This paper presents Callinicos, a robust storage system with a novel transaction protocol that generalizes minitransactions. This protocol allows Callinicos to cope with Byzantine failures, support cross-partition communication with transactions, and implement on-demand contention management. We have evaluated Callinicos with a set of micro-benchmarks, and two realistic applications: a Twitter-like social network and a distributed message queue. Our experiments show that: (i) cross-partition communication improves performance by reducing the number of aborts, and (ii) the conflict resolution protocol results in no aborts in the presence of contention and no overhead in the absence of contention.