i need to store some data about match to mark state, and when it reaches certain value i'll take an action. but i am not sure how to store a variable on match instance.

Hi, I'm reading this answer as I'm looking into ways of storing data that is easier to relate to the current match / campaign / world instance than to any player in particular. "runtime collection" seems to imply that this is transient? Though reading through the tutorials I've seen it used to store (alas player specific) race data, and retrieve it later?

Or would it be necessary to setup a fake user ("Gamemaster") to store this type of data? After spending reasonable time reading the documentation it would appear that you're not really supporting this use case.

Why don't you create a runtime collection with the matchId as the key of the document ?

It would not be linked to a user in particular...