Merge branch 'document-activesession' into 'master'

Document ActiveSession

See merge request gitlab-org/gitlab!48579

app/models/active_session.rb

 # frozen_string_literal: true
 
+# Backing store for GitLab session data.
+#
+# The raw session information is stored by the Rails session store
+# (config/initializers/session_store.rb). These entries are accessible by the
+# rack_key_name class method and consistute the base of the session data
+# entries. All other entries in the session store can be traced back to these
+# entries.
+#
+# After a user logs in (config/initializers/warden.rb) a further entry is made
+# in Redis. This entry holds a record of the user's logged in session. These
+# are accessible with the key_name(user_id, session_id) class method. These
+# entries will expire. Lookups to these entries are lazilly cleaned on future
+# user access.
+#
+# There is a reference to all sessions that belong to a specific user. A
+# user may login through multiple browsers/devices and thus record multiple
+# login sessions. These are accessible through the lookup_key_name(user_id)
+# class method.
+#
 class ActiveSession
   include ActiveModel::Model
 
@@ -143,6 +162,10 @@ class ActiveSession
     list(user).reject(&:is_impersonated)
   end
 
+  def self.rack_key_name(session_id)
+    "#{Gitlab::Redis::SharedState::SESSION_NAMESPACE}:#{session_id}"
+  end
+
   def self.key_name(user_id, session_id = '*')
     "#{Gitlab::Redis::SharedState::USER_SESSIONS_NAMESPACE}:#{user_id}:#{session_id}"
   end
@@ -197,7 +220,7 @@ class ActiveSession
   end
 
   def self.rack_session_keys(rack_session_ids)
-    rack_session_ids.map { |session_id| "#{Gitlab::Redis::SharedState::SESSION_NAMESPACE}:#{session_id}" }
+    rack_session_ids.map { |session_id| rack_key_name(session_id)}
   end
 
   def self.raw_active_session_entries(redis, session_ids, user_id)