before_action :set_course_token, only: [:implement]
 
   skip_before_filter :verify_authenticity_token, only: [:import_proforma_xml]
   skip_after_action :verify_authorized, only: [:import_proforma_xml]
@@ -19,6 +20,15 @@ class ExercisesController < ApplicationController
   end
   private :authorize!
 
+  def max_intervention_count
+    3
+  end
+
+
+  def java_course_token
+    "702cbd2a-c84c-4b37-923a-692d7d1532d0"
+  end
+
   def batch_update
     @exercises = Exercise.all
     authorize!
@@ -54,6 +64,20 @@ class ExercisesController < ApplicationController
 
   def create
     @exercise = Exercise.new(exercise_params)
+    collect_set_and_unset_exercise_tags
+    myparam = exercise_params
+    checked_exercise_tags = @exercise_tags.select { | et | myparam[:tag_ids].include? et.tag.id.to_s }
+    removed_exercise_tags = @exercise_tags.reject { | et | myparam[:tag_ids].include? et.tag.id.to_s }
+
+    for et in checked_exercise_tags
+      et.factor = params[:tag_factors][et.tag_id.to_s][:factor]
+      et.exercise = @exercise
+    end
+
+    myparam[:exercise_tags] = checked_exercise_tags
+    myparam.delete :tag_ids
+    removed_exercise_tags.map {|et| et.destroy}
+
     authorize!
     create_and_respond(object: @exercise)
   end
@@ -63,6 +87,7 @@ class ExercisesController < ApplicationController
   end
 
   def edit
+    collect_set_and_unset_exercise_tags
   end
 
   def import_proforma_xml
@@ -118,7 +143,8 @@ class ExercisesController < ApplicationController
   private :user_by_code_harbor_token
 
   def exercise_params
-    params[:exercise].permit(:description, :execution_environment_id, :file_id, :instructions, :public, :hide_file_tree, :allow_file_creation, :allow_auto_completion, :title, files_attributes: file_attributes).merge(user_id: current_user.id, user_type: current_user.class.name)
+    params[:exercise][:expected_worktime_seconds] = params[:exercise][:expected_worktime_minutes].to_i * 60
+    params[:exercise].permit(:description, :execution_environment_id, :file_id, :instructions, :public, :hide_file_tree, :allow_file_creation, :allow_auto_completion, :title, :expected_difficulty, :expected_worktime_seconds, files_attributes: file_attributes, :tag_ids => []).merge(user_id: current_user.id, user_type: current_user.class.name)
   end
   private :exercise_params
 
@@ -139,6 +165,13 @@ class ExercisesController < ApplicationController
 
   def implement
     redirect_to(@exercise, alert: t('exercises.implement.no_files')) unless @exercise.files.visible.exists?
+    user_got_enough_interventions = UserExerciseIntervention.where(exercise: @exercise, user: current_user).count >= max_intervention_count
+    is_java_course = @course_token && @course_token.eql?(java_course_token)
+
+    @show_interventions = (!is_java_course || user_got_enough_interventions) ? "false" : "true"
+
+    @search = Search.new
+    @search.exercise = @exercise
     @submission = current_user.submissions.where(exercise_id: @exercise.id).order('created_at DESC').first
     @files = (@submission ? @submission.collect_files : @exercise.files).select(&:visible).sort_by(&:name_with_extension)
     @paths = collect_paths(@files)
@@ -150,6 +183,44 @@ class ExercisesController < ApplicationController
     end
   end
 
+  def set_course_token
+    lti_parameters = LtiParameter.find_by(external_users_id: current_user.id,
+                                          exercises_id: @exercise.id)
+    if lti_parameters
+      lti_json = lti_parameters.lti_parameters["lis_outcome_service_url"]
+      @course_token =
+          if match = lti_json.match(/^.*courses\/([a-z0-9\-]+)\/sections/)
+            match.captures.first
+          else
+            java_course_token
+          end
+    else
+      # no consumer, therefore implementation with internal user
+      @course_token = java_course_token
+    end
+  end
+  private :set_course_token
+
+  def working_times
+    working_time_accumulated = @exercise.accumulated_working_time_for_only(current_user)
+    working_time_75_percentile = @exercise.get_quantiles([0.75]).first
+    render(json: {working_time_75_percentile: working_time_75_percentile, working_time_accumulated: working_time_accumulated})
+  end
+
+  def intervention
+    intervention = Intervention.find_by_name(params[:intervention_type])
+    unless intervention.nil?
+      uei = UserExerciseIntervention.new(
+          user: current_user, exercise: @exercise, intervention: intervention,
+          accumulated_worktime_s: @exercise.accumulated_working_time_for_only(current_user))
+      uei.save
+      render(json: {success: 'true'})
+    else
+      render(json: {success: 'false', error: "undefined intervention #{params[:intervention_type]}"})
+    end
+
+  end
+
   def index
     @search = policy_scope(Exercise).search(params[:q])
     @exercises = @search.result.includes(:execution_environment, :user).order(:title).paginate(page: params[:page])
@@ -174,6 +245,8 @@ class ExercisesController < ApplicationController
 
   def new
     @exercise = Exercise.new
+    collect_set_and_unset_exercise_tags
+
     authorize!
   end
 
@@ -201,6 +274,16 @@ class ExercisesController < ApplicationController
   end
   private :set_file_types
 
+  def collect_set_and_unset_exercise_tags
+    @search = policy_scope(Tag).search(params[:q])
+    @tags = @search.result.order(:name)
+    checked_exercise_tags = @exercise.exercise_tags
+    checked_tags = checked_exercise_tags.collect{|e| e.tag}.to_set
+    unchecked_tags = Tag.all.to_set.subtract checked_tags
+    @exercise_tags = checked_exercise_tags + unchecked_tags.collect { |tag| ExerciseTag.new(exercise: @exercise, tag: tag)}
+  end
+  private :collect_set_and_unset_exercise_tags
+
   def show
   end
 
@@ -252,7 +335,20 @@ class ExercisesController < ApplicationController
   private :transmit_lti_score
 
   def update
-    update_and_respond(object: @exercise, params: exercise_params)
+    collect_set_and_unset_exercise_tags
+    myparam = exercise_params
+    checked_exercise_tags = @exercise_tags.select { | et | myparam[:tag_ids].include? et.tag.id.to_s }
+    removed_exercise_tags = @exercise_tags.reject { | et | myparam[:tag_ids].include? et.tag.id.to_s }
+
+    for et in checked_exercise_tags
+      et.factor = params[:tag_factors][et.tag_id.to_s][:factor]
+      et.exercise = @exercise
+    end
+
+    myparam[:exercise_tags] = checked_exercise_tags
+    myparam.delete :tag_ids
+    removed_exercise_tags.map {|et| et.destroy}
+    update_and_respond(object: @exercise, params: myparam)
   end
 
   def redirect_after_submit
diff --git a/app/controllers/interventions_controller.rb b/app/controllers/interventions_controller.rb
new file mode 100644
index 00000000..b4b5971e
--- /dev/null
+++ b/app/controllers/interventions_controller.rb
@@ -0,0 +1,55 @@
+class InterventionsController < ApplicationController
+  include CommonBehavior
+
+  before_action :set_intervention, only: MEMBER_ACTIONS
+
+  def authorize!
+    authorize(@intervention || @interventions)
+  end
+  private :authorize!
+
+  def create
+    #@intervention = Intervention.new(intervention_params)
+    #authorize!
+    #create_and_respond(object: @intervention)
+  end
+
+  def destroy
+    destroy_and_respond(object: @intervention)
+  end
+
+  def edit
+  end
+
+  def intervention_params
+    params[:intervention].permit(:name)
+  end
+  private :intervention_params
+
+  def index
+    @interventions = Intervention.all.paginate(page: params[:page])
+    authorize!
+  end
+
+  def new
+    #@intervention = Intervention.new
+    #authorize!
+  end
+
+  def set_intervention
+    @intervention = Intervention.find(params[:id])
+    authorize!
+  end
+  private :set_intervention
+
+  def show
+  end
+
+  def update
+    update_and_respond(object: @intervention, params: intervention_params)
+  end
+
+  def to_s
+    name
+  end
+end
diff --git a/app/controllers/proxy_exercises_controller.rb b/app/controllers/proxy_exercises_controller.rb
new file mode 100644
index 00000000..02fe0220
--- /dev/null
+++ b/app/controllers/proxy_exercises_controller.rb
@@ -0,0 +1,80 @@
+class ProxyExercisesController < ApplicationController
+  include CommonBehavior
+
+  before_action :set_exercise, only: MEMBER_ACTIONS + [:clone, :reload]
+
+  def authorize!
+    authorize(@proxy_exercise || @proxy_exercises)
+  end
+  private :authorize!
+
+  def clone
+    proxy_exercise = @proxy_exercise.duplicate(token: nil, exercises: @proxy_exercise.exercises)
+    proxy_exercise.send(:generate_token)
+    if proxy_exercise.save
+      redirect_to(proxy_exercise, notice: t('shared.object_cloned', model:  ProxyExercise.model_name.human))
+    else
+      flash[:danger] = t('shared.message_failure')
+      redirect_to(@proxy_exercise)
+    end
+  end
+
+  def create
+    myparams = proxy_exercise_params
+    myparams[:exercises] = Exercise.find(myparams[:exercise_ids].reject { |c| c.empty? })
+    @proxy_exercise = ProxyExercise.new(myparams)
+    authorize!
+
+    create_and_respond(object: @proxy_exercise)
+  end
+
+  def destroy
+    destroy_and_respond(object: @proxy_exercise)
+  end
+
+  def edit
+    @search = policy_scope(Exercise).search(params[:q])
+    @exercises = @search.result.order(:title)
+    authorize!
+  end
+
+  def proxy_exercise_params
+    params[:proxy_exercise].permit(:description, :title, :exercise_ids  => [])
+  end
+  private :proxy_exercise_params
+
+  def index
+    @search = policy_scope(ProxyExercise).search(params[:q])
+    @proxy_exercises = @search.result.order(:title).paginate(page: params[:page])
+    authorize!
+  end
+
+  def new
+    @proxy_exercise =  ProxyExercise.new
+    @search = policy_scope(Exercise).search(params[:q])
+    @exercises = @search.result.order(:title)
+    authorize!
+  end
+
+  def set_exercise
+    @proxy_exercise = ProxyExercise.find(params[:id])
+    authorize!
+  end
+  private :set_exercise
+
+  def show
+    @search = @proxy_exercise.exercises.search
+    @exercises = @proxy_exercise.exercises.search.result.order(:title) #@search.result.order(:title)
+  end
+
+  #we might want to think about auth here
+  def reload
+  end
+
+  def update
+    myparams = proxy_exercise_params
+    myparams[:exercises] = Exercise.find(myparams[:exercise_ids].reject { |c| c.blank? })
+    update_and_respond(object: @proxy_exercise, params: myparams)
+  end
+
+end
diff --git a/app/controllers/searches_controller.rb b/app/controllers/searches_controller.rb
new file mode 100644
index 00000000..8af6b364
--- /dev/null
+++ b/app/controllers/searches_controller.rb
@@ -0,0 +1,34 @@
+class SearchesController < ApplicationController
+  include CommonBehavior
+
+  def authorize!
+    authorize(@search || @searchs)
+  end
+  private :authorize!
+
+
+  def create
+    @search = Search.new(search_params)
+    @search.user = current_user
+    authorize!
+
+    respond_to do |format|
+      if @search.save
+        path = implement_exercise_path(@search.exercise)
+        respond_with_valid_object(format, path: path, status: :created)
+      end
+    end
+  end
+
+  def search_params
+    params[:search].permit(:search, :exercise_id)
+  end
+  private :search_params
+
+  def index
+    @search = policy_scope(ProxyExercise).search(params[:q])
+    @searches = @search.result.order(:title).paginate(page: params[:page])
+    authorize!
+  end
+
+end
\ No newline at end of file
diff --git a/app/controllers/sessions_controller.rb b/app/controllers/sessions_controller.rb
index e6bdac8c..8f698d1a 100644
--- a/app/controllers/sessions_controller.rb
+++ b/app/controllers/sessions_controller.rb
@@ -1,7 +1,7 @@
 class SessionsController < ApplicationController
   include Lti
 
