Besides the player triggering quests, there is another type of quest called Achievement. Achievements are some default quests which players complete passively. The requirements of achievement can be completed by the players without even noticing them. For example, an achievement may describe the user to stay in the virtual world for 30 minutes. While the players are playing in the virtual world for half an hour, this achievement is automatically marked as completed.
The following graph is an achievement list from the virtual world Habbo. It is a virtual world that focuses on making friends and socializing. Therefore, the achievements are related to the login time and the number of friends in the buddy list.
As the achievement quests are completed passively without triggering by players, we need to install the achievement quests as in-progress quests in background in order to keep track of the progress of the achievements. This server-side extension in virtualworld...