1. Distributed Systems: Client/Server Computing

2. Client/Server Computing
Major trend in information systems, replacing both the centralized approaches and alternative forms of distributed computing
Main concepts
Clients
Single-user PCs or workstations with user-friendly, usually graphical, interfaces
Servers
Provide a set of shared user services to clients
Example: Database server, controlling a relational database
Allows many clients to share access to the same database
Can use a high-performance computer system to manage the database
Network
Clients and servers are connected to a LAN or WAN or by an internet of networks
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

3. Client/Server Computing (cont.)
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

4. Client/Server Computing (cont.)
Client/server architecture
Platforms and the OSs may be different
Communications protocols and the applications supported must be common
Allocation of application-level tasks between clients and servers is central to the architecture. It must:
Optimize platform and network resources
Optimize the ability of users to perform various tasks and to cooperate with one another
Presentation services in client must provide a user interface (e.g., GUI) that is easy to use, powerful, and flexible
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

5. Client/Server Computing (cont.)
Relational database applications
Server is responsible for maintaining the database using a database management system software module
Client has applications that make use of the database
Client/server interactions are in the form of transactions in which the client makes a database request and receives a database response, e.g., the Structured Query Language (SQL)
Not all the application logic (e.g., data analysis) has to reside in the client
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

6. Client/Server Computing (cont.)
Example: Online access for record lookup
Server is maintaining a database of 1 million records
User wants to perform a search using a certain search criteria (e.g., records older than 1999) and expects to find at most a few records
First query yields a server response of 100,000 records
User sends another query adding new qualifiers
Response returns 1,000 records
Client issues third request with additional qualifiers and receives a single record
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

7. Client/Server Computing (cont.)
Example 2: Online access for record lookup
Server is maintaining a database of 1 million records
Client would like to compute the grant total of a certain field across many records
The query results in 300,000 records transmitted, which would be a misuse of the client/server architecture
Solution: move part of the application logic to the server
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

8. Client/Server Computing (cont.)
Classes of client/server applications
Wide spectrum of implementations in partitioning work between client and server
Host-based processing
All processing done on a central host
User’s station is limited to the role of a terminal emulator, even if it employs a microcomputer
Traditional mainframe computing, not a true client/server architecture
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

9. Client/Server Computing (cont.)
Classes of client/server applications (cont.)
Server-based processing
Most basic class of client/server configuration, early implementations (thin client model)
Client provides a graphical user interface
Server provides all processing
This configuration does not provide significant improvements in the use of resources
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

10. Client/Server Computing (cont.)
Classes of client/server applications (cont.)
Client-based processing
All application processing is done by the client, except for the data validation routines
Advantage: allows users to employ applications best suited for local needs
Most common client-server approach in use today
Fat client model
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

11. Client/Server Computing (cont.)
Classes of client/server applications (cont.)
Cooperative processing
Distribution of application processing is optimized, taking advantage of the strengths of both client and server machines and of the best distribution of data
More difficult to set up and maintain
Can lead to greater network efficiency and highest productivity gains
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

12. Client/Server Computing (cont.)
Three–tier client/server architecture
Application software is distributed between three types of machines: a (thin) client, a middle-tier server, and a backend server
The middle-tier server provides a gateway function between the clients and the different backend servers
Convert protocols and map from one type of database query to another
Can merge/integrate results from different sources
Can serve as a gateway between the new desktop applications and the backend legacy applications
The middle-tier server is both a server and a client: server for the thin client and client for the backend server
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

13. Client/Server Computing (cont.)
File cache consistency
To reduce the performance penalty of accessing remote files, individual systems can use file caches
Typical distribution mechanism for caching files among network workstations: a file access request in the client results in (1) request to client cache (“file traffic”), then to local disk (“disk traffic”), then to file server (“server traffic”), cache and then disk
Problem: caches can become inconsistent when the remote data are changed and local cache is obsolete
Solutions: (a) File locking techniques to prevent simultaneous file access by more than one client, and (b) Sprite approach: on a write by a client, all other clients that opened the file are notified
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

14. Client/Server Computing (cont.)
Middleware
The rapid development and deployment of client/server products has outpaced the standards efforts: difficult to implement an integrated, multi-vendor client/server solution
Developers need a set of tools that provide a uniform means and style of access to system resources across all platforms
Middleware:
A set of standard programming interfaces and protocols that sit between the application above and the communications software and operating system below
Hide the complexities and disparities of different network protocols and operating systems
Client and server vendors provide a number of middleware packages as options
The user selects a middleware strategy and then assembles equipment from different vendors that support that strategy
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

15. Client/Server Computing (cont.)
Middleware architecture
The role of the middleware depends on the style of client/server computing used
There is both a client and a server component of middleware
Goal: allow an application or user at the client to access different services on servers, even though the servers’ implementations may be different
Many relational databases, even though they support Structured Query Language (SQL), they also add proprietary extensions to SQL
Example: A distributed system supporting a personnel department is using both Gupta and Oracle databases
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

16. Client/Server Computing (cont.)
A logical view of a distributed system using middleware
The entire distributed system can be viewed as a set of applications and resources available to its users
Applications run over a uniform Applications Programming Interface (API)
The middleware operates over all client and server platforms and is responsible for routing client requests to the appropriate servers
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]

17. Client/Server Computing (cont.)
An example of the use of middleware to integrate different products
CS-550 (M.Soneru): Distributed Systems - Client/Server Computing: [Sta’01]