-  [:require_oauth_parameters, :require_valid_consumer_key, :require_valid_oauth_signature, :require_unique_oauth_nonce, :require_valid_exercise_token].each do |method_name|
+  [:require_oauth_parameters, :require_valid_consumer_key, :require_valid_oauth_signature, :require_unique_oauth_nonce, :set_current_user, :require_valid_exercise_token].each do |method_name|
     before_action(method_name, only: :create_through_lti)
   end
 
@@ -18,7 +18,6 @@ class SessionsController < ApplicationController
   end
 
   def create_through_lti
-    set_current_user
     store_lti_session_data(consumer: @consumer, parameters: params)
     store_nonce(params[:oauth_nonce])
     redirect_to(implement_exercise_path(@exercise),
diff --git a/app/controllers/tags_controller.rb b/app/controllers/tags_controller.rb
new file mode 100644
index 00000000..ff6925dd
--- /dev/null
+++ b/app/controllers/tags_controller.rb
@@ -0,0 +1,55 @@
+class TagsController < ApplicationController
+  include CommonBehavior
+
+  before_action :set_tag, only: MEMBER_ACTIONS
+
+  def authorize!
+    authorize(@tag || @tags)
+  end
+  private :authorize!
+
+  def create
+    @tag = Tag.new(tag_params)
+    authorize!
+    create_and_respond(object: @tag)
+  end
+
+  def destroy
+    destroy_and_respond(object: @tag)
+  end
+
+  def edit
+  end
+
+  def tag_params
+    params[:tag].permit(:name)
+  end
+  private :tag_params
+
+  def index
+    @tags = Tag.all.paginate(page: params[:page])
+    authorize!
+  end
+
+  def new
+    @tag = Tag.new
+    authorize!
+  end
+
+  def set_tag
+    @tag = Tag.find(params[:id])
+    authorize!
+  end
+  private :set_tag
+
+  def show
+  end
+
+  def update
+    update_and_respond(object: @tag, params: tag_params)
+  end
+
+  def to_s
+    name
+  end
+end
diff --git a/app/models/concerns/user.rb b/app/models/concerns/user.rb
index ee72a715..28601cdf 100644
--- a/app/models/concerns/user.rb
+++ b/app/models/concerns/user.rb
@@ -8,6 +8,11 @@ module User
     has_many :exercises, as: :user
     has_many :file_types, as: :user
     has_many :submissions, as: :user
+    has_many :user_proxy_exercise_exercises, as: :user
+    has_many :user_exercise_interventions, as: :user
+    has_many :interventions, through: :user_exercise_interventions
+    accepts_nested_attributes_for :user_proxy_exercise_exercises
+
 
     scope :with_submissions, -> { where('id IN (SELECT user_id FROM submissions)') }
   end
diff --git a/app/models/exercise.rb b/app/models/exercise.rb
index 29f260c2..15c740f4 100644
--- a/app/models/exercise.rb
+++ b/app/models/exercise.rb
@@ -12,6 +12,15 @@ class Exercise < ActiveRecord::Base
   belongs_to :execution_environment
   has_many :submissions
 
+  has_and_belongs_to_many :proxy_exercises
+  has_many :user_proxy_exercise_exercises
+  has_and_belongs_to_many :exercise_collections
+  has_many :user_exercise_interventions
+  has_many :interventions, through: :user_exercise_interventions
+  has_many :exercise_tags
+  has_many :tags, through: :exercise_tags
+  accepts_nested_attributes_for :exercise_tags
+
   has_many :external_users, source: :user, source_type: ExternalUser, through: :submissions
   has_many :internal_users, source: :user, source_type: InternalUser, through: :submissions
   alias_method :users, :external_users
@@ -48,6 +57,10 @@ class Exercise < ActiveRecord::Base
     return user_count == 0 ? 0 : submissions.count() / user_count.to_f()
   end
 
+  def time_maximum_score(user)
+    submissions.where(user: user).where("cause IN ('submit','assess')").where("score IS NOT NULL").order("score DESC, created_at ASC").first.created_at rescue Time.zone.at(0)
+  end
+
   def user_working_time_query
     """
       SELECT user_id,
@@ -58,7 +71,7 @@ class Exercise < ActiveRecord::Base
          FROM
             (SELECT user_id,
                     id,
-                    (created_at - lag(created_at) over (PARTITION BY user_id
+                    (created_at - lag(created_at) over (PARTITION BY user_id, exercise_id
                                                         ORDER BY created_at)) AS working_time
             FROM submissions
             WHERE exercise_id=#{id}) AS foo) AS bar
@@ -66,6 +79,35 @@ class Exercise < ActiveRecord::Base
     """
   end
 
+  def get_quantiles(quantiles)
+    quantiles_str = "[" + quantiles.join(",") + "]"
+    result = self.class.connection.execute("""
+      SELECT unnest(PERCENTILE_CONT(ARRAY#{quantiles_str}) WITHIN GROUP (ORDER BY working_time))
+      FROM
+          (
+            SELECT user_id,
+                   sum(working_time_new) AS working_time
+            FROM
+              (SELECT user_id,
+                      CASE WHEN working_time >= '0:30:00' THEN '0' ELSE working_time END AS working_time_new
+               FROM
+                  (SELECT user_id,
+                          id,
+                          (created_at - lag(created_at) OVER (PARTITION BY user_id, exercise_id
+                                                              ORDER BY created_at)) AS working_time
+                  FROM submissions
+                  WHERE exercise_id=#{self.id} AND user_type = 'ExternalUser') AS foo) AS bar
+            GROUP BY user_id
+      ) AS foo
+    """)
+    if result.count > 0
+      quantiles.each_with_index.map{|q,i| Time.parse(result[i]["unnest"]).seconds_since_midnight}
+    else
+      quantiles.map{|q| 0}
+    end
+
+  end
+
   def retrieve_working_time_statistics
     @working_time_statistics = {}
     self.class.connection.execute(user_working_time_query).each do |tuple|
@@ -88,23 +130,25 @@ class Exercise < ActiveRecord::Base
     @working_time_statistics[user_id]["working_time"]
   end
 
-  def average_working_time_for_only(user_id)
-    self.class.connection.execute("""
+  def accumulated_working_time_for_only(user)
+    user_type = user.external_user? ? "ExternalUser" : "InternalUser"
+    Time.parse(self.class.connection.execute("""
       SELECT sum(working_time_new) AS working_time
       FROM
         (SELECT CASE WHEN working_time >= '0:30:00' THEN '0' ELSE working_time END AS working_time_new
          FROM
             (SELECT id,
-                    (created_at - lag(created_at) over (PARTITION BY user_id
+                    (created_at - lag(created_at) over (PARTITION BY user_id, exercise_id
                                                         ORDER BY created_at)) AS working_time
             FROM submissions
-            WHERE exercise_id=#{id} and user_id=#{user_id}) AS foo) AS bar
-    """).first["working_time"]
+            WHERE exercise_id=#{id} and user_id=#{user.id} and user_type='#{user_type}') AS foo) AS bar
+    """).first["working_time"] || "00:00:00").seconds_since_midnight
   end
 
   def duplicate(attributes = {})
     exercise = dup
     exercise.attributes = attributes
+    exercise_tags.each  { |et| exercise.exercise_tags << et.dup }
     files.each { |file| exercise.files << file.dup }
     exercise
   end
@@ -162,8 +206,12 @@ class Exercise < ActiveRecord::Base
   end
   private :generate_token
 
