The context data model combines features of all the above models. It can be considered as a collection of object-oriented, network and semistructured models or as some kind of object database. In other words this is a flexible model, you can use any type of database structure depending on task. Such data model has been implemented in DBMS ConteXt.
The fundamental unit of information storage of ConteXt is a CLASS. Class contains METHODS and describes OBJECT. The Object contains FIELDS and PROPERTY. The field may be composite, in this case the field contains SubFields etc. The property is a set of fields that belongs to particular Object. (similar to AVL database). In other words, fields are permanent part of Object but Property is its variable part.
The header of Class contains the definition of the internal structure of the Object, which includes the description of each field, such as their type, length, attributes and name. Context data model has a set of predefined types as well as user defined types. The predefined types include not only character strings, texts and digits but also pointers (references) and aggregate types (structures).
Types of Fields
A context model comprises three main data types: REGULAR, VIRTUAL and REFERENCE. A regular (local) field can be ATOMIC or COMPOSITE. The atomic field has no inner structure. In contrast, a composite field may have a complex structure, and its type is described in the header of Class. The composite fields are divided into STATIC and DYNAMIC. The type of a static composite field is stored in the header and is permanent. Description of the type of a dynamic composite field is stored within the Object and can vary from Object to Object.
Like a NETWORK database, apart from the fields containing the information directly, context database has fields storing a place where this information can be found, i.e. POINTER (link, reference) which can point to an Object in this or another Class. Because main addressed unit of context database is an Object, the pointer is made to Object instead of a field of this Object. The pointers are divided on STATIC and DYNAMIC. All pointers that belong to a particular static pointer type point to the same Class (albeit, possibly, to different Object). In this case, the Class name is an integral part of the that pointer type. A dynamic pointer type describes pointers that may refer to different Classes. The Class, which may be linked through a pointer, can reside on the same or any other computer on the local area network. There is no hierarchy between Classes and the pointer can link to any Class, including its own.
In contrast to pure object-oriented databases, context databases is not so coupled to the programming language and doesn't support methods directly. Instead, method invocation is partially supported through the concept of VIRTUAL fields.
A VIRTUAL field is like a regular field: it can be read or written into. However, this field is not physically stored in the database, and in it does not have a type described in the scheme. A read operation on a virtual field is intercepted by the DBMS, which invokes a method associated with the field and the result produced by that method is returned. If no method is defined for the virtual field, the field will be blank. The METHODS is a subroutine written in C++ by an application programmer. Similarly, a write operation on a virtual field invokes an appropriate method, which can changes the value of the field. The current value of virtual fields is maintained by a run-time process; it is not preserved between sessions. In object-oriented terms, virtual fields represent just two public methods: reading and writing. Experience shows, however, that this is often enough in practical applications. From the DBMS point of view, virtual fields provide transparent interface to such methods via an aplication written by application programer.
A context database that does not have composite or pointer fields and property is essentially RELATIONAL. With static composite and pointer fields, context database become OBJECT-ORIENTED. If the context database has only Property in this case it is an ENTITY-ATTRIBUTE-VALUE database. With dynamic composite fields, a context database becomes what is now known as a SEMISTRUCTURED database. If the database has all available types... in this case it is ConteXt database!
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Ruchika Mandore [ MCA ]
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