Emily Gore et al. have published "The Digital Public Library of America and the National Digital Platform" in D-Lib Magazine.
Here's an excerpt:
The Digital Public Library of America brings together the riches of America's libraries, archives, and museums, and makes them freely available to the world. In order to do this, DPLA has had to build elements of the national digital platform to connect to those institutions and to serve their digitized materials to audiences. In this article, we detail the construction of two critical elements of our work: the decentralized national network of "hubs," which operate in states across the country; and a version of the Hydra repository software that is tailored to the needs of our community. This technology and the organizations that make use of it serve as the foundation of the future of DPLA and other projects that seek to take advantage of the national digital platform.