Information Technology Reference
In-Depth Information
The algorithm of method interpreter is:
Begin
Query Method definition from Method class;
If Method is DML command
Then call DML interpreter
Else begin
Execute Method definition;
If execution error
Then return with error
Else return with result
End;
End;
6.3.2
Frame Model Metadata
The frame model metadata consists of two classes: static classes and active classes.
Static classes represent factual data entities and active classes represent rule enti-
ties. An active class is event driven, obtaining data from the database when invoked
by a certain event. The static class stores data in its own database. The two classes
use the same structure. Combining these two types of objects within the inheritance
hierarchy structure enables the frame model to represent heterogeneous knowledge.
With database gateways translating OSQL to SQL, the frame model metadata sys-
tem runs through OSQL to access heterogeneous RDBs. The frame model captures
the semantics of heterogeneous RDB schemas after schema translation. With the
help of an object agent, the frame model metadata can be processed with an object-
oriented view. The frame model metadata handles complex data such as class, con-
straints, and methods. The object agent precompiles methods and stores methods
as stored procedures, and invokes method and constraint class in the frame model
metadata. The users, the object agent, the frame model metadata, and the RDBMS
form an OODBC. The users issue an OSQL to access a RDB via the OODBC. The
RDBMS linked with an OODBC can interoperate with each other through OSQL.
The frame model metadata is a metadata consisting of four main parts: header, attri-
butes, methods, and constraints (Huang 1994). The detail can be referred to Fig. 8.2
in Chap. 8.
6.3.3
Translate Query Transaction from OSQL to SQL
After schema mapping from OODB to RDB, we can translate OSQL query transac-
tion to SQL query transaction. The technique is to convert the database access path
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