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XML has become a crucial tool for data storage and exchange. In this chapter, after a brief introduction on the basic structure of XML, we illustrate the most important characteristics of access control models. We then discuss two models for XML documents, pointing out their main characteristics. We finally present other proposals, describing their main features and their innovation compared to the previous two models. [ABSTRACT FROM AUTHOR]

Database outsourcing is becoming increasingly popular introducing a new paradigm, called database-as-a-service, where a client's database is stored at an external service provider. Outsourcing databases to external providers promises higher availability and more effective disaster protection than in-house operations. This scenario presents new research challenges on which the usability of the system is based. In particular, one important aspect is the metadata that must be provided to support the proper working of the system. In this paper, we illustrate the metadata that are needed, at the client and server, to store and retrieve mapping information for processing a query issued by a client application to the server storing the outsourced database. We also present an approach to develop an efficient access control technique and the corresponding metadata needed for its enforcement. [ABSTRACT FROM AUTHOR]