Lecture 22: Client-Server Software Engineering

Today’s Topics Basic Principles of C/S Systems Configurations, Subsystem Linking Object Request Broker (ORB) SE Paradigms for C/S Systems Testing C/S Systems

Client-Server SE “A dominant architecture for information processing” Collective result of many improvements in computing: Network bandwidth Database technology Internet & WWW What are the related SE issues?

Basic Elements Root System Servers Network Clients (individual PCs) Mainframe or DB server Repository for corporate data Servers Update & request corporate data Implement departmental systems Network Connectivity via LAN, WAN, Internet Clients (individual PCs)

Generic Client-Server Architecture [From SEPA 5/e]

Client-Server Options File Servers file request / data response Database Servers SQL request / table response Transaction Servers remote procedure request Groupware Servers client connectivity [From SEPA 5/e]

C/S Software Components “Several distinct subsystems that can be allocated to client or server” “Distributed among machines” multi-tier, networked architecture

Components [2] Classic 3-tier architecture: User interaction/presentation subsystem Application subsystem Database management subsystem Web Browser Server DB Java Servlet Client 1 Server 1 Server 2 Web Example

Middleware Software components that exist on both client & server: Networking (e.g., TCP/IP) Application Connectivity (e.g. HTTP, CORBA) “The nervous system of a client-server system” [Orfali, et al. ’99]

Distributing Functionality Fat Server design most functionality on the server e.g., file server, DB server Fat Client design most functionality on the client e.g., message forwarding system

Distributed Configurations Distributed Presentation client handles display only Remote Presentation client prepares display from data Distributed Logic client handles data entry & update Remote Data Management client processes data (e.g. DSS) Distributed Databases client manages data

Guidelines Presentation on the client Shared DBs on a server Windows/PC-based Offload GUI processing to client Shared DBs on a server DBMS, physical database Static / frequently-used data stored on the client Minimize unnecessary network traffic

Linking C/S Subsystems Pipes (Unix-based) One process can share data with another (across machines, OS) Remote Procedure Call (RPC) One process can invoke another (across machines, OS) Client/Server SQL A process can query / update data from a central RDBMS

Web Linking Hypertext Transfer Protocal (HTTP) Web browser can retrieve data (HTML) from a web server Common Gateway Interface (CGI) Web browser can invoke a process (script, servlet, etc.) on a web server

Object Request Broker Middleware Coordinates message-passing between client and server objects Finds location of server object(s) Invokes method(s) Returns resulting data to client Examples: COM, CORBA, EJB

ORB Client/Server Connectivity [Vogel & Duddy, ’98] Stubs give a client a generic way of requesting services, regardless of how the client is implemented Skeletons give a server a generic way to provide services, regardless of how they are implemented

CORBA Architecture [Orfali & Harkey ’98]

CORBA Architecture [From SEPA 5/e] IDL: Interface Description Language (objects, attributes, & methods) IDL stubs act as a gateway for C/S communication Object implementations are hidden, can vary (Java, C++, etc.)

SE Paradigms for C/S Systems Many are possible, two are common: Evolutionary Paradigm Event-based and/or object-based Component-Based Paradigm COTS & in-house components Or, some hybrid of both…

Analysis Modeling Constraints of the design & implementation can influence analysis phase e.g., qualification step from CBSE e.g., architectural constraints

Design Phase Data & architectural design dominate the process Behavioral modeling important for event-driven systems User interface design a key activity for client side Object-oriented approach is usually the most appropriate

Architectural Design Key criterion: scalability Synchronous vs. asynchronous communication ORB architecture can be used as an adaptive infrastructure for integrating multiple COTS modules

Sharing Data with the Client Manual extract User initiates a static, local copy Snapshot Automatic extraction of local copy Replication Fully-coordinated, real-time duplication Fragmentation Distributed storage of virtual DB

Designing a C/S Business Process [Porter ’95] Identify files created, updated, referenced, deleted Define components or objects For each component, define business rules & information Decompose rules down to methods Define additional components necessary to support the methods

Structure Chart Notation [Porter ‘95] [From SEPA 5/e]

Testing Issues Distributed software is difficult to test! Client GUI Environment / platform diversity Networked databases Distributed processing Non-robust target environment Non-linear performance relationships

Testing Phases Clients tested in isolation Client & server tested in isolation (test or dummy networking) Complete architecture is tested (including network & real-time performance)

Types of Tests Application function tests client functionality, standalone Server tests response time, throughput Database tests accuracy, integrity, archiving Transaction tests correctness, performance Network communication tests connectivity, traffic, security

Operational Profile Common usage patterns Abnormal usage patterns Vary per user, time period, task, etc. Based on user scenarios Abnormal usage patterns Average- vs. worst-case performance Malicious attacks (web systems) Global Expectations maximum acceptable downtime, delay, etc.

Questions?