What it is:
The description of the system, the databases, transactions, tele-communications network, and of the users are all together in dictionary. The repository:
a) States the characteristics of objects like terminals and databases.
b) Cross-references such objects.
c) Records all natural language (example: German) explanations of the meaning and the utilization of objects.
Whenever the system arrives, the dictionary consists of only a few definitions of transactions (generally utilities), states some distinguished users (that is, operator, database administrator and so on) and defines some special terminals (that is, master console). The system administrator carries on defining new terminals, transactions, users and the databases. (The system administrator function comprises data base administration (or DBA) and data communications network administration (or DCA). As well, the system administrator might modify the existing definitions to match the real system or to reflect modifications. This addition and modification procedure is treated as the editing operation.
For illustration, one defines a latest user by entering the ‘define’ transaction and choosing USER from the menu of the definable kinds. This reasons a form to be exhibited that consists of a field for each and every attribute of the user. Definer fills in this form and proposes it to the dictionary. When the form is wrongly filled out, then this is redisplayed and the definer corrects it. Redefinition obeys an identical pattern; the present form is exhibited, edited and then submitted. (There is as well a non-interactive interface to dictionary for programs instead of people).
All modifications are validated through the dictionary for syntactic and semantic exactness. The capability to set up the exactness of a definition is similar to the capability of a compiler to identify the exactness of a program. That is, lots of semantic errors go unnoticed. Such errors are a noteworthy problem.
Aside from the validating and storing the definitions, dictionary gives a query facility that answers questions like: ‘Which transactions use record type A of files B?’ or, ‘What are the attributes of terminal 34261’.
The dictionary performs one further service, that of compiling the definitions into a "machine readable" form more directly usable by the other system components. For illustration, a terminal definition is transformed from a variable length character string to the fixed format ‘descriptor’ providing the terminal attributes in the non-symbolic form.
Dictionary is a database all along with the set of transactions to manipulate such database. Some of the systems incorporate the dictionary with the data management system and hence the data definition and data manipulation interface are both homogeneous. This consists of the virtue of sharing big bodies of code and of giving a consistent interface to the user. Ingress and System R are illustrations of these systems.
In history, the argument against employing the database for dictionary has been performance. There is much high read traffic on dictionary throughout the normal operation of system. The user logon needs observing the definitions of the user, his terminal, his class and of the session which his logon establishes. The invocation of transaction needs examining his authorization, transaction, and transaction descriptor (that is, to build the transaction). In turn, the transaction definition might reference databases and queues that might in turn reference files, fields and records. The performance of such accesses is vital as they appear in the processing of each and every transaction. Such performance constraints joined with the fact that the access is predominantly read-only have caused most of the systems to special-case the dictionary. The dictionary statements and their compiled descriptors are stored through the data base management component. Dictionary compiled descriptors are stored on the special device and a cache of them is sustained in high-speed storage on an LRU (or Least Recently Used) basis. This method usually employs a coarse granularity of locks and since operations are read merely it keeps no log. The revises to the descriptors are made at times while the system is quiesced.
Descriptors in the dictionary are importunate. Throughout operation, lots of other short-lived descriptors are made for short-lived objects like cursors, processes and the messages. Most of such descriptors are as well kept in the descriptor cache.
The dictionary is a natural expansion of the catalog or file system present in the operating systems. The dictionary just joins more semantics to the objects it stores and more influential operators on such objects.
Readers well-known with the literature might find a striking resemblance between the dictionary and the notion of theoretical schema that is ‘a model of the enterprise’. The dictionary is the theoretical schema devoid of its artificial intelligence features. In time, the dictionary component will develop in the direction recommended by papers on the theoretical schema.
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