Architecture and structure of hypertext
Each unit of information in a hypertext system stored in a node and each node may have a pointer to other nodes. These pointers are called links. A hypertext link connects two nodes, in other words, it points from one node called the anchor node to another node called the destination node.
A link is anchored at a particular location in a departure node while its destination is an entire node. A typical application of this is to have a link anchored at a specific word in a node and whenever a user clicks on this word the link takes the user to another node.
As a hypertext system consists of nodes and links between the nodes a hypertext document can be viewed as a graph. The edges of the graph represent the links between graph nodes. There may be a number of links between two nodes and a particular destination node may have a number of sources or anchors.
The internal architecture of a hypertext systems can be examined in terms of three levels (Campbell & Goodman 1988). These are the Database level the Hypertext Abstract Machine (HAM) level and the User Interface level
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The database level deals with information storage issues and is responsible for ensuring a specified piece of information is made available when requested. At this level links and nodes are data objects with no particular meaning and the database level does not need to know that it is serving a hypertext system. Handling concurrent access, data integrity, backup and security are all issues associated with the database level |
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This level lies between the user interface level and the database level. It defines objects within the hypertext system and maintains relationships between them. These objects can be graphs (networks of nodes and links containing one or more contexts), contexts (partitions of data within a graph), nodes, links and attributes. Also defined are operators on the objects such as get, create and delete. |
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The user interface level deals with the presentation of the information in the HAM. This will cover issues such as which buttons or links should be made available to the user, how they should be displayed, which document, fragment of a document or diagrams should be displayed and so on. |
The WWW maps to the three-level architecture of hypertext in the following way. The database level consists of the Internet and all computers which supply materials to others over the WWW. These computers act as Web servers. All the servers provide their data to the client software in a standardised format called HTML via a standard communication protocol HTTP (HyperText Transfer Protocol). The combination of HTML and HTTP constitutes the hypertext abstract machine (HAM) level. The presentational level is handled by the client viewer or browser running on the users machine.
Authored by Serengul Smith
E-mail to:
serengul1@mdx.ac.uk
School of Computing Science Middlesex University
Revised: September 1998