I am looking into how to design an instance system for the game I am working on. I have always wondered how these are created in games like World of Warcraft, where instances == dungeons/raids/etc). Areas that are separated from players other than those in your group, but have specific logic to them. Specifically how can you reuse your existing code base and not have a bunch of checks everywhere ?

if (isInstance) do x; else do y;

I don't know if this will make too much of a difference on any answers, but we're using a pretty classic "Object as pure aggregation" component system for our entities.