A N S I S P A R C Architecture

Three Level ANSI-SPARC Architecture

The model provides the basis for understanding some of the functionality of a DBMS.

In 1975 American National Standards Institute (ANSI) Standards Planning and Requirement Committee (SPARC) recognized the need for three level architecture.

Three Level ANSI-SPARC Architecture-I

Three level architecture consist of:

1) External Level

2) Conceptual Level

3) Internal Level

External Level

The users view of the database. This level describes that part of the database that is relevant to each user.

The external level consists of a number of different external views of the database.

Different views may have different representation of same data.

External Level-II

For example, one user may view dates in the form (day, month, year), while another may view dates as (year, month, day).

Some views might include derived or calculated data. (Example DOB Age)

Conceptual Level

The middle level in the three level architecture is the conceptual level.

This level is “community view” of data , includes a description of all available data

The conceptual level provides both the mapping and the desired independence between the external and internal levels.

Conceptual Level (cont…)

The conceptual level represents:

1. All entities, their attributes and their relationships.

2. The constraints on the data.

3. Security and integrity information.

Conceptual Level (cont…)

The conceptual level supports each external view, in that any data available to a user must be contained in or derivable from the conceptual level.

However this level must not contain any storage-dependent details.

External and conceptual layers

Internal Level

The physical representation of the database on the computer. This level describes how the data is stored in the database.

It covers the data structure and file organizations used to store data on storage devices.

Internal Level-I

The internal level is concerned:

•Storage space allocation for data and indexes.•Record descriptions for storage (with stored sizes for data items).•Record placement.•Data compression and data encryption techniques.

Representation of data at different levels of data base Architecture and at the physical level at bottom

Database Schemas

The overall description of the database is called the database schema.

The schema is specified during the database design process and is not expected to change frequently.

There are three different types of schema in the database. These are defined according to three level of architecture.

External Schema

At the highest level we have multiple external schemas (also called subschemas) that correspond to different views of data.

Conceptual Schema

At the conceptual level we have conceptual schema, which describe all the entities, attributes and relationships together with integrity constraints.

Internal SchemaAt lowest level we have internal schema which is the complete description of internal model.

Containing the definition of stored record, the methods of representation, the data fields and indexes or hashing scheme.

There is only one conceptual schema and one internal schema per database.

Mappings

The DBMS is responsible for mapping between these three types of schema.

It must also check schemas for consistency.

Each external schema is derivable from the conceptual schema and it must use the information in the conceptual schema to map between each external schema and the internal schema

Mappings-I

The conceptual schema related to the internal schema through a conceptual/internal mapping.

External schema is related to the conceptual schema by the external/conceptual mapping.

Physical level is managed by OS under direction of DBMS.

Stored Record Interface B/w Int. & Physical Level, provided by OS to DBMS

Database Instance

The data in the database at any particular point in time is called a database instance.

Actual data in database may change frequently.

Therefore many database instances correspond to the permanent structure of DB.

The permanent structure is sometimes called the intension of the database while an instance is called and extension or state of the database.

Three Level Architecture Objectives

•Each user should be able to access the same data but have a different customize view of the data.

•User should not have to deal directly with physical database storage detail.

•The DBA should be able to change the database storage structure without affecting the users views.

Three Level Architecture Objectives

•The internal structure of the database should be unaffected by changes to the physical aspects of storage.

•The DBA should be able to change the conceptual structure of the database without affecting all users.

Data Independence

A major objective for three-level architecture is to provide data independence.

Which means that upper levels are unaffected by changes to lower levels.

Two kinds of data independence:

1)Logical Data Independence

2)Physical Data Independence

Logical Data Independence

Logical data independence refers to the immunity (protection) of the external schemas to changes in the conceptual schema.

Changes to the conceptual schema such as the addition or removal of new entities, attributes or relationships should be possible without having to change existing external schemas or having rewrite the application programs.

Physical Data Independence

Physical data independence refers to the immunity of the conceptual schema to changes in the internal schema.

Changes to the internal schema such as using different file organizations or storage structures, using different storage devices, modifying indexes or hashing algorithms should be possible without having to change the conceptual or external schema.

