Computer manufacturers, software developers, and service providers spend significant time and resources to ensure that they can optimally support one class of applications. However, they cannot cope with the dynamic and continuous changes in applications and workload types, and consequently their offered services become unstable, fragile, and cannot guarantee the required Quality of Service (QoS). Furthermore, it becomes prohibitively expensive to build data centers (DCs) that are optimized for fixed types of workloads and businesses or to scale up for accommodating new growth in heterogeneous workload demands. Consequently, it is critically important that the architecture of next generation DCs is dynamically customizable to support a wide range of application types or businesses under constrained resources. In this paper, we present some design concepts for our Just In Time Architecture (JITA), a novel data center design to overcome the composable DC challenges by dynamically interconnecting DC components into a virtual DC (VDC) that is optimized to the service level objectives for each class of applications. We present how we can use optical waveguide links to build a passive crossbar that directly interconnects all DC resources at the required throughput and latency.