Split up SystemNoteService30000-split-up-system-note-service

Extracted topic related services but kept 
SystemNoteService in place for now.

9 files changed, 811 insertions, 629 deletions

diff --git a/app/models/note.rb b/app/models/note.rb
index 62b3f47fadd..0faaf5aa490 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -215,7 +215,7 @@ class Note < ApplicationRecord
     if force_cross_reference_regex_check?
       matches_cross_reference_regex?
     else
-      SystemNoteService.cross_reference?(note)
+      IssuablesSystemNoteService.cross_reference?(note)
     end
   end
   # rubocop: enable CodeReuse/ServiceClass
diff --git a/app/services/system_note_service.rb b/app/services/system_note_service.rb
index 1b48b20e28b..9e565ca98ee 100644
--- a/app/services/system_note_service.rb
+++ b/app/services/system_note_service.rb
@@ -7,702 +7,178 @@
 module SystemNoteService
   extend self
 
-  # Called when commits are added to a Merge Request
-  #
-  # noteable         - Noteable object
-  # project          - Project owning noteable
-  # author           - User performing the change
-  # new_commits      - Array of Commits added since last push
-  # existing_commits - Array of Commits added in a previous push
-  # oldrev           - Optional String SHA of a previous Commit
-  #
-  # See new_commit_summary and existing_commit_summary.
-  #
-  # Returns the created Note object
   def add_commits(noteable, project, author, new_commits, existing_commits = [], oldrev = nil)
-    total_count  = new_commits.length + existing_commits.length
-    commits_text = "#{total_count} commit".pluralize(total_count)
-
-    text_parts = ["added #{commits_text}"]
-    text_parts << commits_list(noteable, new_commits, existing_commits, oldrev)
-    text_parts << "[Compare with previous version](#{diff_comparison_path(noteable, project, oldrev)})"
-
-    body = text_parts.join("\n\n")
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'commit', commit_count: total_count))
+    service = CommitSystemNoteService.new(noteable, project, author)
+    service.add_commits(new_commits, existing_commits, oldrev)
   end
 
-  # Called when a commit was tagged
-  #
-  # noteable  - Noteable object
-  # project   - Project owning noteable
-  # author    - User performing the tag
-  # tag_name  - The created tag name
-  #
-  # Returns the created Note object
   def tag_commit(noteable, project, author, tag_name)
-    link = url_helpers.project_tag_path(project, id: tag_name)
-    body = "tagged commit #{noteable.sha} to [`#{tag_name}`](#{link})"
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'tag'))
-  end
-
-  # Called when the assignee of a Noteable is changed or removed
-  #
-  # noteable - Noteable object
-  # project  - Project owning noteable
-  # author   - User performing the change
-  # assignee - User being assigned, or nil
-  #
-  # Example Note text:
-  #
-  #   "removed assignee"
-  #
-  #   "assigned to @rspeicher"
-  #
-  # Returns the created Note object
+    service = CommitSystemNoteService.new(noteable, project, author)
+    service.tag_commit(tag_name)
+  end
+
   def change_assignee(noteable, project, author, assignee)
-    body = assignee.nil? ? 'removed assignee' : "assigned to #{assignee.to_reference}"
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'assignee'))
-  end
-
-  # Called when the assignees of an issuable is changed or removed
-  #
-  # issuable - Issuable object (responds to assignees)
-  # project  - Project owning noteable
-  # author   - User performing the change
-  # assignees - Users being assigned, or nil
-  #
-  # Example Note text:
-  #
-  #   "removed all assignees"
-  #
-  #   "assigned to @user1 additionally to @user2"
-  #
-  #   "assigned to @user1, @user2 and @user3 and unassigned from @user4 and @user5"
-  #
-  #   "assigned to @user1 and @user2"
-  #
-  # Returns the created Note object
+    service = IssuablesSystemNoteService.new(noteable, project, author)
+    service.change_assignee(assignee)
+  end
+
   def change_issuable_assignees(issuable, project, author, old_assignees)
-    unassigned_users = old_assignees - issuable.assignees
-    added_users = issuable.assignees.to_a - old_assignees
-    text_parts = []
-
-    Gitlab::I18n.with_default_locale do
-      text_parts << "assigned to #{added_users.map(&:to_reference).to_sentence}" if added_users.any?
-      text_parts << "unassigned #{unassigned_users.map(&:to_reference).to_sentence}" if unassigned_users.any?
-    end
-
-    body = text_parts.join(' and ')
-
-    create_note(NoteSummary.new(issuable, project, author, body, action: 'assignee'))
-  end
-
-  # Called when the milestone of a Noteable is changed
-  #
-  # noteable  - Noteable object
-  # project   - Project owning noteable
-  # author    - User performing the change
-  # milestone - Milestone being assigned, or nil
-  #
-  # Example Note text:
-  #
-  #   "removed milestone"
-  #
-  #   "changed milestone to 7.11"
-  #
-  # Returns the created Note object
+    service = IssuablesSystemNoteService.new(issuable, project, author)
+    service.change_issuable_assignees(old_assignees)
+  end
+
   def change_milestone(noteable, project, author, milestone)
-    format = milestone&.group_milestone? ? :name : :iid
-    body = milestone.nil? ? 'removed milestone' : "changed milestone to #{milestone.to_reference(project, format: format)}"
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'milestone'))
-  end
-
-  # Called when the due_date of a Noteable is changed
-  #
-  # noteable  - Noteable object
-  # project   - Project owning noteable
-  # author    - User performing the change
-  # due_date  - Due date being assigned, or nil
-  #
-  # Example Note text:
-  #
-  #   "removed due date"
-  #
-  #   "changed due date to September 20, 2018"
-  #
-  # Returns the created Note object
+    service = IssuablesSystemNoteService.new(noteable, project, author)
+    service.change_milestone(milestone)
+  end
+
   def change_due_date(noteable, project, author, due_date)
-    body = due_date ? "changed due date to #{due_date.to_s(:long)}" : 'removed due date'
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'due_date'))
-  end
-
-  # Called when the estimated time of a Noteable is changed
-  #
-  # noteable      - Noteable object
-  # project       - Project owning noteable
-  # author        - User performing the change
-  # time_estimate - Estimated time
-  #
-  # Example Note text:
-  #
-  #   "removed time estimate"
-  #
-  #   "changed time estimate to 3d 5h"
-  #
-  # Returns the created Note object
+    service = TimeTrackingSystemNoteService.new(noteable, project, author)
+    service.change_due_date(due_date)
+  end
+
   def change_time_estimate(noteable, project, author)
-    parsed_time = Gitlab::TimeTrackingFormatter.output(noteable.time_estimate)
-    body = if noteable.time_estimate == 0
-             "removed time estimate"
-           else
-             "changed time estimate to #{parsed_time}"
-           end
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'time_tracking'))
-  end
-
-  # Called when the spent time of a Noteable is changed
-  #
-  # noteable   - Noteable object
-  # project    - Project owning noteable
-  # author     - User performing the change
-  # time_spent - Spent time
-  #
-  # Example Note text:
-  #
-  #   "removed time spent"
-  #
-  #   "added 2h 30m of time spent"
-  #
-  # Returns the created Note object
-  def change_time_spent(noteable, project, author)
-    time_spent = noteable.time_spent
-
-    if time_spent == :reset
-      body = "removed time spent"
-    else
-      spent_at = noteable.spent_at
-      parsed_time = Gitlab::TimeTrackingFormatter.output(time_spent.abs)
-      action = time_spent > 0 ? 'added' : 'subtracted'
-
-      text_parts = ["#{action} #{parsed_time} of time spent"]
-      text_parts << "at #{spent_at}" if spent_at
-      body = text_parts.join(' ')
-    end
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'time_tracking'))
-  end
-
-  # Called when the status of a Noteable is changed
-  #
-  # noteable - Noteable object
-  # project  - Project owning noteable
-  # author   - User performing the change
-  # status   - String status
-  # source   - Mentionable performing the change, or nil
-  #
-  # Example Note text:
-  #
-  #   "merged"
-  #
-  #   "closed via bc17db76"
-  #
-  # Returns the created Note object
-  def change_status(noteable, project, author, status, source = nil)
-    body = status.dup
-    body << " via #{source.gfm_reference(project)}" if source
+    service = TimeTrackingSystemNoteService.new(noteable, project, author)
+    service.change_time_estimate
+  end
 
