02-06-2012, 01:37 PM
Distributed Programming Model
Distributed Programming Model.ppt (Size: 500.5 KB / Downloads: 39)
What is Distributed Systems
A distributed system is one in which components located at networked computers communicate and coordinate their actions only by passing messages.
The sharing of resources is a main motivation for constructing distributed systems. Eg Web.
Resources may be managed by servers and accessed by clients or they may be encapsulated as objects and accessed by other client objects.
Significant Consequences of DS
Concurrency
The coordination of concurrently executing programs that share resources is an important one.
No global clock
Close coordination depends on a shared idea of the time at which the programs actions occur.
The only communication is by sending messages through a network
Independent failures
Fault in the network.
Unexpected termination of a program somewhere in the system /crash.
The programs may not be able to detect whether the network has failed or has become unusually slow.
The Internet
The Internet is a very large distributed system
The Internet is a vast interconnected collection of computer networks of many different types.
Programs running on the computers connected to it interact by passing messages, employing a common means of communication.
The internet protocols enabling a program running anywhere to address messages to program anywhere else.
Intranets
Sub networks operated by companies and other organizations
Internet Service Providers
Companies that provide modem links and other types of connection to individual users and small organizations
Enabling them to access services anywhere in the internet as well as providing local services such as email and web hosting
Backbone
A network link with a high transmission capacity
Employing satellite connections, fibre optic cables and other high-bandwidth circuits.
Intranets
An intranet is a portion of the Internet that is separately administered and has a boundary that can be configured to enforce local security policies.
It is composed of several local area networks (LAN) linked by backbone connections.
The network configuration of a particular intranet is the responsibility of the organization that administers it.
It is connected to the internet via a router.
The role of the firewall is to protect an intranet by preventing unauthorized messages leaving or entering.
It is implemented by filtering incoming and outgoing messages
The main issues arising in the design of components for use in intranets are: file services, firewalls, cost of software installation and support.
Some organizations do not wish to connect their internal networks to the internet at all:
Police and other security
Law enforcement agencies
Military organizations