Help:Using SPARQL and RDF stores

By default, Semantic MediaWiki (SMW) stores all data in the same relational database (usually, a MySQL database) that is used by MediaWiki. This ensures a simple setup, but a relational database is not an ideal type of storage for semantic data. A more natural data model for SMW data is RDF, a data format that organizes information in graphs rather than in fixed database tables. Fortunately, it is possible to use RDF-based systems, in conjunction with the standard SQL database, to manage and query SMW's data. This page explains the details.

Pros and cons of using an RDF database
Whether or not to use an RDF store in a specific wiki depends on a number of factors, including the specific RDF database being used. Nonetheless, we can reasonably hope for the following advantages: RDF stores are designed to answer queries in the SPARQL query language. SMW queries can be expressed in this language much more naturally than in the SQL query language of relational databases. In this sense, SMW queries are a mostly typical use case for RDF database systems while they are a rather exotic use case for relational database systems. Moreover, many important optimization methods for relational database queries are useless or misleading for SMW queries. It can therefore be expected that RDF stores should provide superior query performance. RDF stores that support the SPARQL standard also allow other applications to ask SPARQL queries against their data without going via the SMW web frontend. This allows efficient use of wiki data in other applications. Some SPARQL-capable databases further support (parts of) the OWL ontology language and provide according interfaces to query the stored data (e.g. via the OWL Link protocol). Semantic Web applications also use a number of common programming libraries (such as librdf or the OWL API) that can be useful for integrating them with other tools on a lower level. Semantic Web languages such as RDF Schema and OWL provide additional expressive features for modeling, for example by allowing the declaration of derived classes or the declaration of further property characteristics (e.g. transitivity of properties). Some SPARQL-capable databases can evaluate these features for query answering, e.g. for ontology-based data access (OBDA), the method of creating "virtual views" on data by means of semantic modeling constructs. It is possible to store additional data in the RDF database that is updated by SMW. In this way, the RDF store can act as a platform for data integration and ontology re-use. Using a database backend that is not the same as in MediaWiki provides an easy way to distribute tasks across multiple servers. In particular, complex queries can thus be prevented from affecting the basic operation of the wiki, even if they unexpectedly consume a prohibitive amount of computing power, i.e. if they kill the server that hosts the RDF database.
 * Better query performance
 * Additional interfaces
 * Reasoning features and ontology-based data access
 * Data integration and ontology re-use
 * Physical separation of computing resources

Nevertheless, there are a number of possible drawbacks as well: The data is only mirrored in RDF databases, not removed from SQL. Hence additional storage space is required. The setup of RDF backends in SMW is easy, but there is still some effort in running an additional database-management system. There are a number of industry-strength RDF databases available today, some of them free/open source. Yet, the experience of using these systems with SMW is still limited, so some testing is helpful before deciding on a particular backend for a large-scale SMW application.
 * Higher storage requirements
 * Additional maintenance effort
 * Questions regarding performance and stability

Luckily, it is possible to switch back and forth between SQL-based and RDF-based storage backends without major effort, so that the decision can be revisited after trying it for a while.

Deciding on an RDF database
In principle, SMW supports any database that supports the SPARQL query language and SPARUL (SPARQL/Update) as introduced in SPARQL 1.1. In, stores are required to accept updates and queries that do not specify a graph but it is planned to remove this limitation in the future. One list of RDF stores is maintained at w3.org. See also a list of SPARQL triple-store vendors which is maintained by the SWM Project.

Moving data to the new database
After the configuration was changed, there is no data yet in the RDF database. To fill it with the current content of the wiki, it is necessary to refresh all data. See the help page on repairing SMW's data for details. Any method that refreshes the data will work. All SMW queries (inline or semantic search) will be executed against the RDF database, so their results will only be correct when all data has been refreshed.

Known limitations
There are still a few features that are not supported when using query answering via an RDF database:
 * Concept queries: There is no support for concepts in RDF stores yet.