Versions of this problem include adapting using only unlabeled target domain data Blitzer et al., 2007;Jiang and Zhai, 2007), adapting using a limited amount of target domain labeled data (Daumé, 2007;Finkel and Manning, 2009), and learning across multiple domains simultaneously in an online setting (Dredze and Crammer, 2008b).
in practical settings, we do not know if the data distribution will change, and certainly not when.