Book excerpt: Security assurance is the guarantee provided with regard to access control, security privileges, and enforcement over time as users interact with an application. For a big-data application that shares and exchanges information from multiple sources in different formats, security assurance must reconcile local security capabilities to meet stakeholder needs. This chapter presents a role-based access control (RBAC) approach to modeling a global security policy and generating an enforcement mechanism for a big-data application by integrating the local policies of the sources, which are assumed to communicate via XML, the de facto standard for information sharing/exchange. Towards this goal, the Unified Modeling Language (UML) is extended to define new diagrams to capture XML for RBAC security and for policy modeling. To illustrate, we use a big-data application in law enforcement for motor vehicle crashes, showing how global security can be achieved in a repository that links different crash data repositories from multiple sources.