-    action = status == 'reopened' ? 'opened' : status
+  def change_time_spent(noteable, project, author)
+    service = TimeTrackingSystemNoteService.new(noteable, project, author)
+    service.change_time_spent
+  end
 
-    create_note(NoteSummary.new(noteable, project, author, body, action: action))
+  def change_status(noteable, project, author, status, source = nil)
+    service = IssuablesSystemNoteService.new(noteable, project, author)
+    service.change_status(status, source)
   end
 
-  # Called when 'merge when pipeline succeeds' is executed
   def merge_when_pipeline_succeeds(noteable, project, author, sha)
-    body = "enabled an automatic merge when the pipeline for #{sha} succeeds"
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'merge'))
+    service = MergeRequestsSystemNoteService.new(noteable, project, author)
+    service.merge_when_pipeline_succeeds(sha)
   end
 
-  # Called when 'merge when pipeline succeeds' is canceled
   def cancel_merge_when_pipeline_succeeds(noteable, project, author)
-    body = 'canceled the automatic merge'
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'merge'))
+    service = MergeRequestsSystemNoteService.new(noteable, project, author)
+    service.cancel_merge_when_pipeline_succeeds
   end
 
-  # Called when 'merge when pipeline succeeds' is aborted
   def abort_merge_when_pipeline_succeeds(noteable, project, author, reason)
-    body = "aborted the automatic merge because #{reason}"
-
-    ##
-    # TODO: Abort message should be sent by the system, not a particular user.
-    # See https://gitlab.com/gitlab-org/gitlab-ce/issues/63187.
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'merge'))
+    service = MergeRequestsSystemNoteService.new(noteable, project, author)
+    service.abort_merge_when_pipeline_succeeds(reason)
   end
 
   def handle_merge_request_wip(noteable, project, author)
-    prefix = noteable.work_in_progress? ? "marked" : "unmarked"
-
-    body = "#{prefix} as a **Work In Progress**"
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'title'))
+    service = MergeRequestsSystemNoteService.new(noteable, project, author)
+    service.handle_merge_request_wip
   end
 
   def add_merge_request_wip_from_commit(noteable, project, author, commit)
-    body = "marked as a **Work In Progress** from #{commit.to_reference(project)}"
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'title'))
+    service = MergeRequestsSystemNoteService.new(noteable, project, author)
+    service.add_merge_request_wip_from_commit(commit)
   end
 
   def resolve_all_discussions(merge_request, project, author)
-    body = "resolved all threads"
-
-    create_note(NoteSummary.new(merge_request, project, author, body, action: 'discussion'))
+    service = MergeRequestsSystemNoteService.new(merge_request, project, author)
+    service.resolve_all_discussions
   end
 
   def discussion_continued_in_issue(discussion, project, author, issue)
-    body = "created #{issue.to_reference} to continue this discussion"
-    note_attributes = discussion.reply_attributes.merge(project: project, author: author, note: body)
-
-    note = Note.create(note_attributes.merge(system: true, created_at: issue.system_note_timestamp))
-    note.system_note_metadata = SystemNoteMetadata.new(action: 'discussion')
-
-    note
+    service = MergeRequestsSystemNoteService.new(nil, project, author)
+    service.discussion_continued_in_issue(discussion, issue)
   end
 
   def diff_discussion_outdated(discussion, project, author, change_position)
-    merge_request = discussion.noteable
-    diff_refs = change_position.diff_refs
-    version_index = merge_request.merge_request_diffs.viewable.count
-    position_on_text = change_position.on_text?
-    text_parts = ["changed this #{position_on_text ? 'line' : 'file'} in"]
-
-    if version_params = merge_request.version_params_for(diff_refs)
-      repository = project.repository
-      anchor = position_on_text ? change_position.line_code(repository) : change_position.file_hash
-      url = url_helpers.diffs_project_merge_request_path(project, merge_request, version_params.merge(anchor: anchor))
-
-      text_parts << "[version #{version_index} of the diff](#{url})"
-    else
-      text_parts << "version #{version_index} of the diff"
-    end
-
-    body = text_parts.join(' ')
-    note_attributes = discussion.reply_attributes.merge(project: project, author: author, note: body)
-
-    note = Note.create(note_attributes.merge(system: true))
-    note.system_note_metadata = SystemNoteMetadata.new(action: 'outdated')
-
-    note
-  end
-
-  # Called when the title of a Noteable is changed
-  #
-  # noteable  - Noteable object that responds to `title`
-  # project   - Project owning noteable
-  # author    - User performing the change
-  # old_title - Previous String title
-  #
-  # Example Note text:
-  #
-  #   "changed title from **Old** to **New**"
-  #
-  # Returns the created Note object
-  def change_title(noteable, project, author, old_title)
-    new_title = noteable.title.dup
-
-    old_diffs, new_diffs = Gitlab::Diff::InlineDiff.new(old_title, new_title).inline_diffs
-
-    marked_old_title = Gitlab::Diff::InlineDiffMarkdownMarker.new(old_title).mark(old_diffs, mode: :deletion)
-    marked_new_title = Gitlab::Diff::InlineDiffMarkdownMarker.new(new_title).mark(new_diffs, mode: :addition)
-
-    body = "changed title from **#{marked_old_title}** to **#{marked_new_title}**"
+    service = MergeRequestsSystemNoteService.new(nil, project, author)
+    service.diff_discussion_outdated(discussion, change_position)
+  end
 
