DB Databases M Client/Server architectures G -...

Introduction to databases Client-server architecture

© 2017 Politecnico di Torino 1

DBMG

Databases

Client/Server architectures

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Centralized architecture

An architecture is centralized when data and applications (programs) reside in a single processing node

The intelligence is in the central elaborator (host or mainframe)

processing

node

applications

Centralized

DBs

user

terminals



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Distributed architecture

An architecture is distributed when at least one of these two conditions is verified

Distributed processing: applications, cooperating among themselves, reside on more than one processing node

Distributed database: Data are hosted on more than one processing node

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4

processing node

applications

local DBs

user

terminals

processing node

applications processing

node

applications



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It is a set of processing nodes linked in a determined way

LAN: Local Area Network

WAN: Wide Area Network

Each node is able to communicate with some other nodes

Programs in execution on each node are able to share their information and to ask for the execution of other programs by other nodes

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Different complexity levels

It depends on independence level of nodes

Benefits

Improved performances

Greater availability

Improved reliability



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Important properties

Portability

Capacity to move an application from a system to another one

Guaranteed by SQL standard

Interoperability

Different DBMSs are able to cooperate in the execution of a given work

Interaction protocols are needed

ODBC

X-Open-DTP

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Applications are characterized by the role they perform in the system

Client: when the application uses services that are made available by other applications

Server: when the application provides services used by other applications

Actor: when the application assume both roles of the client and of the server, in different contexts

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The client-server paradigm

Client: service user

Server: service supplier

Client and server have no meaning without a communication protocol that

Defines possible interactions between client and server

Specifies details of each interaction

Defines error conditions and related actions to perform

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Protocols example

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Service Client Protocol

World Wide Web

Mozilla Firefox, Internet Explorer, Google Chrome,

Opera, …

Hypertext Transfer Protocol (HTTP)

File transfer UNIX FTP, MS FTP, Filezilla, browser

Web

File Transfer Protocol (FTP)

Electronic mail Eudora, Outlook,

Thunderbird

Simple Mail Transfer Protocol

(SMTP)



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Client-server architecture

Particular case of a distributed system

The easiest and most diffused architecture

A client sends a request to a server for the execution of a task

A task may consist simply on requesting information, or on executing complex elaborations

The server can be the client of another service at the same time

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2-levels architecture (2-Tier)

Thick client

Contains application logic

DBMS server

Permits data access

DB

CLIENT1 CLIENTn

DBMS SERVER



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3-levels architecture (3-Tier)

Thin client

the browser

Application server

It implements application logic

It is usually a web server

DBMS Server

Permits data access

DBMS SERVER

DB

CLIENT1 CLIENTn

APPLICATION SERVER

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SQL execution

Compile & Go

Query is sent to the server

Query is compiled

Generation of the execution plane

Query is executed

Result is returned

Strong for the execution of not repeated queries

Flexible, good for dynamic SQL execution



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SQL execution

Compile & Store

Query is sent to the server

Query is compiled

Generation of the execution plane

The execution plane is stored for a future use

Query is executed

Result is returned

Efficient for repeated queries

Parametric executions of the same query

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C/S architecture: the World Wide Web

The World Wide Web is a graph of documents

Server keeps a local set of documents and sends one of them to the client (browser Web) on demand

text, picture, video, ...

Some types of script ask server for executing a program (e.g. PHP script)

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The World Wide Web

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Internet

LAN

Client

Web & authentication

server(s)

Transaction server(s)

Database Server(s)

ISP

ISP

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Databases

Web applications



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Web application workflow

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User Client-side interface (browser)

Server-side elaboration

Data source

User defines a query using the query interface

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Data source


Query is sent to the agent on server-side

1 2



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Data source



Agent responds to the query using the data

source

1 2 3

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Data source




source

Data source returns the result of the

query

1 2 3

4



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Data source




source


query

Agent on server-side returns the result

of the query

1 2 3

4 5

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Data source




source


query

Agent on server-side returns the result

of the query

Result of the query is shown to the

user

1 2 3

4 5 6



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Web applications perform more or less the same tasks:

Provide an interface for asking informations (query interface);

e.g.: search in a database, file request, purchase a book, booking a flight, …

Transmit user data to the Web server

Process data on server-side, accessing a database if necessary

Transmit query results to the client

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Static transaction

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Client

Web server Internet

URL http request

HTML files

http response display page

TCP/IP

path

browser server file system

HTML

send



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Static transaction: time axis

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Browser

Web server

Disk access

t0

t1

t2

t3

t4

t6

t7

t8

t9

t5

t’0

t’1

total response time

server response time user think time

network transfer time

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Dynamic transaction

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Client

Web server Internet

URL http request & POST data

http response display page

TCP/IP

com- mand

browser server application

HTML

send

Application

para- meters

logic



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Dynamic transaction: time axis

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Browser

Web server

Application server

t0

t1

t2

t3

t5

t6

t7

t8

t9

t4

total response time

application time

total server time

t’0

t’1

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Transaction on a database

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Client

Web server Internet

URL http

& POST

http display page

TCP/IP

com- mand

browser server application

HTML

send

Application

para- meters

database

data

Database

query



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Transaction on a database: time axis

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Browser

Web server

Application server

Database server

t0

t1

t2

t3

t5

t6

t7

t8

t9

t4

t’0

t’1

total response time

application time

total server time

database time

DB Databases M Client/Server architectures G -...

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