A semantic wiki is a wiki that has an underlying model of the knowledge described in its pages. Regular wikis have structured text and untyped hyperlinks (such as the links in this article). Semantic wikis allow the ability to capture or identify further information about the pages' (metadata) and their relations.
Key characteristics Edit
Reliance on Formal Notation Edit
The formal notation may be included in the pages themselves by the users, as in Semantic MediaWiki. Or, it may be derived from the pages or the page names or the means of linking. For instance, using a specific alternative page name might indicate a specific type of link was intended. This is especially common in wikis devoted to code projects. When the formal notation is not written by knowledgeable humans, it should be easy to examine and fix, not least to identify problems in the parsing and conventions.
In either case, providing information through a formal notation allows machines to calculate new facts (e.g. relations between pages) from the facts represented in the knowledge model.
Enables Semantic Web Edit
The technologies developed by the Semantic Web community provide one basis for formal reasoning about the knowledge model that is developed by importing this data. However, there are a vast array of technologies that work on ERD or relational data.
Imagine a semantic wiki devoted solely to foods. The page for an apple would contain, in addition to standard text information, some machine-readable or at least machine-intuitable semantic data. The most basic kind of data would be that an apple is a kind of fruit - what's known as an inheritance relationship. The wiki would thus be able to automatically generate a list of fruits, simply by listing all pages that are tagged as being of type "fruit". Further semantic tags in the "apple" page could indicate other data about apples, including their possible colors and sizes, nutritional information and serving suggestions, and any other data that was considered notable. These tags could be derived from the text but with some chance of error - accordingly they should be presented alongside that data to be easily corrected.
If the wiki exports all this data in RDF or a similar format, it can then be queried in ways a database might - so that an external user or site could, for instance, submit a query to get a list of all fruits that are red and can be baked in a pie.
Use in knowledge management Edit
Where wikis replace older CMS or knowledge management tools, semantic wikis try to serve similar functions: to allow users to make their internal knowledge more explicit and more formal, so that the information in a wiki can be searched in better ways than just with keywords, offering queries similar to structural databases.
Some systems are aimed at personal knowledge management, some more at knowledge management for communities. The amount of formalisation and the way the semantic information is made explicit vary. Existing systems range from primarily content-oriented (like Semantic MediaWiki) where semantics are entered by creating annotated hyperlinks, via approaches mixing content and semantics in plain text (like WikSAR or living ontology), via content-oriented with a strong formal background (like IkeWiki), to systems where the formal knowledge is the primary interest (like Platypus Wiki), where semantics are entered into explicit fields for that purpose.
Other technologies to process typed links between collectively-maintained hypertext pages existed in the pre-web era, and there is no functional difference between some of these and semantic wikis. However, such tools as Project Xanadu, NoteCards, KMS and gIBIS were never used by very large numbers of users with loose coordination on the open Internet. The extensive research on these tools, much of it published by the collaboration software, computer-mediated communication and computer supported cooperative work communities in the 1980s, includes many conventions and principles that are applicable to semantic wikis.
The term Semantic Wiki is first recorded on Usenet, then in several scientific publications:
As the addition of semantic features to large public wiki is relatively new, most of what is known about semantic wikis was actually discovered in this earlier era. Many problems which prevented the deployment of typed links in the original WWW among them.
One of the challenges identified in this research was scale. Although adding link-types to a wiki is straightforward, the number of link types can often be quite large. Again the most useful precedents come from technologies that long predated the World Wide Web.
The Cyc system has over 15,000 different types of links. In order to create the right type of link a set of questions is often used to create the correct link type. Rules can also be added to check that the destination page is appropriate for that link type. For example a link of "capital_of" might only be appropriate when linking a city to a region or a country.
Most early wiki-like technologies, specifically gIBIS and NoteCards, that had direct support for online deliberation, often including argumentation frameworks such as logic trees or more elaborated issue/position/argument structures that were flexible enough to support meeting agendas and support actual online meetings.
In these uses, the link types are deliberately limited to simplify presentation and also to avoid anyone gaining advantage in the debate or meeting by knowing the types better (often thought to be a primary reason why ordinary users strongly resist typed links). For instance, a link to an assertion that "contradicts" another or which "supports" it.
One problem is that users have a natural tendency to undermine or under-represent those positions or arguments that they disagree with, and to question evidence and sources more sharply for those than for those they agree with, and accuse other editors of both insincerity or lack of integrity. See Internet troll for more on this phenomenon as it has carried over into the open web. Semantic wikis have edit wars as well!.
The challenge is to create new adversarial process designs that deal with the power imbalances and rapid pace of change in online forums, and support these with new tools. The open politics theory, for instance, developed some of these for use in politics.
