Database System Architecture
2. DATABASE SYSTEM ARCHITECTURE
2.1 INTRODUCTION
(The architecture of a database system is greatly influenced by the underlying
computer system on which it runs, in particular by such aspects of computer
architecture as networking, parallelism, and distribution :)
2.2 Schemas, Sub-schemas, and Instances
 Schema
 The overall design of the database is called the database schema.
 The logical structure of database
 Example: the database consists of information about a set of customers and
accounts and the relationship between them
 Database system has several schemas Physical schema at the lowest level(physical level)
 Logical schema at the intermediate level(logical level)
 Sub-schema
 A database may also have several schemas at the view level, sometimes
called subschema.
 Subschema at the highest level
 Instances
 The collection of information stored in the database as a particular moment
is called an instance of the database.
 Database change over time as information is inserted and deleted.
OR
 The actual content of the database at a particular point in time
 Analogous to the value of a variable
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Database System Architecture
2.3 Three-level ANSI SPARC database Architecture
 External Level
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Users’ view of the database.
The highest level of abstraction describes only part of the entire database.
Describes that part of database that is relevant to a particular user.
Mostly end users interact with this level most frequently.
This level deals with mainly the end display of the database structure along
with records stored here.
 Conceptual Level
 Community view of the database.
 The next higher level of abstraction.
 Describes what data is stored in database and relationships among the data.
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Database System Architecture
 The logical level of abstraction is used by database administrator who must
decide what information is to be kept in the database.
 It is a “level of indirection” between the internal and external level.
 Internal Level
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Physical representation of the database on the computer.
The lowest level of abstraction.
Describes how the data is stored in the database.
Complex low-level data structures are described in detail.
Differences between Three Levels of ANSI-SPARC Architecture
 Objectives of three-tire architecture
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All users should be able to access same data.
A user’s view is immune to changes made in other views.
Users should not need to know physical database storage details.
DBA should be able to change database storage structures without affecting
the users’ views.
 Internal structure of database should be unaffected by changes to physical
aspects of storage.
 DBA should be able to change conceptual structure of database without
affecting all users.
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Database System Architecture
2.4 Data Independence
 Definition:
The ability to modify a schema definition in one level without affecting a
schema definition in the next higher level is called data independence.
 There are two level of data independence.
 Physical data independence
 Logical data independence
 Physical data independence
 Refers to immunity of conceptual schema to changes in the internal
schema.
 Internal schema changes (e.g. using different file organizations, storage
structures/devices).
 Should not require change to conceptual or external schemas.
 It is occasionally when improve performance.
 Logical data independence
 Refers to immunity of external schemas to changes in conceptual
schema.
 Conceptual schema changes (e.g. addition/removal of entities).
 Should not require changes to external schema or rewrites of application
programs.
 It is done whenever logical structure of the database is altered.
2.5 Mappings
 There are the two types of mappings
 Conceptual/Internal Mapping
 External/Conceptual Mapping
 Conceptual/Internal Mapping
 The conceptual/internal mapping defines the correspondence between
the conceptual view and the store database; it specifies how conceptual
record and fields are represented at the internal level.
 If structure of the store database is changed.
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Database System Architecture
 If changed is made to the storage structure definition-then the
conceptual/internal mapping must be changed accordingly, so that the
conceptual schema can remain invariant.
 External/Conceptual Mapping
 The external/conceptual mapping defines the correspondence between a
particular external view and conceptual view.
 The differences that can exist between these two levels are analogous to
those that can exist between the conceptual view and the stored
database.
 Example: fields can have different data types; fields and record name
can be changed; several conceptual fields can be combined into a single
external field.
 Any number of external views can exist at the same time; any number of
users can share a given external view: different external views can
overlap.
2.6 Structure Components, and Functions of DBMS
 Structure of DBMS
DBMS (Database Management System) acts as an interface between the user and
the database. The user requests the DBMS to perform various operations (insert,
delete, update and retrieval) on the database. The components of DBMS perform
these requested operations on the database and provide necessary data to the users.
