This Department of Energy Exascale Computing Project co-design center is working to overcome the rapidly growing gap between compute speed and storage input/output rates on massively parallel computers by evaluating, deploying and integrating novel online data analysis and reduction methods. Working closely with Exascale Computing Project applications, CODAR undertakes a focused co-design process that targets both common and domain-specific data analysis and reduction methods, with the goal of allowing application developers to choose and configure methods to output just the data needed by the application. CODAR engages directly with providers of ECP hardware, system software, programming models, data analysis and reduction algorithms and applications in order to better understand and guide tradeoffs in the development of exascale systems, applications and software frameworks, given constraints relating to application development costs, application fidelity, performance portability, scalability and power efficiency. CODAR is a collaboration with Argonne, Brookhaven, and Oak Ridge National Laboratories.