-  def maximum_score
-    files.teacher_defined_tests.sum(:weight)
+  def maximum_score(user = nil)
+    if user
+      submissions.where(user: user).where("cause IN ('submit','assess')").where("score IS NOT NULL").order("score DESC").first.score || 0 rescue 0
+    else
+      files.teacher_defined_tests.sum(:weight)
+    end
   end
 
   def set_default_values
diff --git a/app/models/exercise_collection.rb b/app/models/exercise_collection.rb
new file mode 100644
index 00000000..2dca0e9d
--- /dev/null
+++ b/app/models/exercise_collection.rb
@@ -0,0 +1,5 @@
+class ExerciseCollection < ActiveRecord::Base
+
+  has_and_belongs_to_many :exercises
+
+end
\ No newline at end of file
diff --git a/app/models/exercise_tag.rb b/app/models/exercise_tag.rb
new file mode 100644
index 00000000..4b8ab3e5
--- /dev/null
+++ b/app/models/exercise_tag.rb
@@ -0,0 +1,13 @@
+class ExerciseTag < ActiveRecord::Base
+
+  belongs_to :tag
+  belongs_to :exercise
+
+  before_save :destroy_if_empty_exercise_or_tag
+
+  private
+  def destroy_if_empty_exercise_or_tag
+     destroy if exercise_id.blank? || tag_id.blank?
+  end
+
+end
\ No newline at end of file
diff --git a/app/models/intervention.rb b/app/models/intervention.rb
new file mode 100644
index 00000000..a6693450
--- /dev/null
+++ b/app/models/intervention.rb
@@ -0,0 +1,16 @@
+class Intervention < ActiveRecord::Base
+
+  has_many :user_exercise_interventions
+  has_many :users, through: :user_exercise_interventions, source_type: "ExternalUser"
+
+  def to_s
+    name
+  end
+
+  def self.createDefaultInterventions
+    %w(BreakIntervention QuestionIntervention).each do |name|
+      Intervention.find_or_create_by(name: name)
+    end
+  end
+
+end
\ No newline at end of file
diff --git a/app/models/proxy_exercise.rb b/app/models/proxy_exercise.rb
new file mode 100644
index 00000000..11b5a545
--- /dev/null
+++ b/app/models/proxy_exercise.rb
@@ -0,0 +1,220 @@
+class ProxyExercise < ActiveRecord::Base
+
+    after_initialize :generate_token
+    after_initialize :set_reason
+
+    has_and_belongs_to_many :exercises
+    has_many :user_proxy_exercise_exercises
+
+    def count_files
+        exercises.count
+    end
+
+    def set_reason
+      @reason = {}
+    end
+
+    def generate_token
+      self.token ||= SecureRandom.hex(4)
+    end
+    private :generate_token
+
+    def duplicate(attributes = {})
+      proxy_exercise = dup
+      proxy_exercise.attributes = attributes
+      proxy_exercise
+    end
+
+    def to_s
+      title
+    end
+
+    def get_matching_exercise(user)
+      assigned_user_proxy_exercise = user_proxy_exercise_exercises.where(user: user).first
+      recommended_exercise =
+        if (assigned_user_proxy_exercise)
+          Rails.logger.debug("retrieved assigned exercise for user #{user.id}: Exercise #{assigned_user_proxy_exercise.exercise}" )
+          assigned_user_proxy_exercise.exercise
+        else
+          Rails.logger.debug("find new matching exercise for user #{user.id}" )
+          matching_exercise =
+              begin
+                find_matching_exercise(user)
+              rescue #fallback
+                Rails.logger.error("finding matching exercise failed. Fall back to random exercise! Error: #{$!}" )
+                @reason[:reason] = "fallback because of error"
+                @reason[:error] = "#{$!}"
+                exercises.shuffle.first
+              end
+          user.user_proxy_exercise_exercises << UserProxyExerciseExercise.create(user: user, exercise: matching_exercise, proxy_exercise: self, reason: @reason.to_json)
+          matching_exercise
+        end
+      recommended_exercise
+    end
+
+    def find_matching_exercise(user)
+      exercises_user_has_accessed = user.submissions.where("cause IN ('submit','assess')").map{|s| s.exercise}.uniq
+      tags_user_has_seen = exercises_user_has_accessed.map{|ex| ex.tags}.uniq.flatten
+      Rails.logger.debug("exercises_user_has_accessed #{exercises_user_has_accessed.map{|e|e.id}.join(",")}")
+
+      # find execises
+      potential_recommended_exercises = []
+      exercises.each do |ex|
+        ## find exercises which have only tags the user has already seen
+        if (ex.tags - tags_user_has_seen).empty?
+          potential_recommended_exercises << ex
+        end
+      end
+      Rails.logger.debug("potential_recommended_exercises: #{potential_recommended_exercises.map{|e|e.id}}")
+      # if all exercises contain tags which the user has never seen, recommend easiest exercise
+      if potential_recommended_exercises.empty?
+        Rails.logger.debug("matched easiest exercise in pool")
+        @reason[:reason] = "easiest exercise in pool. empty potential exercises"
+        select_easiest_exercise(exercises)
+      else
+        recommended_exercise = select_best_matching_exercise(user, exercises_user_has_accessed, potential_recommended_exercises)
+        recommended_exercise
+      end
+    end
+    private :find_matching_exercise
+
+    def select_best_matching_exercise(user, exercises_user_has_accessed, potential_recommended_exercises)
+      topic_knowledge_user_and_max = get_user_knowledge_and_max_knowledge(user, exercises_user_has_accessed)
+      Rails.logger.debug("topic_knowledge_user_and_max: #{topic_knowledge_user_and_max}")
+      Rails.logger.debug("potential_recommended_exercises: #{potential_recommended_exercises.size}: #{potential_recommended_exercises.map{|p| p.id}}")
+      topic_knowledge_user = topic_knowledge_user_and_max[:user_topic_knowledge]
+      topic_knowledge_max = topic_knowledge_user_and_max[:max_topic_knowledge]
+      current_users_knowledge_lack = {}
+      topic_knowledge_max.keys.each do |tag|
+        current_users_knowledge_lack[tag] = topic_knowledge_user[tag] /  topic_knowledge_max[tag]
+      end
+
+      relative_knowledge_improvement = {}
+      potential_recommended_exercises.each do |potex|
+        tags =  potex.tags
+        relative_knowledge_improvement[potex] = 0.0
+        Rails.logger.debug("review potential exercise #{potex.id}")
+        tags.each do |tag|
+          tag_ratio = potex.exercise_tags.where(tag: tag).first.factor.to_f / potex.exercise_tags.inject(0){|sum, et| sum += et.factor }.to_f
+          max_topic_knowledge_ratio = potex.expected_difficulty * tag_ratio
+          old_relative_loss_tag = topic_knowledge_user[tag] / topic_knowledge_max[tag]
+          new_relative_loss_tag = topic_knowledge_user[tag] / (topic_knowledge_max[tag] + max_topic_knowledge_ratio)
+          Rails.logger.debug("tag #{tag} old_relative_loss_tag #{old_relative_loss_tag}, new_relative_loss_tag #{new_relative_loss_tag}, tag_ratio #{tag_ratio}")
+          relative_knowledge_improvement[potex] += old_relative_loss_tag - new_relative_loss_tag
+        end
+      end
+      highest_difficulty_user_has_accessed =  exercises_user_has_accessed.map{|e| e.expected_difficulty}.sort.last || 0
+      best_matching_exercise = find_best_exercise(relative_knowledge_improvement, highest_difficulty_user_has_accessed)
+      @reason[:reason] = "best matching exercise"
+      @reason[:highest_difficulty_user_has_accessed] = highest_difficulty_user_has_accessed
+      @reason[:current_users_knowledge_lack] = current_users_knowledge_lack
+      @reason[:relative_knowledge_improvement] = relative_knowledge_improvement
+
+      Rails.logger.debug("current users knowledge loss: " + current_users_knowledge_lack.map{|k,v| "#{k} => #{v}"}.to_s)
+      Rails.logger.debug("relative improvements #{relative_knowledge_improvement.map{|k,v| k.id.to_s + ':' + v.to_s}}")
+      best_matching_exercise
