System of Mobile Devices (SyD) is a middleware we developed that can be used for implementing collaborative, mobile, and distributed applications over heterogeneous devices, data stores, and computing environments. Current prototype implementation of SyD consists of five modules. These modules provide ease of programming in the areas of distributed communication, remote method invocation, service publication and discovery, directory services, distributed service invocation and aggregation, event handling, collaborative link creation and enforcement. A central module is SyDLink, which allows SyD-based applications to create "coordination" links. Coordination links represent dependencies among heterogeneous devices and application components. Based on the underlying event-and-trigger mechanism, they allow automatic updates as well as real-time enforcement of global constraints and interdependencies. SyDLink objects provide the underlying mechanism in SyD to enforce atomic execution of distributed transactions. We explain and demonstrate the use of SyDLink objects via a running example, a collaborative SyD calendar application, throughout the paper.