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'title'))
+  def change_title(noteable, project, author, old_title)
+    service = IssuablesSystemNoteService.new(noteable, project, author)
+    service.change_title(old_title)
   end
 
-  # Called when the description of a Noteable is changed
-  #
-  # noteable  - Noteable object that responds to `description`
-  # project   - Project owning noteable
-  # author    - User performing the change
-  #
-  # Example Note text:
-  #
-  #   "changed the description"
-  #
-  # Returns the created Note object
   def change_description(noteable, project, author)
-    body = 'changed the description'
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'description'))
-  end
-
-  # Called when the confidentiality changes
-  #
-  # issue   - Issue object
-  # project - Project owning the issue
-  # author  - User performing the change
-  #
-  # Example Note text:
-  #
-  #   "made the issue confidential"
-  #
-  # Returns the created Note object
-  def change_issue_confidentiality(issue, project, author)
-    if issue.confidential
-      body = 'made the issue confidential'
-      action = 'confidential'
-    else
-      body = 'made the issue visible to everyone'
-      action = 'visible'
-    end
-
-    create_note(NoteSummary.new(issue, project, author, body, action: action))
-  end
-
-  # Called when a branch in Noteable is changed
-  #
-  # noteable    - Noteable object
-  # project     - Project owning noteable
-  # author      - User performing the change
-  # branch_type - 'source' or 'target'
-  # old_branch  - old branch name
-  # new_branch  - new branch name
-  #
-  # Example Note text:
-  #
-  #   "changed target branch from `Old` to `New`"
-  #
-  # Returns the created Note object
-  def change_branch(noteable, project, author, branch_type, old_branch, new_branch)
-    body = "changed #{branch_type} branch from `#{old_branch}` to `#{new_branch}`"
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'branch'))
-  end
-
-  # Called when a branch in Noteable is added or deleted
-  #
-  # noteable    - Noteable object
-  # project     - Project owning noteable
-  # author      - User performing the change
-  # branch_type - :source or :target
-  # branch      - branch name
-  # presence    - :add or :delete
-  #
-  # Example Note text:
-  #
-  #   "restored target branch `feature`"
-  #
-  # Returns the created Note object
-  def change_branch_presence(noteable, project, author, branch_type, branch, presence)
-    verb =
-      if presence == :add
-        'restored'
-      else
-        'deleted'
-      end
-
-    body = "#{verb} #{branch_type} branch `#{branch}`"
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'branch'))
+    service = IssuablesSystemNoteService.new(noteable, project, author)
+    service.change_description
   end
 
-  # Called when a branch is created from the 'new branch' button on a issue
-  # Example note text:
-  #
-  #   "created branch `201-issue-branch-button`"
-  def new_issue_branch(issue, project, author, branch, branch_project: nil)
-    branch_project ||= project
-    link = url_helpers.project_compare_path(branch_project, from: branch_project.default_branch, to: branch)
-
-    body = "created branch [`#{branch}`](#{link}) to address this issue"
-
-    create_note(NoteSummary.new(issue, project, author, body, action: 'branch'))
+  def change_issue_confidentiality(issue, project, author)
+    service = IssuablesSystemNoteService.new(issue, project, author)
+    service.change_issue_confidentiality
   end
 
-  def new_merge_request(issue, project, author, merge_request)
-    body = "created merge request #{merge_request.to_reference(project)} to address this issue"
-
-    create_note(NoteSummary.new(issue, project, author, body, action: 'merge'))
-  end
-
-  # Called when a Mentionable references a Noteable
-  #
-  # noteable  - Noteable object being referenced
-  # mentioner - Mentionable object
-  # author    - User performing the reference
-  #
-  # Example Note text:
-  #
-  #   "mentioned in #1"
-  #
-  #   "mentioned in !2"
-  #
-  #   "mentioned in 54f7727c"
-  #
-  # See cross_reference_note_content.
-  #
-  # Returns the created Note object
-  def cross_reference(noteable, mentioner, author)
-    return if cross_reference_disallowed?(noteable, mentioner)
-
-    gfm_reference = mentioner.gfm_reference(noteable.project || noteable.group)
-    body = cross_reference_note_content(gfm_reference)
-
-    if noteable.is_a?(ExternalIssue)
-      noteable.project.issues_tracker.create_cross_reference_note(noteable, mentioner, author)
-    else
-      create_note(NoteSummary.new(noteable, noteable.project, author, body, action: 'cross_reference'))
-    end
-  end
-
-  # Check if a cross-reference is disallowed
-  #
-  # This method prevents adding a "mentioned in !1" note on every single commit
-  # in a merge request. Additionally, it prevents the creation of references to
-  # external issues (which would fail).
-  #
-  # noteable  - Noteable object being referenced
-  # mentioner - Mentionable object
-  #
-  # Returns Boolean
-  def cross_reference_disallowed?(noteable, mentioner)
-    return true if noteable.is_a?(ExternalIssue) && !noteable.project.jira_tracker_active?
-    return false unless mentioner.is_a?(MergeRequest)
-    return false unless noteable.is_a?(Commit)
-
-    mentioner.commits.include?(noteable)
-  end
-
-  # Check if a cross reference to a noteable from a mentioner already exists
-  #
-  # This method is used to prevent multiple notes being created for a mention
-  # when a issue is updated, for example. The method also calls notes_for_mentioner
-  # to check if the mentioner is a commit, and return matches only on commit hash
-  # instead of project + commit, to avoid repeated mentions from forks.
-  #
-  # noteable  - Noteable object being referenced
-  # mentioner - Mentionable object
-  #
-  # Returns Boolean
-  def cross_reference_exists?(noteable, mentioner)
-    notes = noteable.notes.system
-    notes_for_mentioner(mentioner, noteable, notes).exists?
+  def change_branch(noteable, project, author, branch_type, old_branch, new_branch)
+    service = MergeRequestsSystemNoteService.new(noteable, project, author)
+    service.change_branch(branch_type, old_branch, new_branch)
   end
 