+    end
+    private :select_best_matching_exercise
+
+    def find_best_exercise(relative_knowledge_improvement, highest_difficulty_user_has_accessed)
+      Rails.logger.debug("select most appropiate exercise for user. his highest difficulty was #{highest_difficulty_user_has_accessed}")
+      sorted_exercises = relative_knowledge_improvement.sort_by{|k,v| v}.reverse
+
+      sorted_exercises.each do |ex,diff|
+        Rails.logger.debug("review exercise #{ex.id} diff: #{ex.expected_difficulty}")
+        if (ex.expected_difficulty - highest_difficulty_user_has_accessed) <= 1
+          Rails.logger.debug("matched exercise #{ex.id}")
+          return ex
+        else
+          Rails.logger.debug("exercise #{ex.id} is too difficult")
+        end
+      end
+      easiest_exercise = sorted_exercises.min_by{|k,v| v}.first
+      Rails.logger.debug("no match, select easiest exercise as fallback #{easiest_exercise.id}")
+      easiest_exercise
+    end
+    private :find_best_exercise
+
+    # [score][quantile]
+    def scoring_matrix
+      [
+          [0  ,0  ,0  ,0  ,0  ],
+          [0.2,0.2,0.2,0.2,0.1],
+          [0.5,0.5,0.4,0.4,0.3],
+          [0.6,0.6,0.5,0.5,0.4],
+          [1  ,1  ,0.9,0.8,0.7],
+      ]
+    end
+
+    def scoring_matrix_quantiles
+      [0.2,0.4,0.6,0.8]
+    end
+    private :scoring_matrix_quantiles
+
+    def score(user, ex)
+      points_ratio =  ex.maximum_score(user) / ex.maximum_score.to_f
+      if points_ratio == 0.0
+        Rails.logger.debug("scoring user #{user.id} for exercise #{ex.id}: points_ratio=#{points_ratio} score: 0" )
+        return 0.0
+      end
+      points_ratio_index = ((scoring_matrix.size - 1)  * points_ratio).to_i
+      working_time_user = ex.accumulated_working_time_for_only(user)
+      quantiles_working_time = ex.get_quantiles(scoring_matrix_quantiles)
+      quantile_index = quantiles_working_time.size
+      quantiles_working_time.each_with_index do |quantile_time, i|
+        if working_time_user <= quantile_time
+          quantile_index = i
+          break
+        end
+      end
+      Rails.logger.debug(
+          "scoring user #{user.id} exercise #{ex.id}: worktime #{working_time_user}, points: #{points_ratio}" \
+          "(index #{points_ratio_index}) quantiles #{quantiles_working_time} placed into quantile index #{quantile_index} " \
+          "score: #{scoring_matrix[points_ratio_index][quantile_index]}")
+      scoring_matrix[points_ratio_index][quantile_index]
+    end
+    private :score
+
+    def get_user_knowledge_and_max_knowledge(user, exercises)
+      # initialize knowledge for each tag with 0
+      all_used_tags_with_count = {}
+      exercises.each do |ex|
+        ex.tags.each do |t|
+          all_used_tags_with_count[t] ||= 0
+          all_used_tags_with_count[t] += 1
+        end
+      end
+      tags_counter = all_used_tags_with_count.keys.map{|tag| [tag,0]}.to_h
+      topic_knowledge_loss_user = all_used_tags_with_count.keys.map{|t| [t, 0]}.to_h
+      topic_knowledge_max = all_used_tags_with_count.keys.map{|t| [t, 0]}.to_h
+      exercises_sorted = exercises.sort_by { |ex| ex.time_maximum_score(user)}
+      exercises_sorted.each do |ex|
+        Rails.logger.debug("exercise: #{ex.id}: #{ex}")
+        user_score_factor = score(user, ex)
+        ex.tags.each do |t|
+          tags_counter[t] += 1
+          tag_diminishing_return_factor = tag_diminishing_return_function(tags_counter[t], all_used_tags_with_count[t])
+          tag_ratio = ex.exercise_tags.where(tag: t).first.factor.to_f / ex.exercise_tags.inject(0){|sum, et| sum += et.factor }.to_f
+          Rails.logger.debug("tag: #{t}, factor: #{ex.exercise_tags.where(tag: t).first.factor}, sumall: #{ex.exercise_tags.inject(0){|sum, et| sum += et.factor }}")
+          Rails.logger.debug("tag #{t}, count #{tags_counter[t]}, max: #{all_used_tags_with_count[t]}, factor: #{tag_diminishing_return_factor}")
+          Rails.logger.debug("tag_ratio #{tag_ratio}")
+          topic_knowledge_ratio = ex.expected_difficulty * tag_ratio
+          Rails.logger.debug("topic_knowledge_ratio #{topic_knowledge_ratio}")
+          topic_knowledge_loss_user[t] += (1 - user_score_factor) * topic_knowledge_ratio * tag_diminishing_return_factor
+          topic_knowledge_max[t] += topic_knowledge_ratio * tag_diminishing_return_factor
+        end
+      end
+      {user_topic_knowledge: topic_knowledge_loss_user, max_topic_knowledge: topic_knowledge_max}
+    end
+    private :get_user_knowledge_and_max_knowledge
+
+    def tag_diminishing_return_function(count_tag, total_count_tag)
+      total_count_tag += 1 # bonus exercise comes on top
+      return 1/(1+(Math::E**(-3/(0.5*total_count_tag)*(count_tag-0.5*total_count_tag))))
+    end
+
+    def select_easiest_exercise(exercises)
+      exercises.order(:expected_difficulty).first
+    end
+
+end
\ No newline at end of file
diff --git a/app/models/search.rb b/app/models/search.rb
new file mode 100644
index 00000000..f22dbc3e
--- /dev/null
+++ b/app/models/search.rb
@@ -0,0 +1,4 @@
+class Search < ActiveRecord::Base
+  belongs_to :user, polymorphic: true
+  belongs_to :exercise
+end
\ No newline at end of file
diff --git a/app/models/tag.rb b/app/models/tag.rb
new file mode 100644
index 00000000..002ec687
--- /dev/null
+++ b/app/models/tag.rb
@@ -0,0 +1,22 @@
+class Tag < ActiveRecord::Base
+
+  has_many :exercise_tags
+  has_many :exercises, through: :exercise_tags
+
+  validates_uniqueness_of :name
+
+  def destroy
+    if (can_be_destroyed?)
+      super
+    end
+  end
+
+  def can_be_destroyed?
+    !exercises.any?
+  end
+
+  def to_s
+    name
+  end
+
+end
\ No newline at end of file
diff --git a/app/models/user_exercise_feedback.rb b/app/models/user_exercise_feedback.rb
new file mode 100644
index 00000000..d3ec09d5
--- /dev/null
+++ b/app/models/user_exercise_feedback.rb
@@ -0,0 +1,8 @@
+class UserExerciseFeedback < ActiveRecord::Base
+
+  belongs_to :user, polymorphic: true
+  belongs_to :exercise
+
+  validates :user_id, uniqueness: { scope: [:exercise_id, :user_type] }
+
+end
\ No newline at end of file
diff --git a/app/models/user_exercise_intervention.rb b/app/models/user_exercise_intervention.rb
new file mode 100644
index 00000000..60824c34
--- /dev/null
+++ b/app/models/user_exercise_intervention.rb
@@ -0,0 +1,11 @@
+class UserExerciseIntervention < ActiveRecord::Base
+
+  belongs_to :user, polymorphic: true
+  belongs_to :intervention
+  belongs_to :exercise
+
+  validates :user, presence: true
+  validates :exercise, presence: true
+  validates :intervention, presence: true
+
+end
\ No newline at end of file
diff --git a/app/models/user_proxy_exercise_exercise.rb b/app/models/user_proxy_exercise_exercise.rb
new file mode 100644
index 00000000..e7defae6
--- /dev/null
+++ b/app/models/user_proxy_exercise_exercise.rb
@@ -0,0 +1,14 @@
+class UserProxyExerciseExercise < ActiveRecord::Base
+
+  belongs_to :user, polymorphic: true
+  belongs_to :exercise
+  belongs_to :proxy_exercise
+
+  validates :user_id, presence: true
+  validates :user_type, presence: true
+  validates :exercise_id, presence: true
+  validates :proxy_exercise_id, presence: true
+
+  validates :user_id, uniqueness: { scope: [:proxy_exercise_id, :user_type] }
+
+end
\ No newline at end of file
diff --git a/app/policies/exercise_policy.rb b/app/policies/exercise_policy.rb
index 55f7d16b..6377488b 100644
--- a/app/policies/exercise_policy.rb
+++ b/app/policies/exercise_policy.rb
@@ -16,7 +16,7 @@ class ExercisePolicy < AdminOrAuthorPolicy
     define_method(action) { admin? || author?}
   end
 
