• Architecture: The architecture of a system defines its structure:– the components of the system are identified;– the function of each component is specified;– the interrelationships and interactions among the components are defined.

• Reference Model: A conceptual framework whose purpose is to divide standardizationwork into manageable pieces and to show at a general level how these pieces arerelated to each other.A reference model can be thought of as an idealized architectural model of the system.• A reference model can be described according to 3 different approaches:– component-based– function-based– data-based

Components Architecture ofDDBMSComponent Architecture for a DDBMS Local DBMS (LDBMS) component - It has its ownlocal system catalog that stores informationabout the data held at that site. Data communications (DC) component – is thesoftware that enables all sites to communicate witheach other. Global System Catalog (GSC) - The GSC holdsinformation specific to the distributed nature of thesystem, such as the fragmentation and allocationschemas. Distributed DBMS component - is the controllingunit of the entire system.

Components Architecture of DBMS

• Data-based– Identify the different types of the data and specify the functional units that will realizeand/or use data according to these views– Gives central importance to data (which is also the central resource of any DBMS)!Claimed to be the preferable choice for standardization of DBMS– The full architecture of the system is not clear without the description of functionalmodules.– Example: ANSI/SPARC architecture of DBMS

ANSI/SPARC Architecture of DBMS• ANSI/SPARC architecture is based on data

• 3 views of data: external view, conceptual view, internal view

• Defines a total of 43 interfaces between these views

Conceptual schema: Provides enterprise view of entire database

• Internal schema: Describes the storage details of the relations.

External view: Specifies the view of different users/applications

• Function-based– Classes of users are identified together with the functionality that the system willprovide for each class– Typically a hierarchical system with clearly defined interfaces between different layers– The objectives of the system are clearly identified.

– Not clear how to achieve the objectives– Example: ISO/OSI architecture of computer networks

A reference architecture of DDB ●global schema. ●fragmentation schema. ●allocation schema. ●local mapping schema. Some levels may be missing depend on levels of transparency supported. Reference Architecture for DDB ●global schema: defines all the data which are contained in distribut database. ●fragment: each global relation can be split into several non overlapping portion. ●fragmentation schema: the mapping between global relation and fragment.

●allocation schema: define at which site a fragment located. ●local mapping schema: depends on the type of DBMS .