**Abstract**

The 2000s may be the flourishing time of the topic of ontology. Specialists and scholars concentrated to define ontology effectively and formulated uniform ontology protocol. Ontology language can be classified into SHOE, OML, XOL, OIL, OWL, and RDFs by different protocols and syntaxes. As for effective exchange of the different ontology messages in different applications, US bioinformatic community and researcher develop a XML-based ontology language. With the simplified OKBC-Lite protocol and flexible XML syntax, XOL offers the ways to define an ontology with the human-readable XML, simplified protocol, and compatible interface. In this chapter, we will introduce its motivation from history, orientation in development, semantic usage, and interpreted example in detail.

**Keywords:** ontology exchange language, Ontolingua, XOL, open knowledge base, Semantic Web

### **1. Introduction**

Internet had maintained a rapid development between the 1990s and 2000s, which not only gives birth to various applications, abundant network facilities, and diverse websites but also accelerates the next generation of Semantic Web. After Berners-Lee put forward the imaginary structure of Semantic Web in 1998, W3C with many semantic work teams is dedicated to develop the technical standard of Resource Description Framework [1]. As ontology is the essence and basic of a resource, technical combinations of paradigm and languages are used to define it.

#### **1.1 Background**

Knowledge engineering has become an essential part of expert system in artificial intelligence. It is important to define the specific knowledge, also known as domain database or knowledge base, for multiple applications. However, traditional knowledge base just reveals the key and value of the data, thus paying less attention on ontology.

Ontology is the description and formulization of thing. By full-semantic and expressive ontology, more information and relationship are able to excavate. In order to build more humanistic and intelligent system, scholars had developed different ontology languages. Although many ontology languages give methods to solve the ontology definition. However, a new language or ontology protocol should also be formulated to deal with the cross-application problem.

#### **1.2 Motivation**

Accompanying with the development of Internet, more infrastructure, application, and knowledge base are generated. In normal knowledge supported systems, domain expert will first considerate the software environment and self-knowledge background and then choose the suitable knowledge scheme and ontology for the system. However, when it comes to the cross applications or large knowledgeassisted system, ontologies in system need to be reused. First, knowledge scheme in different systems may exist difference from the expert's personal cognize. Second, it offers several ontology languages for each system; thus, different ontology schemes can show in different formats, which make it hard to communicate in different applications. Third, the increasing demand of openness and the sharing lead of ontology could be exchange. Therefore, an ontology exchangeable protocol or new ontology language supporting to exchange should be redefined.

To realize the need of an evaluation on ontology in bioinformatics, several researchers on the US evaluation team developed a new specific ontology language—XOL [2]. By flexible XML expression and simplified protocol, XOL (xml-based ontology exchange language) is able to express and exchange different ontology information across incompatible applications.

#### **1.3 Definition**

XOL is an ontology language developing for exchange ontology in cross applications. It takes inspiration from OKBC (a protocol used for open knowledge base, see in Ref. [3]) and Ontolingua (another ontology used for reusing and editing ontology, see in Ref. [4]). Its syntax is based on human-readable and high compatible XML document. XOL can also respect as one effective intermedia language in ontologies' use, exchange, negotiation, and cocreation.

#### **1.4 A simple example**

```
Note the following XOL definitions:
<class>
<name> [class-name] </name>
</class>
<slot>
<[slot-attribute]></[slot-attribute]>
</slot>
<individual>
<name></name>
<type></type>
<…></…>
</individual>
```
All of above XOL elements are pertained to all ontologies. Between the pair of <class></class> defines the basic information of this ontology, like the name of the class during the tag pair of <name></name>.

Pair of <slot></slot> will depict the attribute and restriction of the class, like value's type of the attribute and the data restriction.

The last tag <individual> </individual> will give an instance of self-class or multiclass. It is not allowed to use the subclass as the individual element.

With the human-readable and self-defined XML syntax, XOL can express the ontology in a concise way. However, it may also lead to the ontology inconformity

**37**

*Ontology Language XOL Used for Cross-Application Communication*

while using XML syntax merely or personally. A more restrictive and stationary tag

Generally speaking, each ontology language makes up for using syntax and language protocol. To realize the essence of XOL, we will show the different classifi-

According to the use of syntax, we can classify the ontology languages into three

XML format: Extensive Markup Language (XML) is a more human-readable and concise document for storing and defining different data. Ontology document made by XML format can easily locate by its hierarchical structure and semantic DTD tag. RDF format: Resource Description Framework (RDF) is a new way to define ontology after XOL. It is a resource model always accompanied by a specific URI and extended specific XML-like label to depict the relation and knowledge model between the resources. It not only specifically and strictly expresses the data but

According to language protocol of these languages, ontology language can divide into first-order predicate logic language, frame-based language, and concept-role

First-order predicate logic language is the most accurate and original language in knowledge representation. The predicate formula is the formula formed by joining some predicates together with the predicate join symbol, like the largest formalized

Frame-based language is a language that includes the aforehand defining framework and simplified first-order logic language. Owing to excessive strict first-order predict logic and unreadable syntax, Ontolingua and frame logic are developed to

Concept-role restriction language is an effort that most language currently adopts. This type of language offers a hierarchy way to represent the hyponymy by concept and the individual's signal. It reveals the relationship and value restrictions

To note the difference between ontology languages in multiple syntax formats, we will give a detailed introduction for some ontology language with the technical

HTML had covered with a long history before the World Wide Web (WWW) appeared and is one of document standards of Standard Generalized Markup Language (SGML). SGML offers a high standard and complicated description about the document resource. As SGML is hard to learn, use, and realize, researcher put forward the HTML in 1989 after considering the computer's ability. HTML is the mere application of SGML in the WWW times. After few years of great development, HTML is widely known in the web document district. Semantic Web, as the next generation of WWW, is also the use of the HTML syntax in the ontology

language. We called this simple HTML ontology language as SHOE.

HTML format: Hypertext Markup Language (HTML) is the basic document mark of the current web. To extend the semantic character of the HTML, ontology language like SHOE offers an effective way to support semantic annotation by more

cations of ontology languages based on the syntax and semantic rules.

also makes the alternation, merging, and inference possible.

between different ontologies by role mark, like OML [7].

*DOI: http://dx.doi.org/10.5772/intechopen.91385*

OKBC-Lite was chosen soon.

types as follows:

extended webpage label.

restriction language.

remedy this defect.

developing route.

**2.1 SHOE (HTML format)**

language Cyclo [5] and KIF [6].

**2. Why is XOL based on XML?**

while using XML syntax merely or personally. A more restrictive and stationary tag OKBC-Lite was chosen soon.
