IE 20303

What is Client/Server?

• Client• Server• Network• How is client/server different from other distributed

computing?– Heavy reliance on user-friendly applications– Emphasis on centralizing databases and management

functions– Commitment to openness/modularity– Networking fundamental to operation

Client/Server Terminology• Application Programming Interface (API): A set of function and call

programs that allow clients and servers to intercommunicate.• Client: A networked information requester (usually a PC or

workstation) that can query a database and/or other information from a server.

• Middleware: A set of drivers, APIs, or other software that improves connectivity between a client application and a server.

• Relational Database: A database in which information access is limited to the selection of rows that satisfy all search criteria.

• Server: A computer, usually a high-powered workstation or a mainframe, that houses information for manipulation by networked clients.

• Structured Query Language (SQL): A language developed by IBM and standardized by ANSI for addressing, creating, updating, or querying relational databases.

Client-Server Environment

Why is Client-Server Different?

• Emphasis on user-friendly client applications• Focus on access to centralized databases• Commitment to open and modular

applications• Networking is fundamental to the organization

Client/Server Applications

• Emphasis on GUI for users• Database Example– Database on server, applications for access on

client, “glue” (like SQL) enables requests)– Application logic can be client-only, or split

between client and server

Classes of Client-Server Applications

• Host-based processing• Server-based processing• Client-based processing• Cooperative processing• “Thick client” vs “Thin server”

3-Tier Client/Server Architecture

Middleware

• Standardized interfaces and protocols between clients and back-end databases

• Hides complexity of data sources from the end-user

• Compatible with a range of client and server options

• All applications operate over a uniform applications programming interface (API).

Middleware Architecture

Logical View of Middleware

Basic Message Passing Primitives

Message Passing Issues

• Reliability vs Unreliability– Reliable facilities guarantee delivery, provide error-

checking, retransmission, etc– Alternatively, the message can be sent without

success/failure; reduces complexity and overhead, passes responsibility for confirmation to application

• Blocking vs Nonblocking– Non-blocking more efficient, but difficult to test and debug

programs– Blocking (synchronous) retains control until

acknowledgment is received

Remote Procedure Call Mechanism

Client/Server Binding

• Nonpersistent binding– Does not maintain state information, connections

re-established as necessary– Inappropriate for RPCs used frequently by same

caller• Persistent binding– Connection sustained until procedure return– Useful for applications making repeated calls to

remote procedures

Object-Oriented Mechanisms

• Clients and servers ship messages between objects.• May rely on an underlying message or RPC structure

or be developed directly on top of object-oriented capabilities in the operating system

• Success depends on standardization of the object mechanism, but competing models exist– COM, OLE, CORBA

Intranets

• Implementation of Internet-based technologies within an organization, rather than for global connectivity

• Immensely successful in corporate computing contexts

Advantages of Intranets

• Rapid prototyping• Scales effectively• Little training

required• Can be

implemented on variety of systems

• Open architecture allows interaction across platforms

• Supports a range of distributed servers

• Allows integration of legacy systems on client and server side

• Supports a range of media types

• Inexpensive to implement

The Intranet Web

• Web Content– The web can be used to effectively distribute content in a

way that requires no new training for end-users

• Web/Database Connectivity– Multiple tools exist to serve as middleware between web

servers and data sources

• Electronic Mail• Network News

Corporate Intranet Example

Web/Database Connectivity

• Advantages– Ease of

administration– Deployment– Development

speed– Flexible

information presentation

• Disadvantages– Limited

functionality– Stateless operation

makes tracking difficult

Intranet Webs vs Traditional Client/Server

• Client/Server Disadvantages Include:1. Long development cycles2. Difficulty in partitioning applications, and modifying based on user feedback3. Effort in distributing upgrades to clients4. Difficult in scaling servers to respond to increased load5. Continuous requirement for more powerful desktop machines

Other Intranet Technologies

• Electronic Mail– Closed internal mail systems (delivery verification,

etc)– Internal mailing lists

• Network news (USENET)– Can be adopted for internal intranet uses

Extranets

• Extends the intranet concept to provide information and services to selected outside populations, such as customers and suppliers

• Enables the sharing of information between companies

• A TCP/IP enabled form of EDI

Advantages of Extranets

• Reduced costs• Coordination• Customer Satisfaction• Expedited communication

Methods for Converting Intranets to Extranets

• Long-distance dial-up access• Internet access to intranet with security• Internet access to an external server that duplicates

some of a company’s intranet data• Internet access to an external server that originates

database queries to internal servers• Virtual private network

Service Oriented Architecture

• Client/server architecture utilized widely by enterprise systems

• Business functions consist of modular structures

SOA Architectural Elements

• Service provider: network node that provides a service interface for a software asset that manages a specific set of tasks.

• Service requestor: network node that discovers and invokes other software services.

• Service broker: specific kind of service provider that acts as a registry and allows lookup of service provider interfaces and service locations.

Key Characterisitics for Effective Services Use

• Coarse-grained• Interface-based design• Discoverable• Single Instance• Loosely Coupled• Asynchronous

SOA Example