Real-time User Presence: How do we scale that?

were buildin a new chat feature and need to show who's online away or offline in real time. wedo expect tons of people usin it at once. so, whats the best way to build this and what database should we pick to keep track of this super fast changing, high volume stuff. r there specific nosql dbs that r good for this, or can we still use a relational db if we index it right