Annotea: an open RDF infrastructure for shared Web annotations

TL;DR: The paper presents the overall design of Annotea and describes some of the issues the project has faced and how it has solved them, including combining RDF with XPointer, XLink, and HTTP.

About: This article is published in Computer Networks.The article was published on 2002-08-05 and is currently open access. It has received 565 citations till now. The article focuses on the topics: Annotea & RDF/XML.

Summary

1. INTRODUCTION

• One of the basic milestones in the road to a Semantic Web [22] is the association of metadata to content.
• Metadata allows the Web to describe properties about some given content, even if the medium of this content does not directly provide the necessary means to do so.
• An interesting side e ect, is that a same piece of metadata can be used not only for describing content, but also to organize and classify it, thus setting up other properties the authors had not thought about at rst.
• One can use it to search for photos of a given location taken at a given time.the authors.
• In order to reach this goal, the authors have been W3C Fellow from Elisa Communications .

2. DESIGN

• In this section the authors describe the architecture of the Annotea system and the RDF annotation schema.
• The authors start with a discussion of the requirements that motivate some of the aspects of their design.

2.1 Requirements

• Since the early design of Annotea, the authors decided to build an infrastructure that was based on generic RDF, with annotations being one possible instantiation of the infrastructure.
• This choice has allowed us to concentrate more on the infrastructure than on the application itself.
• At the same time that an annotation can be seen as metadata related to an annotated document, annotations themselves can have distinct properties.
• Di erent users have di erent views and needs.
• Annotations are stored in generic RDF databases.

2.2 Annotea and its operation

• In Annotea, annotations are described with a dedicated RDF schema and are stored in annotation servers (Fig. 1).
• Section 2.3 describes the annotation schema further in detail.
• The user then publishes the annotation to a given annotation server.
• For each list of annotations that it receives, the browser parses the metadata of each annotation, resolves the XPointer of the annotation and, if successful, highlights the annotated text.
• The motivation for this choice is to reduce the amount of data that is being sent back to the browser.

2.3 RDF Schema for Annotations

• The most important feature of an annotation is that it supports the evolving needs of the collaborating groups.
• In its most simple level, RDF provides (resource, property, value) triples (Fig. 3).
• By Annotation A super class describing the common features of annotations.
• Pointing to ner details after the ID can be done by other XPointer means, such as using text matching.
• Other metadata can be added to the annotation when the working group needs that.

3. ANNOTATIONS IN AMAYA

• Since the beginning of the project, the authors have been implementing both a client and a server prototype.
• Amaya [1] is a full-featured web browser and editor developed by W3C for experimenting and validating web speci - cations at an early stage of their development.
• The relation between an annotation resource and the resource to which the annotation applies.
• It is also possible to specify additional annotation types as an RDF schema that can be can be downloaded at runtime.
• Amaya will use the namespace name to try to retrieve an RDF schema from the Web or the schema content can be cached in a local le and speci ed with the same startup con guration.

3.1 Creating an annotation

• The user has three choices for creating an annotation: annotate a whole document, annotate the position where the caret is, annotate the current selection.
• The annotation window shows the metadata of the annotation, as de ned in Section 2.3, inside a box and the body of the annotation.
• If the user clicks on the Source document eld, Amaya will scroll to the annotated text and highlight it if it is a selection.
• Users can cut and paste fragments from other documents, add links to other documents, and so on.
• When a user creates an annotation, it is con- sidered a local one and will be stored in the user's Amaya directory.

3.2 Browsing annotations

• By means of a setup menu, the user can specify the URIs of the annotation servers he wants to query, as well as the local annotation repository.
• Moreover, if the user does not open all the annotations, the authors save time by not downloading the body.
• When the user clicks once on the A-element, Amaya highlights the target of the annotation.
• Instead, the authors place it as the the beginning of the Math expression.
• The user may then open the orphan annotation and reposition its XPointer or delete it.

3.3 Filtering annotations

• For a heavily annotated document, seeing the A-element icon can make reading the document bothersome.
• This lter menu does not have any e ect on the Links view.
• As an alternative to hiding annotations, the user can also temporarily disable some annotation servers using the conguration menu.
• The Algae language is derived from Algernon [4].
• This customized query interface makes it possible to start ltering the annotations on the server side, for example, by only requesting those done in the past week by a given author and belonging to a given annotation type.

4. RELATED WORK

• This section discusses some previous annotation approaches.
• In a proxy-based approach, annotations are stored and merged with a Web document by a proxy server.
• This approach works with any existing browser.
• Unfortunately, users cannot host their own servers.
• Using XLink provides the means to easily present the annotations in prede ned ways in any browser implementing XLink.

5. CONCLUSIONS AND FUTURE PLANS

• Being able to associate metadata with Web resources is an important milestone for building a Semantic Web.
• Annotea provides a simple infrastructure for associating annotations with Web documents, without having to modify these documents.
• Users can extend it by de ning their own annotation types or by adding other annotation properties.
• All the source code is freely available too.
• If a user edits an annotated document, in some cases, the XPointer of an annotation may point to the wrong place and thus become a misleading annotation.

Figures

Figure 5: The RDF model of an annotation.

Figure 1: The basic architecture of Annotea.

Figure 7: Annotating a paragraph with Amaya.

Figure 2: A basic annotation model with an author making a statement about a document.

Figure 3: RDF triple model.

Figure 8: The Links View showing an orphan annotation and a normal one.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Annotea: an open rdf infrastructure for shared web annotations" ?

One of the goals of this project has been to re-use as much existing W3C technology as possible. The paper presents the overall design of Annotea and describes some of the issues the authors have faced and how they have solved them. 

Q2. What are the future works in "Annotea: an open rdf infrastructure for shared web annotations" ?

Their wish list for future work on Annotea includes: Shared bookmarks. The authors would like to experiment with other ways for displaying annotations. The authors plan to expand the schema so that the author is de ned by another RDF schema and use this property in the Annotation schema. It will then be easy to search for the metadata of the author and, for example, substitute the pencil icon with the photo of the author. 

Q3. How does the browser query annotation metadata?

The browser queries each of the annotation servers, requesting via an HTTP GET method the annotation metadata that is associated with the document's URI. 

Q4. What is the purpose of the annotation download?

Once a document is downloaded, the annotations metadata is downloaded asynchronously, just like images, and merged into the document. 

Q5. What happens when the user decides to post it to an annotation server?

When the user decides to post it to an annotation server, the local annotation will be deleted and subsequent saves will be sent to the server. 

Q6. What is the way to store annotations?

It codes the annotations into an extended URI format and uses local les similar to bookmark les to store and retrieve the annotations. 

Q7. How can the authors prevent unnecessary loss of pointers?

To prevent unnecessary loss of pointers the authors can search for the nearest ID to a parent of the object use it as the starting point for the XPointer path. 

Q8. What is the way to present annotations on a Web page?

An interesting possibility for presenting the annotations on a Web page is to use internal DOM [14] events without actually changing the mark-up of the page. 

Q9. How can the user disable annotation servers?

As an alternative to hiding annotations, the user can also temporarily disable some annotation servers using the conguration menu. 

Q10. How can the authors add new properties to annotation classes?

The authors can also easily add new properties to the annotation classes, for instance, the authors could add a property that de nes an annotation set. 

Q11. What is the way to prevent misleading annotations?

A better XPointer expression would be one that is more tolerant of document changes, but robust enough to prevent misleading annotations.