-  [:implement?, :submit?, :reload?].each do |action|
+  [:implement?, :working_times?, :intervention?, :submit?, :reload?].each do |action|
     define_method(action) { everyone }
   end
 
diff --git a/app/policies/intervention_policy.rb b/app/policies/intervention_policy.rb
new file mode 100644
index 00000000..b3a25667
--- /dev/null
+++ b/app/policies/intervention_policy.rb
@@ -0,0 +1,34 @@
+class InterventionPolicy < AdminOrAuthorPolicy
+  def author?
+    @user == @record.author
+  end
+  private :author?
+
+  def batch_update?
+    admin?
+  end
+
+  def show?
+    @user.internal_user?
+  end
+
+  [:clone?, :destroy?, :edit?, :update?].each do |action|
+    define_method(action) { admin? || author?}
+  end
+
+  [:reload?].each do |action|
+    define_method(action) { everyone }
+  end
+
+  class Scope < Scope
+    def resolve
+      if @user.admin?
+        @scope.all
+      elsif @user.internal_user?
+        @scope.where('user_id = ? OR public = TRUE', @user.id)
+      else
+        @scope.none
+      end
+    end
+  end
+end
diff --git a/app/policies/proxy_exercise_policy.rb b/app/policies/proxy_exercise_policy.rb
new file mode 100644
index 00000000..28de525e
--- /dev/null
+++ b/app/policies/proxy_exercise_policy.rb
@@ -0,0 +1,34 @@
+class ProxyExercisePolicy < AdminOrAuthorPolicy
+  def author?
+    @user == @record.author
+  end
+  private :author?
+
+  def batch_update?
+    admin?
+  end
+
+  def show?
+    @user.internal_user?
+  end
+
+  [:clone?, :destroy?, :edit?, :update?].each do |action|
+    define_method(action) { admin? || author?}
+  end
+
+  [:reload?].each do |action|
+    define_method(action) { everyone }
+  end
+
+  class Scope < Scope
+    def resolve
+      if @user.admin?
+        @scope.all
+      elsif @user.internal_user?
+        @scope.where('user_id = ? OR public = TRUE', @user.id)
+      else
+        @scope.none
+      end
+    end
+  end
+end
diff --git a/app/policies/search_policy.rb b/app/policies/search_policy.rb
new file mode 100644
index 00000000..9da9a641
--- /dev/null
+++ b/app/policies/search_policy.rb
@@ -0,0 +1,34 @@
+class SearchPolicy < AdminOrAuthorPolicy
+  def author?
+    @user == @record.author
+  end
+  private :author?
+
+  def batch_update?
+    admin?
+  end
+
+  def show?
+    @user.internal_user?
+  end
+
+  [:clone?, :destroy?, :edit?, :update?].each do |action|
+    define_method(action) { admin? || author?}
+  end
+
+  [:reload?].each do |action|
+    define_method(action) { everyone }
+  end
+
+  class Scope < Scope
+    def resolve
+      if @user.admin?
+        @scope.all
+      elsif @user.internal_user?
+        @scope.where('user_id = ? OR public = TRUE', @user.id)
+      else
+        @scope.none
+      end
+    end
+  end
+end
diff --git a/app/policies/tag_policy.rb b/app/policies/tag_policy.rb
new file mode 100644
index 00000000..8325b9fa
--- /dev/null
+++ b/app/policies/tag_policy.rb
@@ -0,0 +1,34 @@
+class TagPolicy < AdminOrAuthorPolicy
+  def author?
+    @user == @record.author
+  end
+  private :author?
+
+  def batch_update?
+    admin?
+  end
+
+  def show?
+    @user.internal_user?
+  end
+
+  [:clone?, :destroy?, :edit?, :update?].each do |action|
+    define_method(action) { admin? || author?}
+  end
+
+  [:reload?].each do |action|
+    define_method(action) { everyone }
+  end
+
+  class Scope < Scope
+    def resolve
+      if @user.admin?
+        @scope.all
+      elsif @user.internal_user?
+        @scope.where('user_id = ? OR public = TRUE', @user.id)
+      else
+        @scope.none
+      end
+    end
+  end
+end
diff --git a/app/views/exercises/_editor.html.slim b/app/views/exercises/_editor.html.slim
index 56986673..8a43b613 100644
--- a/app/views/exercises/_editor.html.slim
+++ b/app/views/exercises/_editor.html.slim
@@ -1,7 +1,8 @@
 - external_user_external_id = @current_user.respond_to?(:external_id) ? @current_user.external_id : '' #'tests' #(@current_user.uuid.present? ? @current_user.uuid : '')
 - external_user_id = @current_user.respond_to?(:external_id) ? @current_user.id : '' #'tests' #(@current_user.uuid.present? ? @current_user.uuid : '')
 - consumer_id = @current_user.respond_to?(:external_id) ? @current_user.consumer_id : '' #'tests' #(@current_user.uuid.present? ? @current_user.uuid : '')
-#editor.row data-exercise-id=exercise.id data-message-depleted=t('exercises.editor.depleted') data-message-timeout=t('exercises.editor.timeout',  permitted_execution_time: @exercise.execution_environment.permitted_execution_time) data-errors-url=execution_environment_errors_path(exercise.execution_environment) data-submissions-url=submissions_path data-user-id=@current_user.id data-user-external-id=external_user_external_id
+- show_interventions = @show_interventions || "false"
+#editor.row data-exercise-id=exercise.id data-message-depleted=t('exercises.editor.depleted') data-message-timeout=t('exercises.editor.timeout',  permitted_execution_time: @exercise.execution_environment.permitted_execution_time) data-errors-url=execution_environment_errors_path(exercise.execution_environment) data-submissions-url=submissions_path data-user-id=@current_user.id data-user-external-id=external_user_external_id data-working-times-url=working_times_exercise_path data-intervention-save-url=intervention_exercise_path data-show-interventions=show_interventions
   div id="sidebar" class=(@exercise.hide_file_tree ? 'sidebar-col-collapsed' : 'sidebar-col') = render('editor_file_tree', exercise: @exercise, files: @files)
   div id='output_sidebar' class='output-col-collapsed' = render('exercises/editor_output', external_user_id: external_user_id, consumer_id: consumer_id )
   div id='frames' class='editor-col'
@@ -22,3 +23,4 @@
 
 
 = render('shared/modal', id: 'comment-modal', title: t('exercises.implement.comment.request'), template: 'exercises/_request_comment_dialogcontent')
+= render('shared/modal', id: 'break-intervention-modal', title: t('exercises.implement.break_intervention.title'), template: 'interventions/_break_intervention_modal')
\ No newline at end of file
diff --git a/app/views/exercises/_editor_file_tree.html.slim b/app/views/exercises/_editor_file_tree.html.slim
index 16cc705b..6a90b60d 100644
--- a/app/views/exercises/_editor_file_tree.html.slim
+++ b/app/views/exercises/_editor_file_tree.html.slim
@@ -1,4 +1,4 @@
-div id='sidebar-collapsed' class=(@exercise.hide_file_tree ? '' : 'hidden')
+div id='sidebar-collapsed' class=(@exercise.hide_file_tree ? '' : 'hidden') data-course_token=@course_token
   = render('editor_button', classes: 'btn-block btn-primary btn-sm', data: {:'data-toggle' => 'tooltip', :'data-placement' => 'right'}, icon: 'fa fa-plus-square', id: 'sidebar-collapse-collapsed', label:'', title:t('exercises.editor.expand_action_sidebar'))
 
   - if @exercise.allow_file_creation and not @exercise.hide_file_tree?
@@ -6,6 +6,8 @@ div id='sidebar-collapsed' class=(@exercise.hide_file_tree ? '' : 'hidden')
 
   = render('editor_button', classes: 'btn-block btn-primary btn-sm enforce-top-margin', data: {:'data-toggle' => 'tooltip', :'data-placement' => 'right'}, icon: 'fa fa-download', id: 'download-collapsed', label:'', title: t('exercises.editor.download'))
   = render('editor_button', classes: 'btn-block btn-primary btn-sm enforce-top-margin', data: {:'data-message-confirm' => t('exercises.editor.confirm_start_over'), :'data-url' => reload_exercise_path(@exercise), :'data-toggle' => 'tooltip', :'data-placement' => 'right'}, icon: 'fa fa-history', id: 'start-over-collapsed', label:'', title: t('exercises.editor.start_over'))
+  - if @course_token
+    = render('editor_button', classes: 'btn-block btn-primary btn-sm enforce-top-margin', data: {:'data-toggle' => 'tooltip', :'data-placement' => 'right'}, icon: 'fa fa-search', id: 'sidebar-search-collapsed', label: '', title: t('search.search_in_forum'))
 
 div id='sidebar-uncollapsed' class=(@exercise.hide_file_tree ? 'hidden' : '')
   = render('editor_button', classes: 'btn-block btn-primary btn-sm', icon: 'fa fa-minus-square', id: 'sidebar-collapse', label: t('exercises.editor.collapse_action_sidebar'))
@@ -24,5 +26,16 @@ div id='sidebar-uncollapsed' class=(@exercise.hide_file_tree ? 'hidden' : '')
   = render('editor_button', classes: 'btn-block btn-primary btn-sm enforce-top-margin', icon: 'fa fa-download', id: 'download', label: t('exercises.editor.download'))
   = render('editor_button', classes: 'btn-block btn-primary btn-sm', data: {:'data-message-confirm' => t('exercises.editor.confirm_start_over'), :'data-url' => reload_exercise_path(@exercise)}, icon: 'fa fa-history', id: 'start-over', label: t('exercises.editor.start_over'))
 
