Modern mobile apps need to store and share structured data, but the coarse-grained access-control mechanisms in existing mobile operating systems are inadequate to help apps express and enforce their protection requirements. We design, implement, and evaluate a prototype of Earp, a new mobile platform that uses the relational model as the unified OS-level abstraction for both storage and inter-app services. Earp provides apps with structure-aware, OS-enforced access control, bringing order and protection to the Wild West of mobile data management.