Distributed Processing, Client/Serverand Clusters

Chapter 16

Client/Server Computing

Client machines:

single-user PCs or workstations that provide a highly user-friendly interface to the end user

Each server provides a set of shared user services to the clients

The server enables many clients to share access to the same database and enables the use of a high-performance computer system to manage the database

Client and server platforms/OS may be different

These lower-level differences are irrelevant as long as a client and server share the same communications protocols (ex: TCP/IP) and support the same applications

Actual functions performed by the application can be split up between client and server in a way that optimizes the use of resources

Optimize the ability of users to perform various tasks and to cooperate with one another using shared resources

Heavy emphasis on providing a user-friendly Graphical User Interface (GUI) on the client side (presentation services layer)

Client/Server Applications

Generic Client/Server Architecture

Database Applications

One of the most common families of client/server applications

The server is a database server responsible for maintaining the database

Interaction between client and server is in the form of transactions the client makes a database request and receives a database

response

A variety of different client applications can use the same database server; all using the same interface/protocol

Client/Server Architecture for Database Applications

Client/ServerDatabase Usage

Example: think of an application in which we need to compute the mean of the ages of a certain population and the search criteria returns 300K records heavy network traffic

To optimize performance: server can be equipped with application logic for performing data analysis (computation of mean).

Split-up the application logic

10

Classes of Client/Server Applications

Host-based (dumb terminal) not true client/server computing traditional mainframe environment

Server-based (thin client) server does all the processing User(client) workstation provides a

graphical user interface

Fat client modelsTakes advantage of desktop power and can serve large number of clients

Cooperative application processing is performed in an

optimized fashion complex to set up and maintain but greater user productivity gains and greater

network efficiency

Client-based Most common client/server model all application processing done at the client data validation routines and other database

logic function are done at the server

Three-Tier Client/Server Architecture

Application software distributed among three types of machines

User machine thin client

Middle-tier serverGatewayConverts protocolsMap from one type of

database query to anotherMerge/integrate results from

different data sourcesAssumes both roles:

server & client

Backend server Legacy applications

File Cache Consistency File caches hold recently accessed file records Cache consistency problem:

Caches are consistent when they contain exact copies for remote data Simple solution: File-locking prevents simultaneous access to a file Complicated approach: allow multiple read but one write access; when there is

a write, mark the file as non-cacheable

Middleware Lack of standards for client/server models makes it difficult to implement an integrated,

multivendor, enterprise-wide client/server configuration Middleware: Set of tools that provide a uniform means and style of access to system resources

across different platforms. Goal: to enable an application or user at a client to access a variety of services on servers without being concerned about differences among them

Provides standard programming interfaces/protocols that sit between the application above and the communications software+OS below.

Capability to hide the complexities and disparities of different network protocols and OS Enable programmers to build applications that look and feel the same with little effort Enable programmers to use the same method to access data

LOGICAL VIEW OF MIDDLEWARE

Middleware which cuts across all client and server platforms, is responsible for routing client requests to the appropriate server.

Distributed Message Passing Middleware products are typically based on one of two underlying

mechanisms: Message-passing or RPC (Remote procedure calls) Send and receive messages as used in a single system

OR Remote procedure calls

Reliable Guarantees delivery if possible - Not necessary to let the sending

process know that the message was delivered

Unreliable Send the message out into the communication network without

reporting success or failure - Reduces complexity and overhead

Blocking Send does not return control to the sending process until the

message has been transmitted OR does not return control until an acknowledgment is received Receive does not return until a message has been placed in the

allocated buffer

Nonblocking Process is not suspended as a result of a Send or a Receive Efficient and flexible Difficult to debug

Message-passing schemes

Clusters

Alternative to symmetric multiprocessing (SMP) Group of interconnected, whole computers working together as a

unified computing resource illusion is one machine system can run on its own

Clusters Compared to SMP

SMP is easier to manage and configure

SMP takes up less space and draws less power

Clusters are better for incremental and absolute scalability Add new systems in small increments Can have dozens of machines each of which is a multiprocessor

Clusters are superior in terms of availability Failure of one node does not mean loss of service

Clusters have superior price/performance

Beowulf and Linux Clusters

Mass market commodity components (No custom components) A dedicated, private network (LAN or WAN or internetworked combination) Easy replication from multiple vendors Scalable I/O A freely available software base Returning the design and improvements to the community

Issues on Clusters

Failure management Highly available vs. fault-tolerant clusters

Highly available clusters offers a high probability that all resources will be in service Fault-tolerant cluster ensures that all resources are always available (use of

redundant disks/processors etc.)

Load balancing When a new computer is added to the cluster, the load-balancing

facility should automatically include this computer in scheduling applications

Parallelizing Computation Parallelizing compiler Parallelized application