The various components of DBMS are shown below: -
Fig. 2.1 Structure of DBMS
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Database System Architecture
 DDL Compiler - Data Description Language compiler processes schema
definitions specified in the DDL. It includes metadata information such as the
name of the files, data items, storage details of each file, mapping information
and constraints etc.
 DML Compiler and Query optimizer - The DML commands such as insert,
update, delete, retrieve from the application program are sent to the DML
compiler for compilation into object code for database access. The object code
is then optimized in the best way to execute a query by the query optimizer and
then send to the data manager.
 Data Manager - The Data Manager is the central software component of the
DBMS also knows as Database Control System.
 The Main Functions Of Data Manager Are:–
 Convert operations in user's Queries coming from the application
programs or combination of DML Compiler and Query optimizer which
is known as Query Processor from user's logical view to physical file
system.
 Controls DBMS information access that is stored on disk.
 It also controls handling buffers in main memory.
 It also enforces constraints to maintain consistency and integrity of the
data.
 It also synchronizes the simultaneous operations performed by the
concurrent users.
 It also controls the backup and recovery operations.
 Data Dictionary - Data Dictionary, which stores metadata about the database,
in particular the schema of the database.
 Data - names of the tables, names of attributes of each table, length of
attributes, and number of rows in each table.
 Detailed information on physical database design such as storage structure,
access paths, files and record sizes.
 Usage statistics such as frequency of query and transactions.
 Data dictionary is used to actually control the data integrity, database
operation and accuracy. It may be used as a important part of the DBMS
 Data Files – which store the database itself.
 Compiled DML - The DML complier converts the high level Queries into low
level file access commands known as compiled DML.
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Database System Architecture
 End Users :
 The second class of users then is end user, who interacts with system from
online workstation or terminals.
 Use the interface provided as an integral part of the database system
software.
 Components of DBMS
 Four components of database system.(explain to the CHAPTER -1)
 Function and services of DBMS
 Functions of DBMS
 DBMS free the programmers from the need to worry about the organization
and location of the data i.e. it shields the users from complex hardware
level details.
 DBMS can organize process and present data elements from the database.
This capability enables decision makers to search and query database
contents in order to extract answers that are not available in regular
Reports.
 Programming is speeded up because programmer can concentrate on logic
of the application.
 It includes special user friendly query languages which are easy to
understand by non programming users of the system.
The various common examples of DBMS are Oracle, Access, SQL Server,
Sybase, FoxPro, Dbase etc.
 The service provided by the DBMS includes:
 Authorization services like log on to the DBMS start the database stop the
Database etc.
 Transaction supports like Recovery, Rollback etc,
 Import and Export of Data.
 Maintaining data dictionary
 User's Monitoring
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Database System Architecture
2.7 Data Models
 The structure of the database is called the data models: A Collection of
conceptual tools for describing data, data relationship, data semantic and
consistency constraint. There are three different groups.
 Object-based logical models
 The entity-relationship model
 The object-oriented model
 The semantic data model
 The functional data model
 Record-based logical models
 Relational model
 Network model
 Hierarchical model
 Physical models
 Unifying model
 Frame-memory model
 Record-based logical models
 Record based logical models are used in describing data at the logical
and view levels.
 Record-base models are named as database structure have fixed format
records of several types.
 Each record type define a fixed number of fields or attributes.
 Each attributes and each fields is a usually of a fixed length.
 There are three most widely used record-based models are:
 Relational model
 Network model
 Hierarchical model
 Relational model:
 The relational model uses a collection of tables to represent both
data and the relationships among those data.
 Each table has multiple columns, and each column has unique
name.
 Network model:
 Data in the network model are represented by collections of
record and relationships among data are represented by links,
which can be viewed as pointers.
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Database System Architecture
 The records in the database are organized as collection of
arbitrary graphs
Network model
 Advantages Network model.
