It seems natural to imagine that SDN policy specification and control is distributed, and this paper focuses on the resulting concurrency issues. Indeed, conflicts among concurrent policy updates may result in serious inconsistencies on the data plane, even when each update is installed with perpacket consistent update semantics. This paper introduces the problem of consistent composition of concurrent policy updates. Intuitively, consistent concurrent policy composition must appear as though there is no concurrency neither between any policy updates, nor between a policy update and in-flight packets on the data plane. We propose an elegant policy composition abstraction based on a transactional interface with all-or-nothing semantics: a policy update is either committed, in which case the policy is guaranteed to compose consistently over the entire network and the update is installed in its entirety, or aborted, in which case, no packet is affected by it. Consequently, the control application logic is relieved from the cumbersome and potentially error-prone synchronization and locking tasks, and control applications are kept lightweight. In this paper, we also sketch a simple implementation of the transactional synchronization: our approach is based on fine-grained locking on network components and avoids complex state machine replication.