+  - if @course_token
+    = form_for(@search, multipart: true, target: "_blank") do |f|
+      .input-group.enforce-top-margin
+        = f.hidden_field :exercise_id
+        .enforce-right-margin
+          = f.text_field(:search, class: 'form-control', id: "search-col", required: true, placeholder: t('search.search_in_forum'))
+        .input-group-btn
+          = button_tag(class: 'btn btn-primary', id: 'btn-search-col', model: @search.class.model_name.human) do
+            i.fa.fa-search
+
+
 - if @exercise.allow_file_creation?
   = render('shared/modal', id: 'modal-file', template: 'code_ocean/files/_form', title: t('exercises.editor.create_file'))
\ No newline at end of file
diff --git a/app/views/exercises/_form.html.slim b/app/views/exercises/_form.html.slim
index c4159254..0811b447 100644
--- a/app/views/exercises/_form.html.slim
+++ b/app/views/exercises/_form.html.slim
@@ -32,6 +32,25 @@
     label
       = f.check_box(:allow_auto_completion)
       = t('activerecord.attributes.exercise.allow_auto_completion')
+  .form-group
+    = f.label(t('activerecord.attributes.exercise.difficulty'))
+    = f.number_field :expected_difficulty, in: 1..10, step: 1
+  .form-group
+    = f.label(t('activerecord.attributes.exercise.worktime'))
+    = f.number_field "expected_worktime_minutes", value: @exercise.expected_worktime_seconds / 60, in: 1..1000, step: 1
+  h2 Tags
+  .table-responsive
+    table.table
+      thead
+        tr
+          th = t('activerecord.attributes.exercise.selection')
+          th = sort_link(@search, :title, t('activerecord.attributes.tag.name'))
+          th = t('activerecord.attributes.tag.difficulty')
+        = collection_check_boxes :exercise, :tag_ids, @exercise_tags, :tag_id, :id do |b|
+          tr
+            td = b.check_box
+            td = b.object.tag.name
+            td = number_field "tag_factors[#{b.object.tag.id}]", :factor, :value => b.object.factor, in: 1..10, step: 1
   h2 = t('activerecord.attributes.exercise.files')
   ul#files.list-unstyled.panel-group
     = f.fields_for :files do |files_form|
diff --git a/app/views/exercises/_request_comment_dialogcontent.html.slim b/app/views/exercises/_request_comment_dialogcontent.html.slim
index 677ccb12..0db575e5 100644
--- a/app/views/exercises/_request_comment_dialogcontent.html.slim
+++ b/app/views/exercises/_request_comment_dialogcontent.html.slim
@@ -1,4 +1,7 @@
+h5#rfc_intervention_text style='display: none;' = t('exercises.implement.rfc_intervention.text')
 h5 = t('exercises.implement.comment.question')
+
+
 textarea.form-control#question(style='resize:none;')
 p = ''
 / data-cause='requestComments' is not used here right now, we pass the button #requestComments (not askForCommentsButton) as initiator of the action. 
diff --git a/app/views/exercises/implement.html.slim b/app/views/exercises/implement.html.slim
index 87ff4e1f..300775a6 100644
--- a/app/views/exercises/implement.html.slim
+++ b/app/views/exercises/implement.html.slim
@@ -22,3 +22,4 @@
     #questions-column
       #questions-holder data-url="#{qa_url}/qa/index/#{@exercise.id}/#{@user_id}"
     = qa_js_tag
+
diff --git a/app/views/exercises/index.html.slim b/app/views/exercises/index.html.slim
index 9ad5cc25..bd8fe880 100644
--- a/app/views/exercises/index.html.slim
+++ b/app/views/exercises/index.html.slim
@@ -16,6 +16,9 @@ h1 = Exercise.model_name.human(count: 2)
         th = sort_link(@search, :execution_environment_id, t('activerecord.attributes.exercise.execution_environment'))
         th = t('.test_files')
         th = t('activerecord.attributes.exercise.maximum_score')
+        th = t('activerecord.attributes.exercise.tags')
+        th = t('activerecord.attributes.exercise.difficulty')
+        th = t('activerecord.attributes.exercise.worktime')
         th
           = t('activerecord.attributes.exercise.public')
           - if policy(Exercise).batch_update?
@@ -29,6 +32,9 @@ h1 = Exercise.model_name.human(count: 2)
           td = link_to_if(exercise.execution_environment && policy(exercise.execution_environment).show?, exercise.execution_environment, exercise.execution_environment)
           td = exercise.files.teacher_defined_tests.count
           td = exercise.maximum_score
+          td = exercise.exercise_tags.count
+          td = exercise.expected_difficulty
+          td = (exercise.expected_worktime_seconds / 60).ceil
           td.public data-value=exercise.public? = symbol_for(exercise.public?)
           td = link_to(t('shared.edit'), edit_exercise_path(exercise)) if policy(exercise).edit?
           td = link_to(t('.implement'), implement_exercise_path(exercise)) if policy(exercise).implement?
diff --git a/app/views/exercises/show.html.slim b/app/views/exercises/show.html.slim
index 902f8135..1efbd612 100644
--- a/app/views/exercises/show.html.slim
+++ b/app/views/exercises/show.html.slim
@@ -19,6 +19,9 @@ h1
 = row(label: 'exercise.allow_auto_completion', value: @exercise.allow_auto_completion?)
 = row(label: 'exercise.embedding_parameters') do
   = content_tag(:input, nil, class: 'form-control', readonly: true, value: embedding_parameters(@exercise))
+= row(label: 'exercise.difficulty', value: @exercise.expected_difficulty)
+= row(label: 'exercise.worktime', value: "#{@exercise.expected_worktime_seconds/60} min")
+= row(label: 'exercise.tags', value: @exercise.exercise_tags.map{|et| "#{et.tag.name} (#{et.factor})"}.sort.join(", "))
 
 h2 = t('activerecord.attributes.exercise.files')
 
