The success of MapReduce has sparked the development of a diverse array of cluster computing frameworks. We believe that no single framework will be optimal for all applications, and that organizations will instead want to run multiple frameworks in the same cluster. Furthermore, to ease development of new frameworks, it is critical to identify common abstractions and modularize their architectures. To achieve these goals, we propose Nexus, a low-level substrate that provides isolation and efficient resource sharing across frameworks running on the same cluster, while giving each framework maximum control over the scheduling and execution of its jobs. Nexus fosters innovation in the cloud by letting organizations run new frameworks alongside existing ones and by letting framework developers focus on specific applications rather than building one-size-fits-all frameworks.