Week 2

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Week 2

• Application Layers

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Learning objectives

• Application architecture
• Overview
• Best practices
• Illustration The case of the WWW
• Architecture
• Messages, protocol and other components
• Application exercises

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Course overview
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Application architectures

• Application programs carry out four functions
• Data storage Storage and retrieval of data
• Data access logic Query required to access a
  subset of data
• Application logic Work performed by application
• Presentation logic User interface
• Three basic application architectures determine
  how these functions are split between clients and
  servers
• Host-based architecture
• Client-based architecture
• Client-server architecture
• Application architecture
• Describes how application layer functions are
  spread among computers to deliver service to
  users
• Not the same as network architecture which refers
  to a network physical and logical topology
  (hardware layer)

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Host-based architecture

• Very first application architecture to be
  developed
• The host, usually a mainframe, performs all 4
  application program functions
• The client, usually a terminal or a PC running a
  terminal emulation program sends and receives
  messages to and from host (See figure 2.1)
• Limitations
• Overload on host side leads to increased response
  time
• Upgrades come in large increments and are
  expensive

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Host-based architecture
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Client-based architecture

• AKA peer-to-peer architecture
• Became important in the late 1980s with the
  widespread use of PCs and LANs
• Server is a microcomputer responsible for data
  storage
• Client is a microcomputer responsible for the
  data access, application and presentation logic
  (See figure 2-2)
• Limitations
• No access logic on server side All data must
  travel to client for processing since queries are
  performed by client
• Overload the network circuits ? poor network
  performance

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Client-based architecture
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Client/Server architecture

• Most common application architecture used in
  todays networks
• Server handles data storage and data access logic
• Client takes care of the presentation logic
• Application logic may reside on the client,
  server or be split up between the two (see next
  four slides)
• Benefits
• Network performance processing is distributed
  between client and server
• Lower required capacity (less traffic)
• Middleware software allows softwares from
  different vendors to be used together (softwares
  that conform to the same standard can
  "communicate" with each other without requiring
  complex coding)
• Comes in different versions See next slides

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Two-tier, three-tier and N-tier architectures

• ?Refers to ways the application logic is
  partitioned in C/S architecture
• 2-tier architecture (see Figure 2-3)
• 1 client, 1 server, application logic is
  partitioned between the 2
• Most common architecture
• 3-tier architecture (see Figure 2-4)
• The client handles the presentation logic
• The application server handles the application
  logic
• The database server handles the data storage and
  data access logic
• N-tier architecture (see Figure 2-5)
• More than three sets of computers are used
• Benefits Easy access (if a web server is added)
  and load balancing
• Major limitation complexity

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Client/Server architecture(2-tier C/S
architecture)
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Three-tier architecture
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N-tier architecture
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Thin versus thick clients

• Refers to how much of the application logic
  resides on the client
• Little or no application logic resides on a thin
  client
• All or most of the application logic resides on a
  fat client
• Thin clients are becoming popular because
• Systems are easier to manage since only the
  server application logic generally needs to be
  updated
• Thin clients are cheaper

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More on clients and servers

• http//www.serverwatch.com/
• Technical focus 2.1 p.34
• Assignment1

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Best practice architecture

• Often, the application architecture is given. So
  no decision has to be made
• When it is not given, 3 criteria help select an
  architecture
• Cost of Infrastructure mainframes are expensive,
  which is why client-based and C/S architectures
  have become popular
• Scalability refers to the ability to increase (or
  decrease) computing capacity as network demand
  changes. C/S architectures are the most scalable
  (both clients and servers can be added (reduced)
  to fit network requirements)
• Cost of Development Lower for any architecture
  that uses off-the-shelf software
• ? For most projects, a C/S architecture, usually
  a three-tier thin or thick client, is
  recommended p.41

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Application exerciseMore on C/S architecture

• Rate the C/S architecture using the criteria
  below
• Centralized data storage
• Prevention of unauthorized access to
  file/applications
• Reliability (built-in reliability)
• Network administration
• Performance (processing)
• Number of users
• Relative (total) cost

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Illustration The case of the WWW
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How the WWW works

• Based on a C/S architecture
• Uses the Hypertext Transfer Protocol (HTTP)
• Web clients (browsers) make HTTP requests to Web
  servers. Web servers answer with HTTP responses.
• Clicking on a hyperlink or typing a URL into a
  browser starts an HTTP request-response cycle
• An HTTP request-response cycle can have multiple
  steps
• Case for web pages that contain embedded files,
  each requiring a separate response

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How the WWW works (Figure 2-8)
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HTTP request

• Packet produced by application layer of client
• Has 3 parts
• Request line (required) includes command, URL
  and the http version number
• Request header (optional) includes information
  on the browser, date, and the referring page
• Request body (optional) includes information
  user send to the server

Command URL HTTP version
GET http//www.kelley.indiana.edu/ardennis/home.ht
m HTTP/1.1 Date Mon 06 Aug 2001 173546
GMT User-Agent Mozilla/6.0 Referer
http//www.indiana.edu/aisdept/faculty.htm
Request line
Request Header
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HTTP response

• Packet produced by application layer of server
• Has 3 parts
• Response status (optional) http version number,
  status code (e.g., 200 ok, 404 page not
  found) and reason phrase (description of status
  code)
• Response header (optional) contains information
  on the server, date, exact URL of the retrieved
  page and format used for the body of the response
  (e.g., html)
• Response body (required) contains the requested
  Web page
• See next slide

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HTTP response from a web server Figure 2-10
HTTP ver. Status code Reason
HTTP/1.1 200 OK
Response Status Date Mon 06
Aug 2001 173546 GMT - Date Server NCSA/1.3 -
Web server Location http// www.kelley.indiana.ed
u/adennis/home.htm - URL Content-type
text/html- Type of file lthtmlgt ltheadgt lttitlegtAll
en R. Dennislt/titlegt lt/headgt ltbodygt ltH2gt Allen R.
Dennis lt/H2gt ltPgtWelcome to the home page of Allen
R. Dennislt/Pgt lt/bodygt lt/htmlgt
Response Header
Response Body
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Application exercises

• 1) Read minicase 2 p.60 61
• Which C/S architecture would you suggest? Why?
• 2) Which applications are supported by the
  following protocols
• SMTP
• FTP
• Telnet

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Thank you