Developing a Data View

Stakeholder and Concerns     Modeling the View      Key Issues     Database Management Systems      Data Dictionary/Directory Systems       Data Administration       Data Security


Stakeholder and Concerns

This view should be developed for database engineers of the system.

Major concerns for this view are understanding how to provide data to the right people and applications with the right interfaces at the right time. This view deals with the architecture of the storage, retrieval, processing, archiving and security of data. It looks at the flow of data as it is stored and processed, and at what components will be required to support and manage both storage and processing. In general these stakeholders are concerned with assuring ubiquitous access to high quality data.

Modeling the View

The general architecture of a "database system" can be modeled with ADML (components, ports, connectors, and roles).

The subject of these are database components or components that provide database services.

The modeling of a "database" is typically done with entity-relationship diagrams and schema definitions, including document type definitions.

Key Issues

Data management services may be provided by a wide range of implementations. Some examples are:

Data management services include the storage, retrieval, manipulation, backup, restart/recovery, security, and associated functions for text, numeric data, and complex data such as documents, graphics, images, audio, and video. The operating system provides file management services, but they are considered here because many legacy databases exist as one or more files without the services provided by a DBMS.

Major components that provide data management services that are discussed in this section are:

These are critical aspects of data management for the following reasons. The DBMS is the most critical component of any data management capability, and a data dictionary/directory system is necessary in conjunction with the DBMS as a tool to aid the administration of the database. Data security is a necessary part of any overall policy for security in information processing.

Database Management Systems

A database management system ( DBMS ) provides for the systematic management of data. This data management component provides services and capabilities for defining the data, structuring the data, accessing the data, as well as security and recovery of the data. A DBMS performs the following functions:

A DBMS must provide:

Database Models

The logical data model that underlies the database characterizes a DBMS. The common logical data models are listed below, and discussed in detail in the subsections that follow.

The Relational Model

A relational DBMS (RDBMS) structures data into tables that have certain properties:

A collection of related tables in the relational model makes up a database. The mathematical theory of relations underlies the relational model - both the organization of data and the languages that manipulate the data. Edgar Codd, then at IBM, developed the relational model in 1973. It has been popular, in terms of commercial use, since the early 1980s.

The Hierarchical Model

The hierarchical data model organizes data in a tree structure. There is a hierarchy of parent and child data segments. This structure implies that a record can have repeating information, generally in the child data segments. For example, an organization might store information about an employee, such as name, employee number, department, salary. The organization might also store information about an employee's children, such as name and date of birth. The employee and children data forms a hierarchy, where the employee data represents the parent segment and the children data represents the child segment. If an employee has three children, then there would be three child segments associated with one employee segment. In a hierarchical database the parent-child relationship is one to many. This restricts a child segment to having only one parent segment. Hierarchical DBMSs were popular from the late 1960s, with the introduction of IBM's Information Management System (IMS) DBMS, through the 1970s.

The Network Model

The popularity of the network data model coincided with the popularity of the hierarchical data model. Some data were more naturally modeled with more than one parent per child. So, the network model permitted the modeling of many-to-many relationships in data. In 1971, the Conference on Data Systems Languages (CODASYL) formally defined the network model. The basic data modeling construct in the network model is the set construct. A set consists of an owner record type, a set name, and a member record type. A member record type can have that role in more than one set, hence the multiparent concept is supported. An owner record type can also be a member or owner in another set. The CODASYL network model is based on mathematical set theory.

The Object-Oriented Model

An object-oriented DBMS (OODBMS) must be both a DBMS and an object-oriented system. As a DBMS it must provide the capabilities identified above. OODBMSs typically can model tabular data, complex data, hierarchical data, and networks of data. The following are important features of an object-oriented system:

Flat Files

A flat file system is usually closely associated with a storage access method. An example is IBM's indexed sequential access method (ISAM). The models discussed earlier in this section are logical data models; flat files require the user to work with the physical layout of the data on a storage device. For example, the user must know the exact location of a data item in a record. In addition, flat files do not provide all of the services of a DBMS, such as naming of data, elimination of redundancy, and concurrency control. Further, there is no independence of the data and the application program. The application program must know the physical layout of the data.