Database Languages

A data sublanguage consists of two parts:1)Data Definition Language (DDL)2)Data Manipulation Language (DML)

These languages are called data sublanguages because they do not include constructs (conditional or iterative statements) for all computing needs, which are provided by HLL.

Database Languages (Cont…. )

Many DBMS have a facility for embedding the sublanguage in a high level programming language such as COBOL, Fortran, PASCAL, C, C++, Java, Visual Basic.

In this case high level language refer to as the host language.

The Data Definition Language (DDL)

A language that allows the DBA or user to describe and name the:

Entities

Attributes

Relationships

Integrity and Security Constraints.

The Data Manipulation Language (DML)

Data manipulation operations usually include the following:

•Insertion of new data into the database

•Modification of data stored in the database

•Retrieval of data contained in the database

•Deletion of data from the database

DML Types

Two Types of DML:1)Procedural DML2)Non Procedural DMLProcedural language specify how the output of DML statement must be obtained.While the non-procedural DMLs describe only what output to be obtained. Procedural language treat records individually, while non-procedural languages operate on set of records.

Functions of DBMS

1) Data Storage, retrieval and updateA DBMS must furnish users with the ability to store, retrieve and update data in the database.

2) A user accessible catalogA DBMS must furnish a catalog in which descriptions of data items are stored and which is accessible to users.

Functions of DBMS-I

3) Transaction Support

A DBMS must furnish a mechanism which will ensure that all the updates corresponding to a given transaction are made or that none of them is made.

4) Concurrency control services

A DBMS must furnish a mechanism to ensure that the database is update correctly when multiple users are updating the database concurrently.

Functions of DBMS-II

5) Recovery ServicesA DBMS must furnish a mechanism for recovering the database in the event that the database is damaged in any way.

6) Authorization ServicesA DBMS must furnish a mechanism to ensure that only authorized users can access the database.

Functions of DBMS-III

7) Support for data communication

A DBMS must be capable of integrating with communication software.

8) Integrity services

A DBMS must furnish a means to ensure that both the data in the database and changes to the data follow certain rules.

Functions of DBMS-IV

9) Services to support data independence

A DBMS must include facilities to support the independence of programs from the actual structure of the database.

Functions of DBMS-V

10) Utility Services

A DBMS should provide a set of utility services.

•Import and Export facilities

•Monitoring database facilities

•Examine database performance

DBMS Environments Single User Database Environment

The database environment which supports only one user accessing the database at a specific time.

The DBMS might have a number of users but at a certain time only one user can log into the database system and use it.

This type of DBMS systems are also called Desktop Database systems.

DBMS Environments-I

Multi-User Database systems

The DBMS which can support a number of users simultaneously interacting with the database indifferent ways. A number of environments exist for such DBMS.

– Teleprocessing – File Servers – Client-Server

Teleprocessing

This type of Multi user database systems processes the user requests at a central computer, all requests are carried to the central computer where the database is residing, transactions are carried out and the results transported back to the terminals (literally dumb terminals). It has become obsolete now.

File Servers This type of multi-user database environment assumes another approach for sharing of data for different users.

A file server is used to maintain a connection between the users of the database system.

Each client of the network runs its own copy of the DBMS and the database resides on the file server.

Now whenever a user needs data from the file server it makes a request the whole file containing the required data was sent to the client.

At this stage it is important to see that the user has requested one or two records from the database but the server sends a complete file, which might contain hundreds of records.

Now if the client after making the desired operation on the desired data wants to write back the data on the database he will have to send the whole file back to the server, thus causing a lot of network overhead.

The Good thing about this approach is that the server does not have lots of actions to do.

Client-ServerThis type of multi-user environment is the best implementation of the network and DBMS environments.

It has a DBMS server machine which runs the DBMS and to this machine are connected the clients having application programs running for each user.

Once a users wants to perform a certain operation on data in the database it sends its requests to the DBMS through its machine’s application software; the request is forwarded to the DBMS server which performs the required operation on data in the database stored in the computer and then passes back the result to the user intending the result.

This environment is best suited for large enterprises where bulk of data is processed and requests are very much frequent.