Access layer1-ppt

8/7/2019 Access layer1-ppt

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A DBMS is a set of programs that enables the

creation and maintainence of a collection of relateddata.

A DBMS and associated programaccess,manipulate,protect and manage the data.

WHAT IS DBMS


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A program creates large amount of data throughout

its execution.

Each item of data will have a different lifetime.

OBJECT STORE AND PERSISTENCE


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A database model is a collection of logical constructs

used to represent the data structure and datarelationships with in the database.

DATABASE MODELS


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Hierarchical model:

represents data as a single rooted tree. Network model:

Same but can have more than one parent.

Relational model:

Represents data in the form of tuples.

TYPES


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The interface on database must include data

definition language(DDL),query&DML.

2 approaches for interfaces:

Embed database language in host programinglanguage:

very popular for defining and designingdatabase and its schema.

application programers have to learn and use 2different languages.

DATABASE INTERFACE


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Extend host programing language with database

related constructs

application programmers need to learn theonly new construct of same language ratherthan completely new language


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DML- allows user to access and manipulate data

organization.

SQL-standard DML for relational DBMS. Widely used for query capabilities.

DML AND QUERY CAPABILITIES


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o Logical database organization refers to the

conceptual view of database structure and

relationship within database.

o Ex:OO system represent database composed ofobjects,many multiple db to share info by definingthe same object.

o Physical database refers to how logical component ofdatabase are represented in physical form by OS.

LOGICAL AND PHYSICAL DATABASEORGANIZATION


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Data and objects in database need to be accessed and

shared by different application.

Transaction is a unit of change that occur in whichmany individual modification are aggregated intosingle modification that occurs in it entirely.

Object database support both short and long term

transactions.

Shareability and Transformation


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If multiple application have access to objects

concurrently then conficts over objects access will

arise.

This mechanism is established to control suchconflicts by making policies that indicate how itshould be handled.

CONCURRENCY POLICY


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Portions of database reside on different computers

and disk drives in the network.

Each portion managed by server

Server dispenses information to client applicationand make queries to client applications.

Client reside on nodes of network other than those

which execute

DISTRIBUTED DATABASE


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Logical extension of modular programming Recognizes that all modules need not be executed to

be executed within the same memory space or onsame machine. Calling modules becomes client and called module

becomes server. Another feature is connectivity,that allows

application to communicate with other programs. Key element of connectivity is Network OS(NOS)

called as middleware.

CLIENT-SERVER COMPUTING


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Used in environments which are small with less than to

users.

This result is ineffective system,as server becomeoverwhelmed.

If more than hundreds or thousands of users,we have tomove to a three-tier architecture.

TWO-TIER ARCHITECTURE


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It introduces a server between client and server.

Role of web server

Translation services(adapting application on mainframesystem to client-server environment)

Metering services(acting as transaction monitor to limitthe no of requests to given server).

Intelligent agent services(mapping result to no ofdifferent servers and returning single response to client)

THREE-TIER ARCHITECTURE


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User interface(interact with users,screens,mouse

handling)

Business processing(uses UI data to performbusiness tasks)

Database processing(application code manipulatesdata within application)

CLIENT-SERVER COMPONENTS


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Procesiing that occur on more than one processsor in

order for transaction to be completed.

These processes need not run at the same time.

Ex: processing an order from client,client info mayprocess at one machine & account info on othermachine.

DISTRIBUTED PROCESSING


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Computing that requires two or more distinct

processors to complete single transactions.

Related to both client-server and distributedcomputing.

Programs interact & execute concurrently ondifferent processors.

COOPERATIVE PROCESSING


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Access layer is needed to create set of classes.

These classes must be able to translate the dataretrieved back into appropriate objects.

Main responsibility of access layer is to Provide link between business objects and data storage.

Designing Access layer classes


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Access layer performs 2 major tasks:

Translate the request:access layer must be able to translate any data related request frombusiness layer into appr protocol for access of data.

-Translate the results:

able to translate data retrieved back into appropriate businessobjects and pass those objects back to business layer.


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Provide easy migration to emerging distributed

object technology.

Able to address modest needs of two-tier clientserver architecture &difficult needs of peerdistributed object architectures.

Benefits of Access Layer classes


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If a class interacts with a non human actore eg-

another system,database etc then class should

become automatically become access class.

The Process


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For every business class identified, access class in access

layer package is created. Define relationships

Simplify classes and relationship: main goal is toremove redundant or unnecessary classes.

- Redundant classes- Method classes

iterate and refine

Access layer1-ppt

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