Mobile apps need to manage data, often across devices, to provide users with a variety of features such as seamless access, collaboration, and offline editing. To do so reliably, an app must anticipate and handle a host of local and network failures while preserving data consistency. For mobile environments, frugal usage of cellular bandwidth and device battery are also essential. The above requirements place an enormous burden on the app developer. We built Simba, a data-sync service that provides mobile app developers with a high-level local-programming abstraction unifying tabular and object data – a need common to mobile apps – and transparently handles data storage and sync in a reliable, consistent, and efficient manner. In this paper we present a detailed description of Simba’s client software which acts as the gateway to the data sync infrastructure. Our evaluation shows Simba’s effectiveness in rapid development of robust mobile apps that are consistent under all failure scenarios, unlike apps developed with Dropbox. Simba-apps are also demonstrably frugal with cellular resources.