Client-Server Computing

It imposses hierarchy and a division of labour among processors. Only one designated processor, the master, controls (in a tightly coupled arrangement) slave processors dedicated to specific functions.

It waits for client-initiated requests.

Executes many requests at the same time.

Takes care of VIP clients first.

Initiates and runs background task activity.

Keeps running.

Grown bigger and faster.

With a transaction server, the client invokes remote procedures that reside on the server with an SQL database engine. These remote procedures on the server execute a group of SQL statements. The network exchange consists of a single request/reply message. The SQL statements either all succeed or fail as a unit.

Ubiquitous communications

Network OS extension

Binary large objects (BLOBs)

Global directories and Network yellow pages

Authentication and Authorization services

System management

Network time

Database and transaction services

Internet services

Object- oriented services

ACID is a term coined by Andrew Reuter in 1983, which stands for Atomicity, Consistence, Isolation and Durability.

ACID property is the basic property for transaction processing.

A - atomicity

C - consistency

I - isolation

D - durability

Non-GUI Client: These are applications, generate server requests with a minimal amount of human interaction.

GUI Clients: These are applicatoins, where occassional requests to the server result from a human interacting with a GUI

(Example: Windows 3.x, NT 3.5)

OOUI clients : These are applications, which are highly-iconic, object-oriented user interface that provides seamless access to information in very visual formats.

(Example: MAC OS, Windows 95, NT 4.0)

There is a many-to-one relationship between clients and server. Clients always initiate the dialog by requesting a service. Servers are passively awaiting for requests from clients.

It treats all processors as equal. Any processor can do the work of any other processor. Applications are divided into threads that can run concurrently on any available processor. Any processor in the pool can run the OS kernel and execute user-written threads.

SQL is a powerful set-oriented language which was developed by IBM research for the databases that adhere to the relational model. It consists of a short list of powerful, yet highly flexible, commands that can be used to manipulate information collected in tables. Through SQL, we can manipulate and control sets of records at a time.

Service

Shared resources

Asymmentrical protocols

Transparency of location

Mix-and-match

Message based exchanges

Encapsulation of services

Scalability

Integrity

Client/Server computing is the ultimate "Open platform". It gives the freedom to mix-and-match components of almost any level. Clients and servers are loosely coupled systems that interact through a message-passing mechanism.

It includes the communication stacks, distributed directories, authentication services, network time, RPC, Queuing services along with the network OS extensions such as the distributed file and print services.

With an object server, the Client/Server application is written as a set of communicating objects. Client object communicate with server objects using an Object Request Broker (ORB). The client invokes a method on a remote object. The ORB locates an instance of that object server class, invokes the requested method and returns the results to the client object. Server objects must provide support for concurrency and sharing. The ORB brings it all together.

An object is define ba a distrinct set up attribute that represent a network resources. For example User1 ,user2, group1 group2 etc.

Base services - It is a part of the standard OS.

Extended services - These are add-on modular software components that are layered on top of base service.

General middleware

Service-specific middleware.

TP lite, based on stored procedures.

TP heavy, based on the TP monitors.

File servers are useful for sharing files across a network. With a file server, the client passes requests for file records over nerwork to file server.

The types of transparencies the NOS middleware is expected to provide are:-

Location transparency

Namespace transparency

Logon transparency

Replication transparency

Local/Remote access transparency

Distributed time transparency

Failure transparency and

Administration transparency.