diff --git a/app/views/interventions/_break_intervention_modal.html.slim b/app/views/interventions/_break_intervention_modal.html.slim
new file mode 100644
index 00000000..12f0e314
--- /dev/null
+++ b/app/views/interventions/_break_intervention_modal.html.slim
@@ -0,0 +1 @@
+h5 = t('exercises.implement.break_intervention.text')
diff --git a/app/views/interventions/_form.html.slim b/app/views/interventions/_form.html.slim
new file mode 100644
index 00000000..6ffe7397
--- /dev/null
+++ b/app/views/interventions/_form.html.slim
@@ -0,0 +1,6 @@
+= form_for(@intervention) do |f|
+  = render('shared/form_errors', object: @intervention)
+  .form-group
+    = f.label(:name)
+    = f.text_field(:name, class: 'form-control', required: true)
+  .actions = render('shared/submit_button', f: f, object: @intervention)
diff --git a/app/views/interventions/index.html.slim b/app/views/interventions/index.html.slim
new file mode 100644
index 00000000..fc7afe05
--- /dev/null
+++ b/app/views/interventions/index.html.slim
@@ -0,0 +1,14 @@
+h1 = Intervention.model_name.human(count: 2)
+
+.table-responsive
+  table.table
+    thead
+      tr
+        th = t('activerecord.attributes.intervention.name')
+    tbody
+      - @interventions.each do |intervention|
+        tr
+          td = intervention.name
+          td = link_to(t('shared.show'), intervention)
+
+= render('shared/pagination', collection: @interventions)
diff --git a/app/views/interventions/show.html.slim b/app/views/interventions/show.html.slim
new file mode 100644
index 00000000..f9202240
--- /dev/null
+++ b/app/views/interventions/show.html.slim
@@ -0,0 +1,4 @@
+h1
+  = @intervention.name
+
+= row(label: 'intervention.name', value: @intervention.name)
diff --git a/app/views/proxy_exercises/_form.html.slim b/app/views/proxy_exercises/_form.html.slim
new file mode 100644
index 00000000..bd57bf06
--- /dev/null
+++ b/app/views/proxy_exercises/_form.html.slim
@@ -0,0 +1,24 @@
+= form_for(@proxy_exercise, multipart: true) do |f|
+  = render('shared/form_errors', object: @proxy_exercise)
+  .form-group
+    = f.label(:title)
+    = f.text_field(:title, class: 'form-control', required: true)
+  .form-group
+    = f.label(:description)
+    = f.pagedown_editor :description
+
+  h3 Exercises
+  .table-responsive
+    table.table
+      thead
+        tr
+          th = t('activerecord.attributes.exercise.selection')
+          th = sort_link(@search, :title, t('activerecord.attributes.submission.exercise'))
+          th = sort_link(@search, :created_at, t('shared.created_at'))
+      = collection_check_boxes :proxy_exercise, :exercise_ids, @exercises, :id, :title do |b|
+        tr
+          td = b.check_box
+          td = link_to(b.object, b.object)
+          td = l(b.object.created_at, format: :short)
+
+  .actions = render('shared/submit_button', f: f, object: @proxy_exercise)
\ No newline at end of file
diff --git a/app/views/proxy_exercises/edit.html.slim b/app/views/proxy_exercises/edit.html.slim
new file mode 100644
index 00000000..8aa200c9
--- /dev/null
+++ b/app/views/proxy_exercises/edit.html.slim
@@ -0,0 +1,3 @@
+h1 = t('activerecord.models.proxy_exercise.one', model: ProxyExercise.model_name.human)+ ": " + @proxy_exercise.title
+
+= render('form')
diff --git a/app/views/proxy_exercises/index.html.slim b/app/views/proxy_exercises/index.html.slim
new file mode 100644
index 00000000..2a8067c1
--- /dev/null
+++ b/app/views/proxy_exercises/index.html.slim
@@ -0,0 +1,35 @@
+h1 = ProxyExercise.model_name.human(count: 2)
+
+= render(layout: 'shared/form_filters') do |f|
+  .form-group
+    = f.label(:title_cont, t('activerecord.attributes.proxy_exercise.title'), class: 'sr-only')
+    = f.search_field(:title_cont, class: 'form-control', placeholder: t('activerecord.attributes.proxy_exercise.title'))
+
+.table-responsive
+  table.table
+    thead
+      tr
+        th = sort_link(@search, :title, t('activerecord.attributes.proxy_exercise.title'))
+        th = "Token"
+        th = t('activerecord.attributes.proxy_exercise.files_count')
+        th colspan=6 = t('shared.actions')
+    tbody
+      - @proxy_exercises.each do |proxy_exercise|
+        tr data-id=proxy_exercise.id
+          td = link_to(proxy_exercise.title,proxy_exercise)
+          td = proxy_exercise.token
+          td = proxy_exercise.count_files
+          td = link_to(t('shared.edit'), edit_proxy_exercise_path(proxy_exercise)) if policy(proxy_exercise).edit?
+
+          td
+            .btn-group
+              button.btn.btn-primary-outline.btn-xs.dropdown-toggle data-toggle="dropdown" type="button" = t('shared.actions_button')
+              span.caret
+              span.sr-only Toggle Dropdown
+              ul.dropdown-menu.pull-right role="menu"
+                li = link_to(t('shared.show'), proxy_exercise) if policy(proxy_exercise).show?
+                li = link_to(t('shared.destroy'), proxy_exercise, data: {confirm: t('shared.confirm_destroy')}, method: :delete) if policy(proxy_exercise).destroy?
+                li = link_to(t('.clone'), clone_proxy_exercise_path(proxy_exercise), data: {confirm: t('shared.confirm_destroy')}, method: :post) if policy(proxy_exercise).clone?
+
+= render('shared/pagination', collection: @proxy_exercises)
+p = render('shared/new_button', model: ProxyExercise)
diff --git a/app/views/proxy_exercises/new.html.slim b/app/views/proxy_exercises/new.html.slim
new file mode 100644
index 00000000..ae59a292
--- /dev/null
+++ b/app/views/proxy_exercises/new.html.slim
@@ -0,0 +1,3 @@
+h1 = t('shared.new_model', model: ProxyExercise.model_name.human)
+
+= render('form')
diff --git a/app/views/proxy_exercises/reload.json.jbuilder b/app/views/proxy_exercises/reload.json.jbuilder
new file mode 100644
index 00000000..8e5d4e3c
--- /dev/null
+++ b/app/views/proxy_exercises/reload.json.jbuilder
@@ -0,0 +1,3 @@
+json.set! :files do
+  json.array! @exercise.files.visible, :content, :id
+end
diff --git a/app/views/proxy_exercises/show.html.slim b/app/views/proxy_exercises/show.html.slim
new file mode 100644
index 00000000..c1888d79
--- /dev/null
+++ b/app/views/proxy_exercises/show.html.slim
@@ -0,0 +1,23 @@
+- content_for :head do
+  = javascript_include_tag('http://cdnjs.cloudflare.com/ajax/libs/highlight.js/8.4/highlight.min.js')
+  = stylesheet_link_tag('http://cdnjs.cloudflare.com/ajax/libs/highlight.js/8.4/styles/default.min.css')
+
+h1
+  = @proxy_exercise.title
+  - if policy(@proxy_exercise).edit?
+    = render('shared/edit_button', object: @proxy_exercise)
+
+= row(label: 'exercise.title', value: @proxy_exercise.title)
+= row(label: 'proxy_exercise.files_count', value: @exercises.count)
+= row(label: 'exercise.description', value: @proxy_exercise.description)
+h3 Exercises
+.table-responsive
+  table.table
+    thead
+      tr
+        th = sort_link(@search, :title, t('activerecord.attributes.submission.exercise'))
+        th = sort_link(@search, :created_at, t('shared.created_at'))
+    - @proxy_exercise.exercises.each do |exercise|
+      tr
+        td = link_to(exercise.title, exercise)
+        td = l(exercise.created_at, format: :short)
diff --git a/app/views/searches/destroy.html.erb b/app/views/searches/destroy.html.erb
new file mode 100644
index 00000000..e69de29b
diff --git a/app/views/tags/_form.html.slim b/app/views/tags/_form.html.slim
new file mode 100644
index 00000000..4f02a28f
--- /dev/null
+++ b/app/views/tags/_form.html.slim
@@ -0,0 +1,6 @@
+= form_for(@tag) do |f|
+  = render('shared/form_errors', object: @tag)
+  .form-group
+    = f.label(:name)
+    = f.text_field(:name, class: 'form-control', required: true)
+  .actions = render('shared/submit_button', f: f, object: @tag)
diff --git a/app/views/tags/edit.html.slim b/app/views/tags/edit.html.slim
new file mode 100644
index 00000000..23c76720
--- /dev/null
+++ b/app/views/tags/edit.html.slim
@@ -0,0 +1,3 @@
+h1 = @tag.name
+
+= render('form')
diff --git a/app/views/tags/index.html.slim b/app/views/tags/index.html.slim
new file mode 100644
index 00000000..2d4916af
--- /dev/null
+++ b/app/views/tags/index.html.slim
@@ -0,0 +1,19 @@
+h1 = Tag.model_name.human(count: 2)
+
+.table-responsive
+  table.table
+    thead
+      tr
+        th = t('activerecord.attributes.hint.name')
+        /th = t('activerecord.attributes.hint.locale')
+        /th colspan=3 = t('shared.actions')
+    tbody
+      - @tags.each do |tag|
+        tr
+          td = tag.name
+          td = link_to(t('shared.show'), tag)
+          td = link_to(t('shared.edit'), edit_tag_path(tag))
+          td = link_to(t('shared.destroy'), tag, data: {confirm: t('shared.confirm_destroy')}, method: :delete) if tag.can_be_destroyed?
+
+= render('shared/pagination', collection: @tags)
+p = render('shared/new_button', model: Tag, path: new_tag_path)
diff --git a/app/views/tags/new.html.slim b/app/views/tags/new.html.slim
new file mode 100644
index 00000000..e5dbc4ee
--- /dev/null
+++ b/app/views/tags/new.html.slim
@@ -0,0 +1,3 @@
+h1 = t('shared.new_model', model: Tag.model_name.human)
+
+= render('form')
diff --git a/app/views/tags/show.html.slim b/app/views/tags/show.html.slim
new file mode 100644
index 00000000..81eda745
--- /dev/null
+++ b/app/views/tags/show.html.slim
@@ -0,0 +1,6 @@
+h1
+  = @tag.name
+  = render('shared/edit_button', object: @tag)
+
+= row(label: 'tag.name', value: @tag.name)
+= row(label: 'tag.usage', value: @tag.exercises.count)
diff --git a/config/locales/de.yml b/config/locales/de.yml
index 5b4329df..3b5dd89a 100644
--- a/config/locales/de.yml
+++ b/config/locales/de.yml
@@ -27,6 +27,7 @@ de:
       exercise:
         description: Beschreibung
         embedding_parameters: Parameter für LTI-Einbettung
+        tags: Tags
         execution_environment: Ausführungsumgebung
         execution_environment_id: Ausführungsumgebung
         files: Dateien
@@ -34,10 +35,16 @@ de:
         instructions: Anweisungen
         maximum_score: Erreichbare Punktzahl
         public: Öffentlich
+        selection: Ausgewählt
         title: Titel
         user: Autor
         allow_auto_completion: "Autovervollständigung aktivieren"
         allow_file_creation: "Dateierstellung erlauben"
+        difficulty: Schwierigkeitsgrad
+        worktime: "vermutete Arbeitszeit in Minuten"
+      proxy_exercise:
+        title: Title
+        files_count: Anzahl der Aufgaben
       external_user:
         consumer: Konsument
         email: E-Mail
@@ -68,6 +75,8 @@ de:
         message: Nachricht
         name: Name
         regular_expression: Regulärer Ausdruck
+      intervention:
+        name: Name
       internal_user:
         activated: Aktiviert
         consumer: Konsument
@@ -91,6 +100,10 @@ de:
         files: Dateien
         score: Punktzahl
         user: Autor
+      tag:
+        name: Name
+        usage: Verwendet
+        difficulty: Anteil an der Aufgabe
       file_template:
         name: "Name"
         file_type: "Dateityp"
@@ -111,6 +124,9 @@ de:
       exercise:
         one: Aufgabe
         other: Aufgaben
+      proxy_exercise:
+        one: Proxy Aufgabe
+        other: Proxy Aufgaben
       external_user:
         one: Externer Nutzer
         other: Externe Nutzer
@@ -259,7 +275,12 @@ de:
         removeAllOnLine: Meine Kommentare auf dieser Zeile löschen
         listing: Die neuesten Kommentaranfragen
         request: "Kommentaranfrage stellen"
-        question: "Bitte beschreiben Sie kurz ihre Problem oder nennen Sie den Programmteil, zu dem sie Feedback wünschen."
+        question: "Bitte beschreiben Sie kurz ihre Probleme oder nennen Sie den Programmteil, zu dem Sie Feedback wünschen."
+      rfc_intervention:
+        text: "Es scheint so als würden sie Probleme mit der Aufgabe haben. Wenn Sie möchten, können wir Ihnen helfen!"
+      break_intervention:
+        title: "Pause"
+        text: "Uns ist aufgefallen, dass du schon lange an dieser Aufgabe arbeitest. Möchtest du vielleicht eine Pause machen um auf neue Gedanken zu kommen?"
     index:
       clone: Duplizieren
       implement: Implementieren
@@ -290,6 +311,9 @@ de:
         tests: Unit Tests
         time_difference: 'Arbeitszeit bis hier*'
         addendum: '* Differenzen von mehr als 30 Minuten werden ignoriert.'
+  proxy_exercises:
+      index:
+        clone: Duplizieren
   external_users:
     statistics:
       title: Statistiken für Externe Benutzer
@@ -327,6 +351,8 @@ de:
       success: Sie haben Ihr Passwort erfolgreich geändert.
     show:
       link: Profil
+  search:
+    search_in_forum: "Probleme? Suche hier im Forum"
   locales:
     de: Deutsch
     en: Englisch
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 5541d68f..213624e8 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -48,6 +48,7 @@ en:
       exercise:
         description: Description
         embedding_parameters: LTI Embedding Parameters
+        tags: Tags
         execution_environment: Execution Environment
         execution_environment_id: Execution Environment
         files: Files
@@ -55,10 +56,16 @@ en:
         instructions: Instructions
         maximum_score: Maximum Score
         public: Public
+        selection: Selected
         title: Title
         user: Author
         allow_auto_completion: "Allow auto completion"
         allow_file_creation: "Allow file creation"
+        difficulty: Difficulty
+        worktime: "Expected worktime in minutes"
+      proxy_exercise:
+        title: Title
+        files_count: Exercises Count
       external_user:
         consumer: Consumer
         email: Email
@@ -89,6 +96,8 @@ en:
         message: Message
         name: Name
         regular_expression: Regular Expression
+      intervention:
+        name: Name
       internal_user:
         activated: Activated
         consumer: Consumer
@@ -112,6 +121,10 @@ en:
         files: Files
         score: Score
         user: Author
+      tag:
+        name: Name
+        usage: Used
+        difficulty: Share on the Exercise
       file_template:
         name: "Name"
         file_type: "File Type"
@@ -132,6 +145,9 @@ en:
       exercise:
         one: Exercise
         other: Exercises
+      proxy_exercise:
+        one: Proxy Exercise
+        other: Proxy Exercises
       external_user:
         one: External User
         other: External Users
@@ -281,6 +297,11 @@ en:
         listing: Listing the newest comment requests
         request: "Request Comments"
         question: "Please shortly describe your problem or the program part you would like to get feedback for."
+      rfc_intervention:
+        text: "It looks like you may struggle with this exercise. If you like we can help you out!"
+      break_intervention:
+        title: "Break"
+        text: "We recognized that you are already working quite a while on this exercise. We would like to encourage you to take a break and come back later."
     index:
       clone: Duplicate
       implement: Implement
@@ -311,6 +332,9 @@ en:
         tests: Unit Test Results
         time_difference: 'Working Time until here*'
         addendum: '* Deltas longer than 30 minutes are ignored.'
+  proxy_exercises:
+      index:
+        clone: Duplicate
   external_users:
     statistics:
       title: External User Statistics
@@ -348,6 +372,8 @@ en:
       success: You successfully changed your password.
     show:
       link: Profile
+  search:
+    search_in_forum: "Problems? Search here in forum"
   locales:
     de: German
     en: English
diff --git a/config/routes.rb b/config/routes.rb
index b4606f74..a33369d4 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -60,12 +60,46 @@ Rails.application.routes.draw do
     member do
       post :clone
       get :implement
+      get :working_times
+      post :intervention
       get :statistics
       get :reload
       post :submit
     end
   end
 