-  # Build an Array of lines detailing each commit added in a merge request
-  #
-  # new_commits - Array of new Commit objects
-  #
-  # Returns an Array of Strings
-  def new_commit_summary(new_commits)
-    new_commits.collect do |commit|
-      content_tag('li', "#{commit.short_id} - #{commit.title}")
-    end
-  end
-
-  # Called when the status of a Task has changed
-  #
-  # noteable  - Noteable object.
-  # project   - Project owning noteable
-  # author    - User performing the change
-  # new_task  - TaskList::Item object.
-  #
-  # Example Note text:
-  #
-  #   "marked the task Whatever as completed."
-  #
-  # Returns the created Note object
-  def change_task_status(noteable, project, author, new_task)
-    status_label = new_task.complete? ? Taskable::COMPLETED : Taskable::INCOMPLETE
-    body = "marked the task **#{new_task.source}** as #{status_label}"
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'task'))
-  end
-
-  # Called when noteable has been moved to another project
-  #
-  # direction    - symbol, :to or :from
-  # noteable     - Noteable object
-  # noteable_ref - Referenced noteable
-  # author       - User performing the move
-  #
-  # Example Note text:
-  #
-  #   "moved to some_namespace/project_new#11"
-  #
-  # Returns the created Note object
-  def noteable_moved(noteable, project, noteable_ref, author, direction:)
-    unless [:to, :from].include?(direction)
-      raise ArgumentError, "Invalid direction `#{direction}`"
-    end
-
-    cross_reference = noteable_ref.to_reference(project)
-    body = "moved #{direction} #{cross_reference}"
-
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'moved'))
-  end
-
-  # Called when a Noteable has been marked as a duplicate of another Issue
-  #
-  # noteable        - Noteable object
-  # project         - Project owning noteable
-  # author          - User performing the change
-  # canonical_issue - Issue that this is a duplicate of
-  #
-  # Example Note text:
-  #
-  #   "marked this issue as a duplicate of #1234"
-  #
-  #   "marked this issue as a duplicate of other_project#5678"
-  #
-  # Returns the created Note object
-  def mark_duplicate_issue(noteable, project, author, canonical_issue)
-    body = "marked this issue as a duplicate of #{canonical_issue.to_reference(project)}"
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'duplicate'))
-  end
-
-  # Called when a Noteable has been marked as the canonical Issue of a duplicate
-  #
-  # noteable        - Noteable object
-  # project         - Project owning noteable
-  # author          - User performing the change
-  # duplicate_issue - Issue that was a duplicate of this
-  #
-  # Example Note text:
-  #
-  #   "marked #1234 as a duplicate of this issue"
-  #
-  #   "marked other_project#5678 as a duplicate of this issue"
-  #
-  # Returns the created Note object
-  def mark_canonical_issue_of_duplicate(noteable, project, author, duplicate_issue)
-    body = "marked #{duplicate_issue.to_reference(project)} as a duplicate of this issue"
-    create_note(NoteSummary.new(noteable, project, author, body, action: 'duplicate'))
+  def change_branch_presence(noteable, project, author, branch_type, branch, presence)
+    service = MergeRequestsSystemNoteService.new(noteable, project, author)
+    service.change_branch_presence(branch_type, branch, presence)
   end
 
-  def discussion_lock(issuable, author)
-    action = issuable.discussion_locked? ? 'locked' : 'unlocked'
-    body = "#{action} this #{issuable.class.to_s.titleize.downcase}"
-
-    create_note(NoteSummary.new(issuable, issuable.project, author, body, action: action))
+  def new_issue_branch(issue, project, author, branch, branch_project: nil)
+    service = MergeRequestsSystemNoteService.new(issue, project, author)
+    service.new_issue_branch(branch, branch_project: branch_project)
   end
 
-  def cross_reference?(note_text)
-    note_text =~ /\A#{cross_reference_note_prefix}/i
+  def new_merge_request(issue, project, author, merge_request)
+    service = MergeRequestsSystemNoteService.new(issue, project, author)
+    service.new_merge_request(merge_request)
   end
 
-  def zoom_link_added(issue, project, author)
-    create_note(NoteSummary.new(issue, project, author, _('added a Zoom call to this issue'), action: 'pinned_embed'))
+  def cross_reference(noteable, mentioner, author)
+    service = IssuablesSystemNoteService.new(noteable, nil, author)
+    service.cross_reference(mentioner)
   end
 
-  def zoom_link_removed(issue, project, author)
-    create_note(NoteSummary.new(issue, project, author, _('removed a Zoom call from this issue'), action: 'pinned_embed'))
+  def cross_reference_exists?(noteable, mentioner)
+    service = IssuablesSystemNoteService.new(noteable, nil, nil)
+    service.cross_reference_exists?(mentioner)
   end
 
-  private
-
-  # rubocop: disable CodeReuse/ActiveRecord
-  def notes_for_mentioner(mentioner, noteable, notes)
-    if mentioner.is_a?(Commit)
-      text = "#{cross_reference_note_prefix}%#{mentioner.to_reference(nil)}"
-      notes.where('(note LIKE ? OR note LIKE ?)', text, text.capitalize)
-    else
-      gfm_reference = mentioner.gfm_reference(noteable.project || noteable.group)
-      text = cross_reference_note_content(gfm_reference)
-      notes.where(note: [text, text.capitalize])
-    end
+  def change_task_status(noteable, project, author, new_task)
+    service = IssuablesSystemNoteService.new(noteable, project, author)
+    service.change_task_status(new_task)
   end
-  # rubocop: enable CodeReuse/ActiveRecord
-
-  def create_note(note_summary)
-    note = Note.create(note_summary.note.merge(system: true))
-    note.system_note_metadata = SystemNoteMetadata.new(note_summary.metadata) if note_summary.metadata?
 
-    note
+  def noteable_moved(noteable, project, noteable_ref, author, direction:)
+    service = IssuablesSystemNoteService.new(noteable, project, author)
+    service.noteable_moved(noteable_ref, direction)
   end
 
-  def cross_reference_note_prefix
-    'mentioned in '
+  def mark_duplicate_issue(noteable, project, author, canonical_issue)
+    service = IssuablesSystemNoteService.new(noteable, project, author)
+    service.mark_duplicate_issue(canonical_issue)
   end
 
-  def cross_reference_note_content(gfm_reference)
-    "#{cross_reference_note_prefix}#{gfm_reference}"
+  def mark_canonical_issue_of_duplicate(noteable, project, author, duplicate_issue)
+    service = IssuablesSystemNoteService.new(noteable, project, author)
+    service.mark_canonical_issue_of_duplicate(duplicate_issue)
   end
 
-  # Builds a list of existing and new commits according to existing_commits and
-  # new_commits methods.
-  # Returns a String wrapped in `ul` and `li` tags.
-  def commits_list(noteable, new_commits, existing_commits, oldrev)
-    existing_commit_summary = existing_commit_summary(noteable, existing_commits, oldrev)
-    new_commit_summary = new_commit_summary(new_commits).join
-
-    content_tag('ul', "#{existing_commit_summary}#{new_commit_summary}".html_safe)
+  def discussion_lock(issuable, author)
+    service = IssuablesSystemNoteService.new(issuable, issuable.project, author)
+    service.discussion_lock
   end
 
