Hypertext Abstract Machine (HAM)

One of the first approaches to a generic hypertext implementation model was the Hypertext Abstract Machine (HAM), "a general purpose, transaction-based, multi-user server for a hypertext storage system." ^{[Campbell & Goodman, 1988]} HAM's emphasis was on developing an appropriate storage model. It provided a general and flexible model that could be used in several, different hypertext applications. The Hypertext System Architecture based on HAM contains the following layers (See Figure 1):

• User Interface: A window-based interactive environment for applications to communicate with users.
• Application: The actual application which may or may not run on the same machine as the HAM.
• Hypertext Abstract Machine: An engine which manages all information about the hypertext and communicates with the application through a byte stream protocol.
• Host file system or storage system: A repository to store all the hypertext graphs or databases.

Figure 1: Hypertext Abstract Machine ^{[Campbell & Goodman, 1988]}

The HAM storage model consists of five major objects: graphs (networks of nodes and links containing one or more contexts), contexts (partitions of data within a graph), nodes, links, and attributes carrying semantics. The following operations could be performed on HAM objects: create, delete, destroy, change, get, filter, and special. The HAM architecture provided version control, filtering and data security. The HAM storage model has been successfully tested against systems such as Guide, Intermedia, and NoteCards.