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Application Layer Introduction
Application Architecture - The way in which the functions of the application layer software are spread among the clients & servers on the network. Functions Of Application Layer - Data Storage [Storing of data gener…
Clients & Servers
Clients: Personal Computer, Terminal, Network Computer, Transaction Terminal & Handheld. -> Servers: Mainframe, Personal Computer, Cluster & Virtual Server.
Determined by how functions of application programs are spread among clients & servers - Host-Based Architectures (Server performs almost all functions). Client-Based Architectures (Client performs most functions). Client-Server Architectures (Functions shared between client & server).
Client - Client captures key strokes then sends them to the mainframe. Client displays information according to the server's instructions. -> Server - Presentation Login, Application Logic, Data Access Logic & Data Storage.
Host-Based Architectures Problems
Host becoming a bottleneck - All processing done by the host, which can severely limit network performance. Host upgrades typically expensive and "lumpy" - Available upgrades require large scale and often costly jumps in proces…
Client: Presentation Logic, Application Logic & Data Access Logic. -> Server: Data Storage.
Client-Based Architectures Problems
Data traffic must travel back and forth between server and client - Example: when the client program is making a database query, the ENTIRE database must travel to the client before the query can be processed.
Client: Presentation Logic & Application Logic. -> Server: Application Logic Data Access Logic Data Logic. (Application Logic can be on client and/or server)
Client-Server Architectures Advantages & Disadvantages
Advantages - More efficient because of distributed processing. Allow hardware and software from different vendors to be used together. Enables cloud computing. Disadvantages - Difficulty in getting software from different vendors to work together smoot…
Between Client Application Programs & Server Application Programs. A standard way of translating between software from different vendors. Manages message transfers. Insulates network changes from the clients (e.g., adding a new server). Examples of standards for Middleware:
Involve more than two computers in distributing application program logic - 2-tier architecture (Uses clients and servers in a balance, very popular approach in simple LANs). 3-tier architecture (3 sets of computers involved). N-tier architec…
Client: Presentation Logic. -> Application Server (PC): Application Logic. -> Application Server (PC, minicomputer, or mainframe): Data Access Logic & Data Storage.
Client: Presentation Logic. -> Web Server (PC): Application Logic. -> Application Server (PC): Application Logic. -> Database Server (PC, minicomputer, or mainframe): Data Access Logic & Data Storage.
Multi-tier Architectures Advantages & Disadvantages
Advantages - Better load balancing: More evenly distributed processing. (e.g., application logic distributed between several servers.). More scalable: Only servers experiencing high demand need be upgraded. Disadvantages - Heavily loaded network: More distributed processing necessit…
Thin and Thick Clients
Classification depends on how much of the application logic resides on the client or server. Thin client:
Thin-Client Example: Web Architecture
Client (PC): Presentation Logic. -> Web Server: Application Logic, Data Access Logic & Data Storage.
Peer to Peer Architecture
All computers can serve as a client and a server. Increased popularity in the last decade due to the rise of P2P services such as Napster. Advantages: Data can be stored anywhere on the netw…
Infrastructure Cost -
Criteria for Choosing Architecture
Applications Layer Examples
World Wide Web, E-mail, File Transfer, Videoconferencing & Instant Messaging.
World Wide Web
a collection of electronic files linked together
How The Web Works
Client computer with Web Browser Software sends an HTTP Request through the Internet back & forth between a Server computer with Web Server Software.
HTTP Request Message
REQUIRED: Request line (command, URL, HTTP version number). OPTIONAL: Request header
HTTP Response Message
OPTIONAL: Response status (http version number, status code, reason) & Response header (information on the server, date,
HTML - Hypertext Markup Language
A language used to create Web pages. Also developed at CERN (initially for text files). Tags are embedded in HTML documents -include information on how to format the file. Extensions to HTML needed to fo…