## 4 Semantic Super-Peer Data Cloud
### Semantics of a TLD  
In an SPD \(\pi_{\alpha}\), actual data are stored in a set of fragmented relational data sources _DS\({}_{\alpha}=\{ds_{1},...,ds_{m}\}\)_ of a database. In an outsourcing database, the sensitive relationships (or properties) of the attributes in the tables are identified and segmented into fragmented data sources to ensure the data protection criteria are satisfied [31]. Using GAV local mappings, we associate a set of local peer _P\({}_{\alpha}=\{\textit{peer}_{1},...,\textit{peer}_{n}\}\)_ in \(\pi_{\alpha}\) with each individual ontology schema to the views of the related relational data sources, _i.e._, SQL queries. Furthermore, through LAV semantic mappings, a set of peers' local ontology schemas are also mapped and aligned into a super-peer global view.  
An SPD \(\pi_{\alpha}\in\Pi\) can be defined as \((P_{\alpha},\textit{SPD}_{\alpha},\textit{GS}_{\alpha},\textit{LS}_{\textit{peer }_{1}},\textit{M}_{\alpha},\textit{DS}_{\alpha})\):  
* An _sp\({}_{\alpha}\in\textit{SPD}_{\alpha}\)_ is the only node in an SPD \(\pi_{\alpha}\), which allows an _agent\({}_{\alpha}\)_ to enforce semantic legal policies. This enforcement empowers _agent\({}_{\alpha}\)_ in an _sp\({}_{\alpha}\)_ to facilitate information collection through a conjunctive query _CQ\({}_{\pi_{\alpha}}(\textit{sp}_{\alpha})\)_ posed to the global schema _GS\({}_{\alpha}\)_. A _CQ\({}_{\pi_{\alpha}}(\textit{sp}_{\alpha})\)_ can be defined as a subset of the Datalog program, _i.e._, a _CQ_ containment problem, for querying the relational database [32].
* Through local LAV mapping assertions, a global schema _GS\({}_{\alpha}\)_ provides an integrated view for a set of peers from _P\({}_{\alpha}\)_ in \(\pi_{\alpha}\). We propose that every LAV assertion has the form \(V_{\textit{LS}_{\textit{peer}_{i}}}\rightsquigarrow CQ_{\pi_{\alpha}}(\textit{ sp}_{\alpha})\), where \(V_{\textit{LS}_{\textit{peer}_{i}}}\) provides the views of the _CQ\({}_{\pi_{\alpha}}(\textit{sp}_{\alpha})\)_ over the global schema _GS\({}_{\alpha}\)_ at an _sp\({}_{\alpha}\)_ for _peer\({}_{i}\)  
Figure 2: The semantics of a super-peer data cloud is described as the policy ontology, which includes two modular concepts (**a**) SPD; (**b**) domain and data policy. An SPD domain includes a super-peer and various peers. A domain policy first decides whether a data request is permitted to enter a TLD. Then, a data policy is used for querying data from the super-peer.  
* A set of peers from \(P_{\alpha}\) are mediators. \(\textit{Peer}_{i}\in P_{\alpha}\) maps its local ontology schema, \(\textit{LS}_{\textit{peer}}\), to a set of fragmented relational data sources, \(\textit{ds}_{i}\), from \(\textit{DS}_{\alpha}\) in \(\pi_{\alpha}\). Therefore, a query uses unfolding GAV mapping assertions \(V_{\textit{LS}_{\textit{peer}_{i}}}\leadsto\textit{CQ}_{\pi_{\alpha}}(\textit{ DS}_{\alpha})\), where \(V_{\textit{LS}_{\textit{peer}_{i}}}\) is the vocabulary of an ontology local schema of \(\textit{peer}_{i}\) that maps to the SQL of \(\textit{CQ}_{\pi_{\alpha}}(\textit{DS}_{\alpha})\) over a set of fragmented data sources, \(\textit{ds}_{i}\), from \(\textit{DS}_{\alpha}\).
* A set of local mapping assertions, \(\textit{M}_{\alpha}\), created from a mapping language, \(\textit{ML}\), are used to semantically link between \(\textit{sp}_{\alpha}\) and a set of peers from \(P_{\alpha}\) in \(\pi_{\alpha}\). The semantics of a set of global mapping assertions created from a Datalog rule language among super-peers are addressed in Section 4.2.
* A set of local data sources, \(\textit{ds}_{i}\), from \(\textit{DS}_{\alpha}\), are fragmented relational structured data that store the materialized instances.