 Conceptual Simplicity
 Ease of data access
 Data Integrity and capability to handle more relationship types
 Data independence
 Database standards
 Disadvantages Network model.
 System complexity
 Absence of structural independence
 Hierarchical model:
 In hierarchical model the data and relationships among the data
are represented by records and links.
 It is same as network model but differs in terms of organization
of records as collections of trees rather than graphs.
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Database System Architecture
Hierarchical model
 Advantages Hierarchical model
 Simplicity
 Data Security and Data Integrity
 Efficiency
 Disadvantages Hierarchical model
 Implementation Complexity
 Lack of structural independence
 Programming complexity
 Object-based logical models
 Object-based logical models are used in describing data at the logical
and the view levels. They provide fairly flexible structuring capabilities
and allow data constraints to be specified explicitly. There are many
different models more widely know ones are;
 Entity-relationship model
 Object-oriented model
 Semantic model
 Functional data model
 Physical data model:
 This model is used to describe the data at the lowest level. Two widely
know ones are the unifying model and the frame-memory model.
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Database System Architecture
 Relational model
 The relational model uses a collection of tables to represent both data
and the relationships among those data. Each table has multiple
columns, and each column has a unique name.
 It does not use pointers or links.
 This model relates record by values that they contain.
Customer-ID
C00001
C00002
Customer-name
Smith
Jones
Customer-street
4 north St
3 main St
Customer-city
Rye
Harrison
(a) Customer table
Account-number
A-101
A-215
Balance
500
700
(b) Account table
Customer-ID
C00001
C00002
Account-number
A-101
A-215
(c) Depositor table
 Entity-Relational model
 Models an enterprise as a collection of entities and relationships
 Entity: a “thing” or “object” in the enterprise that is distinguishable
from other objects
E.g. each person is an entity, bank account is an entity.
 Attribute: Described by a set of attributes
E.g. the attributes account-number and balance describes one
particular account
 Relationship: an association among several entities
E.g. Depositor relationship associates a customer with each account
 Entity set and Relationship: The set of all entities of the same type are
called entity set and the set of relationship of the same type are called
the relationship set.
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Database System Architecture
 Represented diagrammatically by an entity-relationship diagram:
(A sample E-R diagram)
 Rectangles, which represent entity sets
 Ellipses, which represent attributes
 Diamonds, which represent relationships among entity sets
 Lines, which link attributes to entity sets and entity sets to relationships
Each component is labelled with the entity or the relationship that it
represents.
 Object- Oriented Data Model
 Object corresponds to an entity in the E-R model
 Object has associated with it
 A set of variables that contain the data for the object; variables
correspond to attributes in the E-R model.
 A set of messages to which the object responds; each message may
have zero, one, or more parameters
 A set methods, each of which is a body of code to implement a
message; a method returns a value as the response to the message.
Class employee {
/* Variables */
string name;
string address;
date start-date;
int salary;
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Database System Architecture
/*Messages*/
int annual-salary();
string get-name();
string get-address();
int set-address(string new-address);
int employment-length();
};
The methods for handling the message are usually defined separately
from the class definition. The methods get-address() and setaddress() would be defined.
string get-address(){
return address;
}
int set-address(string new-address){
address=new-address;
}
2.8 Types of Database System
 Centralized Database Systems
A Centralized Computer System
 Run on a single computer system and do not interact with other computer
systems.
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Database System Architecture
 General-purpose computer system: one to a few CPUs and a number of
device controllers that are connected through a common bus that provide
access to shared memory.
 Single-user system (e.g., personal computer or workstation): desk-top unit,
single user, usually has only one CPU and one or two hard disks; the OS
may support only one user.
 Multi-user system: more disks, more memory, multiple CPUs, and a multiuser OS. Serve a large number of users who are connected to the system vie
terminals. Often called server systems.
 Parallel Database Systems
 Parallel database systems consist of multiple processors and multiple disks
connected by a fast interconnection network.