Distributed DBMSs

A distributed DBMS manages a database that is spread over more than one platform. The database can be based on any of the data models discussed above (except the flat file). The database can be replicated, partitioned, or a combination of both. A replicated database is one in which full or partial copies of the database exist on the different platforms. A partitioned database is one in which part of the database is on one platform and parts are on other platforms. The partitioning of a database can be vertical or horizontal. A vertical partitioning puts some fields and the associated data on one platform and some fields and the associated data on another platform. For example, consider a database with the following fields: employee ID, employee name, department, number of dependents, project assigned, salary rate, tax rate. One vertical partitioning might place employee ID, number of dependents, salary rate, and tax rate on one platform and employee name, department, and project assigned on another platform. A horizontal partitioning might keep all the fields on all the platforms but distribute the records. For example, a database with 100,000 records might put the first 50,000 records on one platform and the second 50,000 records on a second platform.

Whether the distributed database is replicated or partitioned, a single DBMS manages the database. There is a single schema (description of the data in a database in terms of a data model, e.g., relational) for a distributed database. The distribution of the database is generally transparent to the user. The term "distributed DBMS" implies homogeneity.

Distributed Heterogeneous DBMSs

A distributed, heterogeneous database system is a set of independent databases, each with its own DBMS, presented to users as a single database and system. "Federated" is used synonymously with "distributed heterogeneous." The heterogeneity refers to differences in data models (e.g., network and relational), DBMSs from different suppliers, different hardware platforms or other differences. The simplest kinds of federated database systems are commonly called gateways. In a gateway, one vendor (e.g., Oracle) provides single-direction access through its DBMS to another database managed by a different vendor's DBMS (e.g., IBM's DB2). The two DBMSs need not share the same data model. For example, many RDBMS vendors provide gateways to hierarchical and network DBMSs.

There are federated database systems both on the market and in research that provide more general access to diverse DBMSs. These systems generally provide a schema integration component to integrate the schemas of the diverse databases and present them to the users as a single database, a query management component to distribute queries to the different DBMSs in the federation, and a transaction management component, to distribute and manage the changes to the various databases in the federation.

Data Dictionary/Directory Systems

The second component providing data management services, the data dictionary/directory system (DD/DS), consists of utilities and systems necessary to catalogue, document, manage, and use metadata (data about data). An example of metadata is the following definition: a 6-character long alphanumeric string, for which the first character is a letter of the alphabet and each of the remaining 5 characters is an integer between 0 and 9; the name for the string is employee ID. The DD/DS utilities make use of special files that contain the database schema. (A schema, using metadata, defines the content and structure of a database.) This schema is represented by a set of tables resulting from the compilation of data definition language (DDL) statements. The DD/DS is normally provided as part of a DBMS but is sometimes available from alternate sources. In the management of distributed data, distribution information may also be maintained in the network directory system. In this case, the interface between the DD/DS and the network directory system would be through the API of the network services component on the platform.

In current environments, data dictionaries are usually integrated with the DBMS, and directory systems are typically limited to a single platform. Network directories are used to expand the DD/DS realms. The relationship between the DD/DS and the network directory is an intricate combination of physical and logical sources of data.

Data Administration

Data administration properly addresses the data architecture, which is outside the scope of TOGAF. We discuss it briefly here because of areas of overlap. It is concerned with all of the data resources of an enterprise, and as such there are overlaps with data management, which addresses data in databases. Two specific areas of overlap are the repository and database administration, which are discussed briefly below.

Repository

A repository is a system that manages all of the data of an enterprise, which includes data and process models and other enterprise information. Hence, the data in a repository is much more extensive than that in a DD/DS, which generally defines only the data making up a database.

Database Administration

Data administration and database administration are complementary processes. Data administration is responsible for data, data structure, and integration of data and processes. Database administration, on the other hand, includes the physical design, development, implementation, security, and maintenance of the physical databases. Database administration is responsible for managing and enforcing the enterprise's policies related to individual databases.

Data Security

The third component providing data management services is data security. This includes procedures and technology measures implemented to prevent unauthorized access, modification, use, and dissemination of data stored or processed by a computer system. Data security also includes data integrity (i.e., preserving the accuracy and validity of the data), and protecting the system from physical harm (including preventative measures and recovery procedures).

Authorization control allows only authorized users to have access to the database at the appropriate level. Guidelines and procedures can be established for accountability, levels of control, and type of control. Authorization control for database systems differs from that in traditional file systems because, in a database system, it is not uncommon for different users to have different rights to the same data. This requirement encompasses the ability to specify subsets of data and to distinguish between groups of users. In addition, decentralized control of authorizations is of particular importance for distributed systems.

Data protection is necessary to prevent unauthorized users from understanding the content of the database. Data encryption, as one of the primary methods for protecting data, is useful for both information stored on disk and for information exchanged on a network.


Copyright The Open Group, 1998, 2000