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DISTRIBUTED SYSTEMS

 

ABSTRACT:

Distributed System is one in which components located at networked computers communicate and coordinate their actions only by passing messages. Characteristics of Distributed Systems include Concurrency of components, lack of global clock and independent failures of components.

Sharing of resources is a main motivation for constructing distributed systems. Resources may be managed by servers and accessed by clients or they may be encapsulated as objects and accessed by other client objects .

Examples of Distributed Systems are: Internet, Intranet, Mobile computing

Advantages of Distributed Systems over Centralized are: Economics, Speed, Inherent distribution, Reliability, Incremental growth.

The Challenges of Distributed Systems are: Heterogeneity of its components, openness, security, scalability, failure handling concurrency of components and transparency.

 

 

INTRODUCTION:

The development in the Computer Systems lead to the invention of high-speed computer networks. The Local Area Network(LAN) allow hundreds of machines within a building to be connected in such a way that small amount of information can be moved between machines in a millisecond. The Wide Area Network(WAN) allow millions of machines all over the earth to be connected at different speeds.

The result of these technologies is that it is now easy to put together computing systems composed of large number of CPUs connected by a high-speed network. They are usually called Distributed Systems.

What is Distributed Systems?

A Distributed System is one in which hardware or software components located at networked computers communicate and coordinate their actions only by passing messages. 

Or

A Distributed System is a collection of independent computers that appear to the users of the system as a single computer.

Challenges:

The Challenges arising from the construction of distributed systems are:

  • Heterogeneity:

The Internet enables users to access services and run application over a heterogeneous collection of computers and networks.

 
  • Openness:

The openness of a computer system is the characteristic that determines whether the system can be extended and re- implemented in various ways. The openness of distributed systems is determined primarily by the degree to which new resource- sharing services can be added and available for use by a variety of client programs.

  • Security:

Many of the information that are made available and maintained in distributed systems have a high intrinsic vale to their users. Their security is therefore of considerable importance. Security for information resources has three components: confidentiality; integrity; and availability

  • Scalability:

Distributed systems operate effectively and efficiently at many different scales. A System is described as Scalable if it will remain effective when there is a significant increase in the number of resources and the number of users.

  • Failure handling:

Recovery from the failures involves redundancy .

  • Concurrency:

Both services and applications provide resources that can be shared by clients in a distributed system. There is therefore as possibility that several clients will attempt to access a shared resource at the same time. Do be safe in a concurrent environment, its operations must be synchronized in such a way that its data remain consistent.

  • Transparency:

Transparency is defined as the concealment from the user and the application programmer of the separation of components in a distributed system, so that system is perceived as a whole rather than as a collection of independent components.

Different kinds of transparency in a distributed system:

  • Location transparency:  

         The users cannot tell where resources are located.

  • Concurrency transparency: 

        Multiple users can share resources automatically.

  • Replication transparency:

        The users cannot tell how many copies exists.

  • Parallelism transparency: 

    Activities can happen in parallel without users knowing.

  • Migration transparency:

    Resources can move without changing their name.

Advantages:

Advantages of distributed systems over centralized systems:

  • Speed A distributed system may have more total computing power than a mainframe.
  • Inherent distribution Some application involve spatially separated machines.
  • Reliability If one machine crashes, the system as a whole can still survive.
  • Incremental Growth Computing power can be added in small increments.

· Advantages of distributed systems over personal computers:

  • Data sharing:

       Allow many users access to a common database.

  • Device sharing

       Allow many users to share expensive peripherals.

  • Flexibility

       Spread the work load over the available machines in the most cost effective way.

Disadvantages:

  • Software

        Little software exists at present for distributed systems.

  • Networking

      The network can saturate or cause other problems.

  • Security

      Easy access also applies to secret data.

Examples of Distributed Systems:

  • Internet

The Internet is a vast interconnected collection of computer networks of many different types. computers connected to it interact by passing messages, employing a common means of communication mechanisms. It is a very large distributed system .

  • Intranet

    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 LANs linked by a backbone connections.

    A Backbone is a network link with a high transmission capacity, employing satellite connections, fiber optic cables and other high bandwidth circuits.

  • Mobile computing

    The portability of devices like laptops, personal digital assistants(PDAs), pagers, mobile phones etc, together with their ability to connect conveniently to networks in different places, makes mobile computing possible.

 

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