+  resources :proxy_exercises do
+    member do
+      post :clone
+      get :reload
+      post :submit
+    end
+  end
+
+  resources :tags do
+    member do
+      post :clone
+      get :reload
+      post :submit
+    end
+  end
+
+  resources :searches do
+    member do
+      post :clone
+      get :reload
+      post :submit
+    end
+  end
+
+  resources :interventions do
+    member do
+      post :clone
+      get :reload
+      post :submit
+    end
+  end
+
   resources :external_users, only: [:index, :show], concerns: :statistics do
     resources :exercises, concerns: :statistics
   end
diff --git a/db/migrate/20170205163247_create_exercise_collections.rb b/db/migrate/20170205163247_create_exercise_collections.rb
new file mode 100644
index 00000000..ef27f756
--- /dev/null
+++ b/db/migrate/20170205163247_create_exercise_collections.rb
@@ -0,0 +1,14 @@
+class CreateExerciseCollections < ActiveRecord::Migration
+  def change
+    create_table :exercise_collections do |t|
+      t.string :name
+      t.timestamps
+    end
+
+    create_table :exercise_collections_exercises, id: false do |t|
+      t.belongs_to :exercise_collection, index: true
+      t.belongs_to :exercise, index: true
+    end
+
+  end
+end
diff --git a/db/migrate/20170205165450_create_proxy_exercises.rb b/db/migrate/20170205165450_create_proxy_exercises.rb
new file mode 100644
index 00000000..fb2704ce
--- /dev/null
+++ b/db/migrate/20170205165450_create_proxy_exercises.rb
@@ -0,0 +1,23 @@
+class CreateProxyExercises < ActiveRecord::Migration
+  def change
+    create_table :proxy_exercises do |t|
+      t.string :title
+      t.string :description
+      t.string :token
+      t.timestamps
+    end
+
+    create_table :exercises_proxy_exercises, id: false do |t|
+      t.belongs_to :proxy_exercise, index: true
+      t.belongs_to :exercise, index: true
+      t.timestamps
+    end
+
+    create_table :user_proxy_exercise_exercises do |t|
+      t.belongs_to :user, polymorphic: true, index: true
+      t.belongs_to :proxy_exercise, index: true
+      t.belongs_to :exercise, index: true
+      t.timestamps
+    end
+  end
+end
diff --git a/db/migrate/20170205210357_create_interventions.rb b/db/migrate/20170205210357_create_interventions.rb
new file mode 100644
index 00000000..07223e1c
--- /dev/null
+++ b/db/migrate/20170205210357_create_interventions.rb
@@ -0,0 +1,23 @@
+class CreateInterventions < ActiveRecord::Migration
+  def change
+    create_table :user_exercise_interventions do |t|
+      t.belongs_to :user, polymorphic: true
+      t.belongs_to :exercise
+      t.belongs_to :intervention
+      t.integer :accumulated_worktime_s
+      t.text :reason
+      t.timestamps
+    end
+
+    create_table :interventions do |t|
+      t.string :name
+      t.text :markup
+      t.timestamps
+    end
+
+    Intervention.createDefaultInterventions
+
+  end
+
+
+end
diff --git a/db/migrate/20170206141210_add_tags.rb b/db/migrate/20170206141210_add_tags.rb
new file mode 100644
index 00000000..8c0c129a
--- /dev/null
+++ b/db/migrate/20170206141210_add_tags.rb
@@ -0,0 +1,19 @@
+class AddTags < ActiveRecord::Migration
+
+  def change
+    add_column :exercises, :expected_worktime_seconds, :integer, default: 60
+    add_column :exercises, :expected_difficulty, :integer, default: 1
+
+    create_table :tags do |t|
+      t.string :name, null: false
+      t.timestamps
+    end
+
+    create_table :exercise_tags do |t|
+      t.belongs_to :exercise
+      t.belongs_to :tag
+      t.integer :factor, default: 1
+    end
+  end
+
+end
diff --git a/db/migrate/20170206152503_add_user_feedback.rb b/db/migrate/20170206152503_add_user_feedback.rb
new file mode 100644
index 00000000..f62ccd9d
--- /dev/null
+++ b/db/migrate/20170206152503_add_user_feedback.rb
@@ -0,0 +1,11 @@
+class AddUserFeedback < ActiveRecord::Migration
+  def change
+    create_table :user_exercise_feedbacks do |t|
+      t.belongs_to :exercise, null: false
+      t.belongs_to :user, polymorphic: true, null: false
+      t.integer :difficulty
+      t.integer :working_time_seconds
+      t.string :feedback_text
+    end
+  end
+end
diff --git a/db/migrate/20170228165741_add_search.rb b/db/migrate/20170228165741_add_search.rb
new file mode 100644
index 00000000..a36d94ff
--- /dev/null
+++ b/db/migrate/20170228165741_add_search.rb
@@ -0,0 +1,10 @@
+class AddSearch < ActiveRecord::Migration
+  def change
+    create_table :searches do |t|
+      t.belongs_to :exercise, null: false
+      t.belongs_to :user, polymorphic: true, null: false
+      t.string :search
+      t.timestamps
+    end
+  end
+end
diff --git a/deleteme.txt b/deleteme.txt
new file mode 100644
index 00000000..e69de29b
diff --git a/spec/concerns/lti_spec.rb b/spec/concerns/lti_spec.rb
index 95181d3b..c03ef9a5 100644
--- a/spec/concerns/lti_spec.rb
+++ b/spec/concerns/lti_spec.rb
@@ -165,6 +165,7 @@ describe Lti do
 
     it 'stores data in the session' do
       controller.instance_variable_set(:@current_user, FactoryGirl.create(:external_user))
+      controller.instance_variable_set(:@exercise, FactoryGirl.create(:fibonacci))
       expect(controller.session).to receive(:[]=).with(:consumer_id, anything)
       expect(controller.session).to receive(:[]=).with(:external_user_id, anything)
       controller.send(:store_lti_session_data, consumer: FactoryGirl.build(:consumer), parameters: parameters)
@@ -172,6 +173,8 @@ describe Lti do
 
     it 'it creates an LtiParameter Object' do
       before_count = LtiParameter.count
+      controller.instance_variable_set(:@current_user, FactoryGirl.create(:external_user))
+      controller.instance_variable_set(:@exercise, FactoryGirl.create(:fibonacci))
       controller.send(:store_lti_session_data, consumer: FactoryGirl.build(:consumer), parameters: parameters)
       expect(LtiParameter.count).to eq(before_count + 1)
     end
diff --git a/spec/controllers/sessions_controller_spec.rb b/spec/controllers/sessions_controller_spec.rb
index ffa957ea..50fef8d6 100644
--- a/spec/controllers/sessions_controller_spec.rb
+++ b/spec/controllers/sessions_controller_spec.rb
@@ -28,6 +28,7 @@ describe SessionsController do
 
   describe 'POST #create_through_lti' do
     let(:exercise) { FactoryGirl.create(:dummy) }
+    let(:exercise2) { FactoryGirl.create(:dummy) }
     let(:nonce) { SecureRandom.hex }
     before(:each) { I18n.locale = I18n.default_locale }
 
@@ -129,6 +130,23 @@ describe SessionsController do
         request
         expect(controller).to redirect_to(implement_exercise_path(exercise.id))
       end
+
+      it 'redirects to recommended exercise if requested token of proxy exercise' do
+        FactoryGirl.create(:proxy_exercise, exercises: [exercise])
+        post :create_through_lti, custom_locale: locale, custom_token: ProxyExercise.first.token, oauth_consumer_key: consumer.oauth_key, oauth_nonce: nonce, oauth_signature: SecureRandom.hex, user_id: user.external_id
+        expect(controller).to redirect_to(implement_exercise_path(exercise.id))
+      end
+
+      it 'recommends only exercises who are 1 degree more complicated than what user has seen' do
+        # dummy user has no exercises finished, therefore his highest difficulty is 0
+        FactoryGirl.create(:proxy_exercise, exercises: [exercise, exercise2])
+        exercise.expected_difficulty = 3
+        exercise.save
+        exercise2.expected_difficulty = 1
+        exercise2.save
+        post :create_through_lti, custom_locale: locale, custom_token: ProxyExercise.first.token, oauth_consumer_key: consumer.oauth_key, oauth_nonce: nonce, oauth_signature: SecureRandom.hex, user_id: user.external_id
+        expect(controller).to redirect_to(implement_exercise_path(exercise2.id))
+      end
     end
   end
 
diff --git a/spec/factories/proxy_exercise.rb b/spec/factories/proxy_exercise.rb
new file mode 100644
index 00000000..9c9974d6
--- /dev/null
+++ b/spec/factories/proxy_exercise.rb
@@ -0,0 +1,7 @@
+FactoryGirl.define do
+  factory :proxy_exercise, class: ProxyExercise do
+    token 'dummytoken'
+    title 'Dummy'
+  end
+
+end