-  # Build a single line summarizing existing commits being added in a merge
-  # request
-  #
-  # noteable         - MergeRequest object
-  # existing_commits - Array of existing Commit objects
-  # oldrev           - Optional String SHA of a previous Commit
-  #
-  # Examples:
-  #
-  #   "* ea0f8418...2f4426b7 - 24 commits from branch `master`"
-  #
-  #   "* ea0f8418..4188f0ea - 15 commits from branch `fork:master`"
-  #
-  #   "* ea0f8418 - 1 commit from branch `feature`"
-  #
-  # Returns a newline-terminated String
-  def existing_commit_summary(noteable, existing_commits, oldrev = nil)
-    return '' if existing_commits.empty?
-
-    count = existing_commits.size
-
-    commit_ids = if count == 1
-                   existing_commits.first.short_id
-                 else
-                   if oldrev && !Gitlab::Git.blank_ref?(oldrev)
-                     "#{Commit.truncate_sha(oldrev)}...#{existing_commits.last.short_id}"
-                   else
-                     "#{existing_commits.first.short_id}..#{existing_commits.last.short_id}"
-                   end
-                 end
-
-    commits_text = "#{count} commit".pluralize(count)
-
-    branch = noteable.target_branch
-    branch = "#{noteable.target_project_namespace}:#{branch}" if noteable.for_fork?
-
-    branch_name = content_tag('code', branch)
-    content_tag('li', "#{commit_ids} - #{commits_text} from branch #{branch_name}".html_safe)
+  def cross_reference_disallowed?(noteable, mentioner)
+    service = IssuablesSystemNoteService.new(noteable, nil, nil)
+    service.cross_reference_disallowed?(mentioner)
   end
 
-  def url_helpers
-    @url_helpers ||= Gitlab::Routing.url_helpers
+  def zoom_link_added(issue, project, author)
+    service = ZoomSystemNoteService.new(issue, project, author)
+    service.zoom_link_added
   end
 
-  def diff_comparison_path(merge_request, project, oldrev)
-    diff_id = merge_request.merge_request_diff.id
-
-    url_helpers.diffs_project_merge_request_path(
-      project,
-      merge_request,
-      diff_id: diff_id,
-      start_sha: oldrev
-    )
+  def zoom_link_removed(issue, project, author)
+    service = ZoomSystemNoteService.new(issue, project, author)
+    service.zoom_link_removed
   end
 
