Ongoing research

The database group has focused in the last few years on the following research issues.

• Optimization of XQuery with updates. XQuery is a standard language, to query XML data, defined by the W3C. The W3C is currently defining an extension of XQuery to update XML data. This extension heavily affects the language semantics: the extended language acquires some imperative features, which heavily impact query optimization. The group is working at the definition of a formal semantics for the extended language and of an algebra to support its compilation and optimization. The group is also working at the definition of optimization rules based on this extended framework.

• Spatial logics for SSD. The ambient logic is a modal logic proposed to describe the structural and computational properties of distributed and mobile computation. The structural part of the ambient logic is, essentially, a logic of labeled trees, hence it turns out to be a good foundation for query languages for semistructured data, much in the same way as first order logic is a fitting foundation for relational query languages. We have defined and implemented a query language for semistructured data that is based on the ambient logic TQL. The language is quite expressive, and its strong foundations are helpful in the definition of the language semantics and execution model.
• A peer-to-peer distributed system for managing SSD. Peer-to-peer is a term used with different meanings. We use it for a distributed system where every node is both a client and a server, and where nodes can freely join and freely leave the system. This implies that, if some peers perform special administrative task, the system must be able to dynamically substitute them, whenever they just ``go away''. The potential availability of many servers, and the tolerance to any sudden server disconnection, makes this architecture a good foundation for systems which may be extremely robust and scalable.

  The huge popularity of p2p systems is mainly due the diffusion of some file-sharing and file-transfer protocols, which proved that such systems can actually be efficient and robust in face of very high dynamicity. However, such systems are extremely limited in the kind of queries they can support.

  Much of the research on the construction of real p2p databases is currently aimed at the p2p decentralization of data integration mediators. The XPeer system we are designing, instead, addresses application fields where schema integration is not an issue, such as communities where a well-known common schema is exploited, or situations where nobody is going to define a schema mapping anyway. In this context, dynamicity and scalability are the central concerns.

  The architecture of XPeer is based on two innovative concepts: the presence of two distinct subsystems devoted, respectively, to the management of queries and of schema update requests; and the use of cloning in order to distribute load among the administrative peers. By exploiting these key features, the actual workload processing power of our system can scale linearly in the number of peers in the system.