Sesame Architecture

A schematic representation of Sesame's architecture is shown in Figure 1 below. Following is a brief overview of the main components.

Figure 1 - Sesame Architecture

The Sesame framework is as a loosely coupled set of components, where alternative implementations can be exchanged easily. Sesame comes with a variety of Storage And Inference Layer (SAIL) implementations that a user can select for the desired behavior (in memory storage, file-system, relational database, etc). OWLIM is a plug-in SAIL component for the Sesame framework.

Applications will normally communicate with Sesame through the Repository API. This provides a high enough level of abstraction so that the details of particular underlying components remain hidden, i.e. different components can be swapped in without requiring modification of the application.

The Repository API has several implementations, one of which uses HTTP to communicate with a remote repository that exposes the Repository API via HTTP.

The SAIL API and OWLIM

The SAIL API is a set of Java interfaces that support the storage and retrieval of RDF statements. The main characteristics of the SAIL API are:

• It is the basic interface for storing/retrieving/deleting RDF data;
• It is used to abstract from the actual storage mechanism, e.g. an implementation can use relational databases, file systems, in-memory storage, etc;
• It is flexible enough to support small and simple implementations, but also offers sufficient freedom for optimizations that huge amounts of data can be handled efficiently on enterprise-level machines;
• It is extendable to other RDF-based languages;
• It supports stacking of SAILs, where the SAIL at the top can perform some action when the modules make calls to it, and then forward these calls to the SAIL beneath it. This process continues until one of the SAILs finally handles the actual retrieval request, propagating the result back up again;
• It handles concurrency and takes care of read and write locks on repositories. This setup allows for supporting concurrency control for any type of repository.

Other proposals for RDF APIs are currently under development. The most prominent of these are the Jena toolkit and the Redland Application Framework. The SAIL shares many characteristics with both approaches, however an important difference between these two proposals and SAIL, is that the SAIL API specifically deals with RDFS on the retrieval side: it offers methods for querying class and property subsumption, and domain and range restrictions. In contrast, both Jena and Redland focus exclusively on the RDF triple set, leaving interpretation of these triples to the user application. In SAIL, these RDFS inferencing tasks are handled internally. The main reason for this is that there is a strong relationship between the efficiency of inference and the actual storage model being used. Since any particular SAIL implementation has a complete understanding of the storage model (e.g. the database schema in the case of an RDBMS), this knowledge can be exploited to infer, for example, class subsumption more efficiently.

Another difference between SAIL and other RDF APIs is that SAIL is considerably more lightweight: only four basic interfaces are provided, offering basic storage and retrieval functionality and transaction support. This minimal set of interfaces promotes flexibility and looser coupling between components.

The current Sesame framework offers several implementations of the SAIL API. The most important of these is the SQL92SAIL, which is a generic implementation for SQL92, ISO99 [1]. This allows for connecting to any RDBMS without having to re-implement a lot of code. In the SQL92SAIL, only the definitions of the data-types (which are not part of the SQL92 standard) have to be changed when switching to a different database platform. The SQL92SAIL features an inferencing module for RDFS, based on the RDFS entailment rules as specified in the RDF Model Theory [2]. This inferencing module computes the closure of the data schema and asserts these implications as derived statements. For example, whenever a statement of the form (foo, rdfs:domain, bar) is encountered, the inferencing module asserts that (foo, rdf:type, property) is an implied statement. The SQL92SAIL has been tested in use with several DBMSs, including PostgreSQL8 and MySQL9 [3].

OWLIM is a high-performance semantic repository, implemented in Java and packaged as a Storage and Inference Layer (SAIL) for the Sesame RDF database. OWLIM is based on Ontotexts Triple Reasoning and Rule Entailment Engine (TRREE) - a native RDF rule-entailment engine. The supported semantics can be configured through the definition of rule-sets. The most expressive pre-defined rule-set combines unconstrained RDFS and OWL-Lite. Custom rule-sets allow tuning for optimal performance and expressivity. OWLIM supports RDFS, OWL DLP, OWL Horst, most of OWL Lite and OWL2 RL.

The two editions of OWLIM are SwiftOWLIM and BigOWLIM. In SwiftOWLIM, reasoning and query evaluation are performed in-memory, while, at the same time, a reliable persistence strategy assures data preservation, consistency, and integrity. BigOWLIM is the high-performance "enterprise" edition that scales to massive quantities of data. Typically, SwiftOWLIM can manage millions of explicit statements on desktop hardware, whereas BigOWLIM can manage billions of statements and multiple simultaneous user sessions.

The KRI in MOLTO uses BigOWLIM Version 3.3.