 A coarse-grain parallel machine consists of a small number of powerful
processors
 A massively parallel or fine grain parallel machine utilizes thousands of
smaller processors.
 Two main performance measures:
 throughput --- the number of tasks that can be completed in a given
time interval
 response time --- the amount of time it takes to complete a single task
from the time it is submitted
 Speedup: a fixed-sized problem executing on a small system is given to a
system which is N-times larger.
 Measured by:
Speedup = small system elapsed time
Large system elapsed time
 Speedup is linear if equation equals N.
Speedup
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Database System Architecture
 Scaleup: increase the size of both the problem and the system
 N-times larger system used to perform N-times larger job
 Measured by:
Scaleup = small system small problem elapsed time
Big system big problem elapsed time
 Scale up is linear if equation equals 1.
Scaleup
 Batch scaleup:
 A single large job; typical of most decision support queries and
scientific simulation.
 Use an N-times larger computer on N-times larger problem.
 Transaction scaleup:
 Numerous small queries submitted by independent users to a shared
database; typical transaction processing and timesharing systems.
 N-times as many users submitting requests (hence, N-times as many
requests) to an N-times larger database, on an N-times larger
computer.
 Well-suited to parallel execution.
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Database System Architecture
 Client-Server Database Systems
 Server systems satisfy requests generated at m client systems, whose
general structure is shown below:
 Database functionality can be divided into:
 Back-end: manages access structures, query evaluation and
optimization, concurrency control and recovery.
 Front-end: consists of tools such as forms, report-writers, and
graphical user interface facilities.
 The interface between the front-end and the back-end is through SQL or
through an application program interface.
 Advantages of replacing mainframes with networks of workstations or
personal computers connected to back-end server machines:
 better functionality for the cost
 flexibility in locating resources and expanding facilities
 better user interfaces
 easier maintenance
 Server systems can be broadly categorized into two kinds:
 transaction servers which are widely used in relational database
systems, and
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Database System Architecture
 data servers, used in object-oriented database systems
 Distributed Database Systems
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Data spread over multiple machines (also referred to as sites or nodes).
Network interconnects the machines
Data shared by users on multiple machines
Homogeneous distributed databases
 Same software/schema on all sites, data may be partitioned among sites
 Goal: provide a view of a single database, hiding details of distribution
Heterogeneous distributed databases
 Different software/schema on different sites
 Goal: integrate existing databases to provide useful functionality
Differentiate between local and global transactions
 A local transaction accesses data in the single site at which the
transaction was initiated.
 A global transaction either accesses data in a site different from the one
at which the transaction was initiated or accesses data in several
different sites.
Sharing data – users at one site able to access the data residing at some
other sites.
Autonomy – each site is able to retain a degree of control over data stored
locally.
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Database System Architecture
 Disadvantage: added complexity required to ensure proper coordination
among sites.
 Software development cost.
 Greater potential for bugs.
 Increased processing overhead.
 Atomicity needed even for transactions that update data at multiple sites
REVIEW QUESTION
1)
2)
3)
4)
Explain Schemas, Sub-schemas, and Instances.
Explain Three-level ANSI SPARC database Architecture.
Explain External Level, Conceptual Level, And Internal Level.
Explain Data Independence OR Difference between Physical data
independence and Logical data independence.
5) Explain to the Mapping OR Explain Conceptual/Internal Mapping and
External/Conceptual Mapping
6) Explain structure of DBMS.
7) Explain components of DBMS.
8) Explain Function and services of DBMS.
9) Explain DATA Model.
10) Explain Object-based logical models.
11) Explain entity-relationship model.
12) Explain object-oriented model.
13) Explain Record-based logical models.
14) Explain Relational model.
15) Explain Network model.
16) Explain Hierarchical model.
17) Explain Physical models
18) Comparison between Data Models
19) Types of Database System any one will be explain?
20) Explain Centralized Database System
21) Explain Parallel Database System
22) Explain Client / Server Database System
23) Explain Distributed Database System
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