-  def content_tag(*args)
-    ActionController::Base.helpers.content_tag(*args)
+  # TODO: Just added for testing
+  def new_commit_summary(new_commits)
+    CommitSystemNoteService.new(nil, nil, nil).new_commit_summary(new_commits)
   end
 end
diff --git a/app/services/system_notes/base_system_note_service.rb b/app/services/system_notes/base_system_note_service.rb
new file mode 100644
index 00000000000..3abd82e9935
--- /dev/null
+++ b/app/services/system_notes/base_system_note_service.rb
@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+class BaseSystemNoteService
+  attr_reader :noteable, :project, :author
+
+  def initialize(noteable, project, author)
+    @noteable = noteable
+    @project = project
+    @author = author
+  end
+
+  def create_note(note_summary)
+    note = Note.create(note_summary.note.merge(system: true))
+    note.system_note_metadata = SystemNoteMetadata.new(note_summary.metadata) if note_summary.metadata?
+
+    note
+  end
+
+  def content_tag(*args)
+    ActionController::Base.helpers.content_tag(*args)
+  end
+
+  def url_helpers
+    @url_helpers ||= Gitlab::Routing.url_helpers
+  end
+end
diff --git a/app/services/system_notes/commit_system_notes_service.rb b/app/services/system_notes/commit_system_notes_service.rb
new file mode 100644
index 00000000000..fc64e0c8d66
--- /dev/null
+++ b/app/services/system_notes/commit_system_notes_service.rb
@@ -0,0 +1,117 @@
+# frozen_string_literal: true
+
+class CommitSystemNoteService < BaseSystemNoteService
+  # Called when commits are added to a Merge Request
+  #
+  # noteable         - Noteable object
+  # project          - Project owning noteable
+  # author           - User performing the change
+  # new_commits      - Array of Commits added since last push
+  # existing_commits - Array of Commits added in a previous push
+  # oldrev           - Optional String SHA of a previous Commit
+  #
+  # See new_commit_summary and existing_commit_summary.
+  #
+  # Returns the created Note object
+  def add_commits(new_commits, existing_commits = [], oldrev = nil)
+    total_count  = new_commits.length + existing_commits.length
+    commits_text = "#{total_count} commit".pluralize(total_count)
+
+    text_parts = ["added #{commits_text}"]
+    text_parts << commits_list(noteable, new_commits, existing_commits, oldrev)
+    text_parts << "[Compare with previous version](#{diff_comparison_path(noteable, project, oldrev)})"
+
+    body = text_parts.join("\n\n")
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'commit', commit_count: total_count))
+  end
+
+  # Called when a commit was tagged
+  #
+  # noteable  - Noteable object
+  # project   - Project owning noteable
+  # author    - User performing the tag
+  # tag_name  - The created tag name
+  #
+  # Returns the created Note object
+  def tag_commit(tag_name)
+    link = url_helpers.project_tag_path(project, id: tag_name)
+    body = "tagged commit #{noteable.sha} to [`#{tag_name}`](#{link})"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'tag'))
+  end
+
+  # Build an Array of lines detailing each commit added in a merge request
+  #
+  # new_commits - Array of new Commit objects
+  #
+  # Returns an Array of Strings
+  def new_commit_summary(new_commits)
+    new_commits.collect do |commit|
+      content_tag('li', "#{commit.short_id} - #{commit.title}")
+    end
+  end
+
+  # Builds a list of existing and new commits according to existing_commits and
+  # new_commits methods.
+  # Returns a String wrapped in `ul` and `li` tags.
+  def commits_list(noteable, new_commits, existing_commits, oldrev)
+    existing_commit_summary = existing_commit_summary(noteable, existing_commits, oldrev)
+    new_commit_summary = new_commit_summary(new_commits).join
+
+    content_tag('ul', "#{existing_commit_summary}#{new_commit_summary}".html_safe)
+  end
+
+  private
+
+  # Build a single line summarizing existing commits being added in a merge
+  # request
+  #
+  # noteable         - MergeRequest object
+  # existing_commits - Array of existing Commit objects
+  # oldrev           - Optional String SHA of a previous Commit
+  #
+  # Examples:
+  #
+  #   "* ea0f8418...2f4426b7 - 24 commits from branch `master`"
+  #
+  #   "* ea0f8418..4188f0ea - 15 commits from branch `fork:master`"
+  #
+  #   "* ea0f8418 - 1 commit from branch `feature`"
+  #
+  # Returns a newline-terminated String
+  def existing_commit_summary(noteable, existing_commits, oldrev = nil)
+    return '' if existing_commits.empty?
+
+    count = existing_commits.size
+
+    commit_ids = if count == 1
+                   existing_commits.first.short_id
+                 else
+                   if oldrev && !Gitlab::Git.blank_ref?(oldrev)
+                     "#{Commit.truncate_sha(oldrev)}...#{existing_commits.last.short_id}"
+                   else
+                     "#{existing_commits.first.short_id}..#{existing_commits.last.short_id}"
+                   end
+                 end
+
+    commits_text = "#{count} commit".pluralize(count)
+
+    branch = noteable.target_branch
+    branch = "#{noteable.target_project_namespace}:#{branch}" if noteable.for_fork?
+
+    branch_name = content_tag('code', branch)
+    content_tag('li', "#{commit_ids} - #{commits_text} from branch #{branch_name}".html_safe)
+  end
+
+  def diff_comparison_path(merge_request, project, oldrev)
+    diff_id = merge_request.merge_request_diff.id
+
+    url_helpers.diffs_project_merge_request_path(
+      project,
+      merge_request,
+      diff_id: diff_id,
+      start_sha: oldrev
+    )
+  end
+end
diff --git a/app/services/system_notes/issuables_system_note_service.rb b/app/services/system_notes/issuables_system_note_service.rb
new file mode 100644
index 00000000000..bd2fae5c65d
--- /dev/null
+++ b/app/services/system_notes/issuables_system_note_service.rb
@@ -0,0 +1,329 @@
+# frozen_string_literal: true
+
+class IssuablesSystemNoteService < BaseSystemNoteService
+  # Called when the assignee of a Noteable is changed or removed
+  #
+  # assignee - User being assigned, or nil
+  #
+  # Example Note text:
+  #
+  #   "removed assignee"
+  #
+  #   "assigned to @rspeicher"
+  #
+  # Returns the created Note object
+  def change_assignee(assignee)
+    body = assignee.nil? ? 'removed assignee' : "assigned to #{assignee.to_reference}"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'assignee'))
+  end
+
+  # Called when the assignees of an issuable is changed or removed
+  #
+  # assignees - Users being assigned, or nil
+  #
+  # Example Note text:
+  #
+  #   "removed all assignees"
+  #
+  #   "assigned to @user1 additionally to @user2"
+  #
+  #   "assigned to @user1, @user2 and @user3 and unassigned from @user4 and @user5"
+  #
+  #   "assigned to @user1 and @user2"
+  #
+  # Returns the created Note object
+  def change_issuable_assignees(old_assignees)
+    unassigned_users = old_assignees - noteable.assignees
+    added_users = noteable.assignees.to_a - old_assignees
+    text_parts = []
+
+    Gitlab::I18n.with_default_locale do
+      text_parts << "assigned to #{added_users.map(&:to_reference).to_sentence}" if added_users.any?
+      text_parts << "unassigned #{unassigned_users.map(&:to_reference).to_sentence}" if unassigned_users.any?
+    end
+
+    body = text_parts.join(' and ')
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'assignee'))
+  end
+
+  # Called when the milestone of a Noteable is changed
+  #
+  # milestone - Milestone being assigned, or nil
+  #
+  # Example Note text:
+  #
+  #   "removed milestone"
+  #
+  #   "changed milestone to 7.11"
+  #
+  # Returns the created Note object
+  def change_milestone(milestone)
+    format = milestone&.group_milestone? ? :name : :iid
+    body = milestone.nil? ? 'removed milestone' : "changed milestone to #{milestone.to_reference(project, format: format)}"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'milestone'))
+  end
+
+  # Called when the title of a Noteable is changed
+  #
+  # old_title - Previous String title
+  #
+  # Example Note text:
+  #
+  #   "changed title from **Old** to **New**"
+  #
+  # Returns the created Note object
+  def change_title(old_title)
+    new_title = noteable.title.dup
+
+    old_diffs, new_diffs = Gitlab::Diff::InlineDiff.new(old_title, new_title).inline_diffs
+
+    marked_old_title = Gitlab::Diff::InlineDiffMarkdownMarker.new(old_title).mark(old_diffs, mode: :deletion)
+    marked_new_title = Gitlab::Diff::InlineDiffMarkdownMarker.new(new_title).mark(new_diffs, mode: :addition)
+
