The OPERAS Metrics Service enables the gathering of open access book metrics into a central OPERAS database, and then the display of those metrics on a customer's site via a centrally-managed OPERAS widget. This document describes how the different parts of the metrics service interact, the work that was carried out during the OPERAS-PLUS project to add scalability by updating the underlying code and architecture, and provides information on the running costs along with suggestions on how these could be covered. This deliverable was approved by the European Commission.