The success of MapReduce has sparked many efforts to design cluster computing frameworks. We argue that no single framework will be optimal for all applications, and that we should instead enable organizations to run multiple frameworks effciently in the same cloud. 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 freedom to implement its own programming model and fully control the 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.