According to me, the mmorpgs'' design should be departed into two parts, one is server-side design, another is client-side design.

The significant differences between such 2 parts is active or passive. At client side, the main loop first check user''s inputs, no matter how inputs presents or not, it should update & render after the check. But when server side, if no remote inputs presented, it almost stay silence, except a low frequence loop(we called it heart-beats) occur.

A loop of heart-beat process such two things:
-Process NPCs AI.
-Process cached events.

At this point, the client-side''s design, we called it Active Model, the other server-side''s design, we called it Passive Model,