+    body = "changed title from **#{marked_old_title}** to **#{marked_new_title}**"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'title'))
+  end
+
+  # Called when the description of a Noteable is changed
+  #
+  # noteable  - Noteable object that responds to `description`
+  # project   - Project owning noteable
+  # author    - User performing the change
+  #
+  # Example Note text:
+  #
+  #   "changed the description"
+  #
+  # Returns the created Note object
+  def change_description
+    body = 'changed the description'
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'description'))
+  end
+
+  # Called when a Mentionable references a Noteable
+  #
+  # noteable  - Noteable object being referenced
+  # mentioner - Mentionable object
+  # author    - User performing the reference
+  #
+  # Example Note text:
+  #
+  #   "mentioned in #1"
+  #
+  #   "mentioned in !2"
+  #
+  #   "mentioned in 54f7727c"
+  #
+  # See cross_reference_note_content.
+  #
+  # Returns the created Note object
+  def cross_reference(mentioner)
+    return if cross_reference_disallowed?(mentioner)
+
+    gfm_reference = mentioner.gfm_reference(noteable.project || noteable.group)
+    body = cross_reference_note_content(gfm_reference)
+
+    if noteable.is_a?(ExternalIssue)
+      noteable.project.issues_tracker.create_cross_reference_note(noteable, mentioner, author)
+    else
+      create_note(NoteSummary.new(noteable, noteable.project, author, body, action: 'cross_reference'))
+    end
+  end
+
+  # Check if a cross-reference is disallowed
+  #
+  # This method prevents adding a "mentioned in !1" note on every single commit
+  # in a merge request. Additionally, it prevents the creation of references to
+  # external issues (which would fail).
+  #
+  # noteable  - Noteable object being referenced
+  # mentioner - Mentionable object
+  #
+  # Returns Boolean
+  def cross_reference_disallowed?(mentioner)
+    return true if noteable.is_a?(ExternalIssue) && !noteable.project.jira_tracker_active?
+    return false unless mentioner.is_a?(MergeRequest)
+    return false unless noteable.is_a?(Commit)
+
+    mentioner.commits.include?(noteable)
+  end
+
+  # Called when the status of a Task has changed
+  #
+  # new_task  - TaskList::Item object.
+  #
+  # Example Note text:
+  #
+  #   "marked the task Whatever as completed."
+  #
+  # Returns the created Note object
+  def change_task_status(new_task)
+    status_label = new_task.complete? ? Taskable::COMPLETED : Taskable::INCOMPLETE
+    body = "marked the task **#{new_task.source}** as #{status_label}"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'task'))
+  end
+
+  # Called when noteable has been moved to another project
+  #
+  # direction    - symbol, :to or :from
+  # noteable     - Noteable object
+  # noteable_ref - Referenced noteable
+  # author       - User performing the move
+  #
+  # Example Note text:
+  #
+  #   "moved to some_namespace/project_new#11"
+  #
+  # Returns the created Note object
+  def noteable_moved(noteable_ref, direction)
+    unless [:to, :from].include?(direction)
+      raise ArgumentError, "Invalid direction `#{direction}`"
+    end
+
+    cross_reference = noteable_ref.to_reference(project)
+    body = "moved #{direction} #{cross_reference}"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'moved'))
+  end
+
+  # Called when the confidentiality changes
+  #
+  # issue   - Issue object
+  # project - Project owning the issue
+  # author  - User performing the change
+  #
+  # Example Note text:
+  #
+  #   "made the issue confidential"
+  #
+  # Returns the created Note object
+  def change_issue_confidentiality
+    if noteable.confidential
+      body = 'made the issue confidential'
+      action = 'confidential'
+    else
+      body = 'made the issue visible to everyone'
+      action = 'visible'
+    end
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: action))
+  end
+
+  # Called when the status of a Noteable is changed
+  #
+  # noteable - Noteable object
+  # project  - Project owning noteable
+  # author   - User performing the change
+  # status   - String status
+  # source   - Mentionable performing the change, or nil
+  #
+  # Example Note text:
+  #
+  #   "merged"
+  #
+  #   "closed via bc17db76"
+  #
+  # Returns the created Note object
+  def change_status(status, source = nil)
+    body = status.dup
+    body << " via #{source.gfm_reference(project)}" if source
+
+    action = status == 'reopened' ? 'opened' : status
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: action))
+  end
+
+  # Check if a cross reference to a noteable from a mentioner already exists
+  #
+  # This method is used to prevent multiple notes being created for a mention
+  # when a issue is updated, for example. The method also calls notes_for_mentioner
+  # to check if the mentioner is a commit, and return matches only on commit hash
+  # instead of project + commit, to avoid repeated mentions from forks.
+  #
+  # noteable  - Noteable object being referenced
+  # mentioner - Mentionable object
+  #
+  # Returns Boolean
+  def cross_reference_exists?(mentioner)
+    notes = noteable.notes.system
+    notes_for_mentioner(mentioner, noteable, notes).exists?
+  end
+
+  # Called when a Noteable has been marked as a duplicate of another Issue
+  #
+  # noteable        - Noteable object
+  # project         - Project owning noteable
+  # author          - User performing the change
+  # canonical_issue - Issue that this is a duplicate of
+  #
+  # Example Note text:
+  #
+  #   "marked this issue as a duplicate of #1234"
+  #
+  #   "marked this issue as a duplicate of other_project#5678"
+  #
+  # Returns the created Note object
+  def mark_duplicate_issue(canonical_issue)
+    body = "marked this issue as a duplicate of #{canonical_issue.to_reference(project)}"
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'duplicate'))
+  end
+
+  # Called when a Noteable has been marked as the canonical Issue of a duplicate
+  #
+  # noteable        - Noteable object
+  # project         - Project owning noteable
+  # author          - User performing the change
+  # duplicate_issue - Issue that was a duplicate of this
+  #
+  # Example Note text:
+  #
+  #   "marked #1234 as a duplicate of this issue"
+  #
+  #   "marked other_project#5678 as a duplicate of this issue"
+  #
+  # Returns the created Note object
+  def mark_canonical_issue_of_duplicate(duplicate_issue)
+    body = "marked #{duplicate_issue.to_reference(project)} as a duplicate of this issue"
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'duplicate'))
+  end
+
+  def discussion_lock
+    action = noteable.discussion_locked? ? 'locked' : 'unlocked'
+    body = "#{action} this #{noteable.class.to_s.titleize.downcase}"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: action))
+  end
+
+  private
+
+  def cross_reference_note_content(gfm_reference)
+    "#{self.class.cross_reference_note_prefix}#{gfm_reference}"
+  end
+
+  # rubocop: disable CodeReuse/ActiveRecord
+  def notes_for_mentioner(mentioner, noteable, notes)
+    if mentioner.is_a?(Commit)
+      text = "#{self.class.cross_reference_note_prefix}%#{mentioner.to_reference(nil)}"
+      notes.where('(note LIKE ? OR note LIKE ?)', text, text.capitalize)
+    else
+      gfm_reference = mentioner.gfm_reference(noteable.project || noteable.group)
+      text = cross_reference_note_content(gfm_reference)
+      notes.where(note: [text, text.capitalize])
+    end
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
+
+  def self.cross_reference_note_prefix
+    'mentioned in '
+  end
+
+  def self.cross_reference?(note_text)
+    note_text =~ /\A#{cross_reference_note_prefix}/i
+  end
+end
diff --git a/app/services/system_notes/merge_requests_system_note_service.rb b/app/services/system_notes/merge_requests_system_note_service.rb
new file mode 100644
index 00000000000..02f52b050a0
--- /dev/null
+++ b/app/services/system_notes/merge_requests_system_note_service.rb
@@ -0,0 +1,143 @@
+# frozen_string_literal: true
+
+class MergeRequestsSystemNoteService < BaseSystemNoteService
+  # Called when 'merge when pipeline succeeds' is executed
+  def merge_when_pipeline_succeeds(sha)
+    body = "enabled an automatic merge when the pipeline for #{sha} succeeds"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'merge'))
+  end
+
+  # Called when 'merge when pipeline succeeds' is canceled
+  def cancel_merge_when_pipeline_succeeds
+    body = 'canceled the automatic merge'
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'merge'))
+  end
+
