Yeah - if I had to handle requirements more towards the painful end of the scale I would probably say Agent + Event Store. Ie, ephemeral process(es) representing the user on the server side, with the ability to cache up to a certain amount of outgoing messages, respond to user's acks, and also to be wound down if the client hasn't consumed for too long. The Agent is a natural place for decisions about how, or if, to attempt to get the user 'caught up.'