Life Cycle of an MOA2 Object

Introduction: The MOA2 XML Standard.

The MOA2 XML standard is set forth in the MOA2.DTD. It provides a standard means for fully describing the parts and structure of a digital document or entity (such as a digitized book, diary, photoalbum, etc.). Such a description can include descriptive metadata, a file inventory, administrative metadata, and structural metadata. The standard can be applied to unary materials (single photographs, for example), or complex, hierarchically structured materials (such as a book whose pages are organized into chapters). The standard provides for a uniform presentation of digitized materials, a basis for archiving these materials, and for sharing them across institutional boundaries.

MOA2 Object Presentation and Navigation: Architecture Overview.

The architecture that supports the presentation of MOA2 Objects in the Library at UCB has five main Components:

MOA2 XML Objects. These are XML documents that have been encoded according to the MOA2.DTD standard. Each MOA2 XML Object represents a digital entity such as a digitized book, diary, photoalbum, etc.
MOA2 XML Repository. This repository on disk contains the MOA2 XML Objects.

MOA2 Java Objects. Before it can be used, each MOA2 XML Object must be converted to an MOA2 Java Object. The MOA2 Java Object contains the same information as the source MOA2 XML Object from which it derives. But this information is arranged in a format optimized for queries and navigation requests issued by the MOA2 Servlet.

MOA2 Java Object Repository and Repository Manager. The MOA2 Repository Manager is a Java program that receives all of the requests users issue via the MOA2 Servlet to view MOA2 XML objects. The Manager converts each MOA2 XML Object requested for use by the Servlet into a MOA2 Java Object with which the Servlet can then communicate directly. The Repository Manager also stores each MOA2 Java Object it generates in an MOA2 Java Object Repository. This will allow the Repository Manager to bypass the MOA2 XML Object to MOA2 Java Object conversion step the next time the MOA2 XML Object is requested for use. (See MOA2 Object Life Cycle below)

MOA2 Servlet. The MOA2 Servlet is a Java servlet that provides users with the means for activating and navigating MOA2 Objects using a web browser.

MOA2 Object Life Cycle.

The stages of an MOA2 Object's life cycle are listed and described below.

Data pertaining to a digitized document is gathered. Using GenDB, an Access database developed at UC Berkeley, library staff members collect the metadata needed fully to describe a digital document or entity.
MOA2 XML Object is generated. After all of the necessary metadata pertaining to a digital document has been entered into the GenDB database, a Java application called GenX queries the GenDB database and creates an MOA2.DTD compliant .xml source file for the digital document or entity. This is the MOA2 XML Object.
MOA2 XML Object is deposited in the MOA2 XML Repository. After it has been generated, the MOA2 XML Object is deposited in the MOA2 XML Repository. Technical service staff then adds links (in the form of URLs) to Finding Aid(s) and/or Catalog records that point to the MOA2 XML Objects in the MOA2 XML Repository. (These links refer to the MOA2 XML Objects indirectly, however, via the MOA2 Servlet)
MOA2 Java Object created and activated. The steps leading to the activation of an MOA2 Java Object are outlined below.

User asks to view an MOA2 document. User clicks on a link that leads to the MOA2 XML Object via the MOA2 Servlet. The user may encounter this link in a finding aid, an online catalog record, or another source.
Servlet sends object activation request to the Repository Manager. The MOA2 Servlet links to the MOA2 Repository Manager using RMI, and asks for a link to the MOA2 Java Object corresponding to the MOA2 XML Object requested by the user.
MOA2 Repository Manager activates the MOA2 Java Object. The MOA2 Repository Manager looks in the MOA2 Java Object Repository to see if the MOA2 Java Object corresponding to the requested MOA2 XML Object is already available.

MOA2 Java Object is already available. If the MOA2 Java Object corresponding to the requested MOA2 XML Object is already available in the MOA2 Java Object Repository, the Repository Manager simply loads this MOA2 Java Object into memory, and provides the MOA2 Servlet with an RMI connection to it.
MOA2Object is not already available. If the desired MOA2 Java Object is not already available in the MOA2 Java Object Repository, the MOA2 Repository Manager loads the requested MOA2 XML Object from the MOA2 XML Repository. Using an XML parser it then processes this XML Object and uses the parsed file to create an MOA2 Java Object in memory. It then provides the MOA2 Servlet with an RMI connection to this MOA2 Java Object. The Repository Manager also saves a copy of the MOA2 Java Object in the MOA2 Java Object Repository (for possible subsequent use).

Servlet obtains table of contents, initial image and linking info from MOA2 Java Object. Using its RMI connection, the Servlet obtains the information it needs to provide an initial display of the requested MOA2 Object to the user. In particular it obtains:

The document "table of contents", which it displays in a tree structure in the upper left-hand frame of the users browser display;
The URL of the first image to be displayed, which it passes on to the browser for display in the right-hand frame;
The links to associated descriptive and administrative metadata, which it formats for display in the lower left-hand frame.

Servlet uses its RMI connection to the MOA2 Java Object to fulfill users navigation requests. The MOA2 Servlet transmits each of the user's subsequent navigation requests to the MOA2 Java Object via RMI. It obtains and formats the results of these navigation requests for display to the user.
User finishes with MOA2Object. When the user has finished using an MOA2 Java Object, the Servlet severs the RMI connection to this object, and the Repository Manager removes the MOA2 Java Object from memory. The MOA2 Java Object is still available in the MOA2 Java Object Repository on disk, however; and this Java Object can be loaded back into memory the next time a user asks to view it.