+  # Called when 'merge when pipeline succeeds' is aborted
+  def abort_merge_when_pipeline_succeeds(reason)
+    body = "aborted the automatic merge because #{reason}"
+
+    ##
+    # TODO: Abort message should be sent by the system, not a particular user.
+    # See https://gitlab.com/gitlab-org/gitlab-ce/issues/63187.
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'merge'))
+  end
+
+  def handle_merge_request_wip
+    prefix = noteable.work_in_progress? ? "marked" : "unmarked"
+
+    body = "#{prefix} as a **Work In Progress**"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'title'))
+  end
+
+  def add_merge_request_wip_from_commit(commit)
+    body = "marked as a **Work In Progress** from #{commit.to_reference(project)}"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'title'))
+  end
+
+  def resolve_all_discussions
+    body = "resolved all threads"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'discussion'))
+  end
+
+  def discussion_continued_in_issue(discussion, issue)
+    body = "created #{issue.to_reference} to continue this discussion"
+    note_attributes = discussion.reply_attributes.merge(project: project, author: author, note: body)
+
+    note = Note.create(note_attributes.merge(system: true, created_at: issue.system_note_timestamp))
+    note.system_note_metadata = SystemNoteMetadata.new(action: 'discussion')
+
+    note
+  end
+
+  def diff_discussion_outdated(discussion, change_position)
+    merge_request = discussion.noteable
+    diff_refs = change_position.diff_refs
+    version_index = merge_request.merge_request_diffs.viewable.count
+    position_on_text = change_position.on_text?
+    text_parts = ["changed this #{position_on_text ? 'line' : 'file'} in"]
+
+    if version_params = merge_request.version_params_for(diff_refs)
+      repository = project.repository
+      anchor = position_on_text ? change_position.line_code(repository) : change_position.file_hash
+      url = url_helpers.diffs_project_merge_request_path(project, merge_request, version_params.merge(anchor: anchor))
+
+      text_parts << "[version #{version_index} of the diff](#{url})"
+    else
+      text_parts << "version #{version_index} of the diff"
+    end
+
+    body = text_parts.join(' ')
+    note_attributes = discussion.reply_attributes.merge(project: project, author: author, note: body)
+
+    note = Note.create(note_attributes.merge(system: true))
+    note.system_note_metadata = SystemNoteMetadata.new(action: 'outdated')
+
+    note
+  end
+
+  # Called when a branch in Noteable is changed
+  #
+  # branch_type - 'source' or 'target'
+  # old_branch  - old branch name
+  # new_branch  - new branch name
+  #
+  # Example Note text:
+  #
+  #   "changed target branch from `Old` to `New`"
+  #
+  # Returns the created Note object
+  def change_branch(branch_type, old_branch, new_branch)
+    body = "changed #{branch_type} branch from `#{old_branch}` to `#{new_branch}`"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'branch'))
+  end
+
+  # Called when a branch in Noteable is added or deleted
+  #
+  # branch_type - :source or :target
+  # branch      - branch name
+  # presence    - :add or :delete
+  #
+  # Example Note text:
+  #
+  #   "restored target branch `feature`"
+  #
+  # Returns the created Note object
+  def change_branch_presence(branch_type, branch, presence)
+    verb =
+      if presence == :add
+        'restored'
+      else
+        'deleted'
+      end
+
+    body = "#{verb} #{branch_type} branch `#{branch}`"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'branch'))
+  end
+
+  # Called when a branch is created from the 'new branch' button on a issue
+  # Example note text:
+  #
+  #   "created branch `201-issue-branch-button`"
+  def new_issue_branch(branch, branch_project: nil)
+    branch_project ||= project
+    link = url_helpers.project_compare_path(branch_project, from: branch_project.default_branch, to: branch)
+
+    body = "created branch [`#{branch}`](#{link}) to address this issue"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'branch'))
+  end
+
+  def new_merge_request(merge_request)
+    body = "created merge request #{merge_request.to_reference(project)} to address this issue"
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'merge'))
+  end
+end
diff --git a/app/services/system_notes/time_tracking_system_note_service.rb b/app/services/system_notes/time_tracking_system_note_service.rb
new file mode 100644
index 00000000000..7c82fa7309b
--- /dev/null
+++ b/app/services/system_notes/time_tracking_system_note_service.rb
@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+class TimeTrackingSystemNoteService < BaseSystemNoteService
+  # Called when the due_date of a Noteable is changed
+  #
+  # noteable  - Noteable object
+  # project   - Project owning noteable
+  # author    - User performing the change
+  # due_date  - Due date being assigned, or nil
+  #
+  # Example Note text:
+  #
+  #   "removed due date"
+  #
+  #   "changed due date to September 20, 2018"
+  #
+  # Returns the created Note object
+  def change_due_date(due_date)
+    body = due_date ? "changed due date to #{due_date.to_s(:long)}" : 'removed due date'
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'due_date'))
+  end
+
+  # Called when the estimated time of a Noteable is changed
+  #
+  # noteable      - Noteable object
+  # project       - Project owning noteable
+  # author        - User performing the change
+  # time_estimate - Estimated time
+  #
+  # Example Note text:
+  #
+  #   "removed time estimate"
+  #
+  #   "changed time estimate to 3d 5h"
+  #
+  # Returns the created Note object
+  def change_time_estimate
+    parsed_time = Gitlab::TimeTrackingFormatter.output(noteable.time_estimate)
+    body = if noteable.time_estimate == 0
+             "removed time estimate"
+           else
+             "changed time estimate to #{parsed_time}"
+           end
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'time_tracking'))
+  end
+
+  # Called when the spent time of a Noteable is changed
+  #
+  # noteable   - Noteable object
+  # project    - Project owning noteable
+  # author     - User performing the change
+  # time_spent - Spent time
+  #
+  # Example Note text:
+  #
+  #   "removed time spent"
+  #
+  #   "added 2h 30m of time spent"
+  #
+  # Returns the created Note object
+  def change_time_spent
+    time_spent = noteable.time_spent
+
+    if time_spent == :reset
+      body = "removed time spent"
+    else
+      spent_at = noteable.spent_at
+      parsed_time = Gitlab::TimeTrackingFormatter.output(time_spent.abs)
+      action = time_spent > 0 ? 'added' : 'subtracted'
+
+      text_parts = ["#{action} #{parsed_time} of time spent"]
+      text_parts << "at #{spent_at}" if spent_at
+      body = text_parts.join(' ')
+    end
+
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'time_tracking'))
+  end
+end
diff --git a/app/services/system_notes/zoom_system_note_service.rb b/app/services/system_notes/zoom_system_note_service.rb
new file mode 100644
index 00000000000..a145cc9c780
--- /dev/null
+++ b/app/services/system_notes/zoom_system_note_service.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class ZoomSystemNoteService < BaseSystemNoteService
+  def zoom_link_added
+    create_note(NoteSummary.new(noteable, project, author, _('added a Zoom call to this issue'), action: 'pinned_embed'))
+  end
+
+  def zoom_link_removed
+    create_note(NoteSummary.new(noteable, project, author, _('removed a Zoom call from this issue'), action: 'pinned_embed'))
+  end
+end
diff --git a/spec/services/system_note_service_spec.rb b/spec/services/system_note_service_spec.rb
index 910fe3b50b7..9c3270c4be6 100644
--- a/spec/services/system_note_service_spec.rb
+++ b/spec/services/system_note_service_spec.rb
@@ -555,7 +555,7 @@ describe SystemNoteService do
 
     context 'when cross-reference disallowed' do
       before do
-        expect(described_class).to receive(:cross_reference_disallowed?).and_return(true)
+        expect_any_instance_of(IssuablesSystemNoteService).to receive(:cross_reference_disallowed?).and_return(true)
       end
 
       it 'returns nil' do
@@ -569,7 +569,7 @@ describe SystemNoteService do
 
     context 'when cross-reference allowed' do
       before do
-        expect(described_class).to receive(:cross_reference_disallowed?).and_return(false)
+        expect_any_instance_of(IssuablesSystemNoteService).to receive(:cross_reference_disallowed?).and_return(false)
       end
 
       it_behaves_like 'a system note' do