Representing action, as opposed to knowledge, has always been relatively difficult since the philosophy of action is relatively immature and has not produced simple notations. There is no good representation of action that is as widely agreed as those for objects. Relationships like "acts-on", "carries-out" and "obeys" can be defined but there are considerable pressures against making these explicit, as they deal with human social relationships. See also social capital, authority and distrust.
However, some semantic wikis dealing with very limited domains such as code or experiments in a specific scientific field, have managed to produce simple grammars that correctly describe the range of actions that the users want to take in the wiki.
Even in Wikipedia, the limited range of actions editors take when they deal with pages has been codified into a set of tags that carry strong hints or instructions for action.
Rights and restrictions Edit
Another range of problems arises from the larger scale and open nature of wiki. In Re-use rights and their relation to customization, the NeOn project deals with questions about customization or adaptation especially in "those cases when someone wants to re-use work of the other people." Questions include:
- "How much detail can one re-use?"
- "Can the level of detail be customized as a particular view of essentially the same ontology?"
Physical location and GPS data are cited as examples where "satellite companies want to differentiate between e.g. the paying customers and free services. The former may be able to express their location in the terminology of 'streets', 'floors', 'rooms'; whereas the latter would see only more general views – 'city', 'country', or 'unavailable'."
User rights Edit
Another focus is on issues of authentication (‘is the user X who she claims she is’), of encryption ('how to prevent information misuse'), and, to a smaller extent, of authorization (‘can user X do a particular action’). The NeOn paper states that "in practice the vocabulary of rights is much richer (e.g. fine-grained, data-, not action-specific authorization levels)." A "knowledge owner (our GPS user) may differ from the knowledge provider (our GPS satellite), and each of them may want to restrict how third parties experience shared knowledge." Thus different lenses or views may need to be provided for different types of user, a problem that ordinary wiki just doesn't have.
Common features Edit
Semantic wikis vary in their degree of formalization. Semantics may be separated from or included in the wiki markup. End users may be supported when adding this content, perhaps with simple autocompletion, or more complex proposal generation or consistency checks. The representation language may be RDF,OWL or some database directly populated by the tool that withdraws the semantics from the raw data. Separate versioning support or correction editing for the formalized content may also be provided. Provenance support for the formalized content, that is, tagging the triples' authors separately from the informal content, varies.
What things can get formalized also varies. One may be able to type pages, categories or paragraphs or sentences (the latter features were more common in the older systems). Links are usually also typed. The source, property and target may be determined by some defaults, e.g. in Platypus Wiki the source is always the current page.
Reflexivity also varies. More reflexive user interfaces provide strong ontology support from within the wiki, and allow it to be loaded, saved, created and changed.
Degree of support for socio-semantic web features also varies. Some inherit their ontology from a pre-existing strong ontology like Cyc and have no room for users to debate, argue or vary. On the other extreme, living ontology relies wholly on its users, though regulated by a strict ruleset that prohibits certain kinds of interference with each other, or by any administrators.
Category and tag integration Edit
Conventional wikis have ways for users to express metadata typically by tagging and categorizing and using namespaces. Sociosemantic features integrate these with other semantic declarations, typically restricting their use. In living ontology, for instance, categories serve as filters while namespaces provide a point of view.
Categories are thus a subset of tags, but namespaces are orthogonal, because the point of a category is to shape people’s language to a particular taxonomy, but the point of a namespace is to differentiate who or what a (social) faction is speaking or asserting.
Some semantic wikis provide reasoning support using a variety of engines. Some properties may be restricted and not accessible. Limits of reasoning may require that all instance data comply with the ontologies.
Most have simple querying support (search for all triples with a certain subject, predicate, object), but degree of advanced query support varies. User interface support to construct these also varies. Visualization of the links especially may be supported.
Some semantic wikis render pages from formalized content, e.g. NoteCards generated a new page showing all the argumentation links as a new page containing that as a graphic.
See also Edit
- RDF, RDFS, OWL, SPARQL
- OntoWiki is a Semantic Wiki, which aims at hiding as much the complexity of knowledge representation formalisms as possible
- Business Intelligence 2.0 (BI 2.0)
- Wikipedia:Semantic MediaWiki
- Semantic wiki article at the Semantic Web community wiki
- First Workshop on Semantic Wikis
- Semantic Wiki State Of The Art, contains a list of existing Semantic Wiki Prototypes
-  is a semantic bio-informatics wiki with  as a front end.
- Semantic MediaWiki is an extension for MediaWiki
- IkeWiki is a Semantic Wiki
- SweetWiki is a Semantic Wiki