System Architecture -- Centralized architecture

Lecture 2 -- part 2

Client-server applications

• Clients
• Interacts with users through a user interface
• Performs application functions
• Interacts with client middleware using middleware API
• Receives response and display it if needed
• Servers
• Implement services
• Invoked by server middleware
• Provide error-recovery and failure handling service

 Overview

• Common communication patterns in distributed applications
• Client-Server
• Group (multicast)
• Function-shipping/Applets
• Client
• Process that requests services
• Server
• Process that provides services
• Details
• Client usually blocks until server responds
• Client usually invoked by end users when hey require services
• Server usually waits for incoming requests
• Server can have many clients making concurrent requests
• Server is usually a program with special privileges

Application Software Architectures

• Many applications can be considered to be made up of three software components or logical tiers

• user interface
• processing layer
• data layer

• Client/server architecture

• Single-physical tiered, two physical tiered
• multi-tiered

• Distributed Data
• e.g., distributed database
• Remote data
• e.g., network file system 
• Distributed programs
• e.g., world wide web
• Remote presentation
• e.g., telnet
• Distributed presentation
• e.g., X Windows

• Motivation for multi-tier architectures

• Frees clients from dependencies on the exact implementation of database
• It allows business logic to be concentrated in one place
• software updates are restricted to middle layer
• Performance improvements possible by batching requests from many clients to database
• Database and business logic tiers could be implemented by multiple servers for scalability