A distributed system is the one in which hardware and software components at networked computers communicate and coordinate their activity only by passing messages. Examples of distributed systems are internet, intranet, electronic banking, mobile and pervasive computing, sensor networks, network of workstations and mobile computing systems. Distributed Systems have evolved rapidly in the past few decades. The unfolding of internet and intranets has made a huge impact on the development of modern software.
Today, distributed systems have become omnipresent. They have found their applications in myriad fields like bank interconnected systems, World Wide Web, network of computers in sophisticated cars.
Internet is a huge collection of networks also called as ‘Information Superhighway’. It’s network of networks particularly used for information exchange and communication. Internet comprises of many heterogeneous networks and applications. Users of internet reside in different physical locations.
The services rendered by internet are open ended for example WWW, ftp, e-mail, CSCW etc. Intranet is a subset of internet which usually falls under proprietorship and is administered individually.
It renders external as well as internal services. Some of the services rendered by intranet are program sharing, print services, internet access (can be connected to internet through router) etc. Intranet can be configured to impose local security insurance.
For instance intranets often use firewalls to block unauthorized information exchange. The advantages of distributed systems include incremental growth, performance, reliability, economy of scale, fault tolerance, distribution of functionality, increased availability, large grained parallelism etc. The key goal of a distributed system is to interconnect resources or users in a scalable, crystalline and open way.
This organization is remarkably more powerful and fault tolerant than assortment of many stand alone systems.
However, if not engineered carefully, distributed systems can lessen overall reliability especially when failure of a node can render other nodes unusable. According to Leslie Lamport “A distributed system is one in which the failure of a computer you didn’t even know existed can render your own computer unusable. ” Distributed Systems suffer from diagnosing and troubleshooting problems as a failure requires investigating communication between various nodes.
The architecture of distributed systems fall into one of basic categories: 3 tier architecture, N tier architecture, Client-server, tight coupling, loose coupling etc. Distributed Computing accelerates computations by the use of multiple computers. Some Examples of areas using Distributed Computing are Network of workstations, grid computing (www. grid. org, using over 2. 5 million CPUs worldwide), or seti@home (project to scan data rendered by a radio telescope to hunt radio signals from some other world).
Cray T3E is an example of Cray Research’s immensely parallel supercomputer architecture. It is an amply distributed memory machine having 4 TB of global memory and high I/O bandwidth (250 GB/s). Domain Name Service comprises of database that performs the mapping of host names to IP addresses and vice versa. Initially all such mappings were stored in file named hosts. txt. But as soon as internet started maturating, the system collapsed because of growing load, difficulty in achieving singularity of names etc. Because of such reasons DNS was born.
Some of DNS characteristics incorporate: Distributed Storage Architecture, hierarchical namespace, server hierarchy (a server preserves only a subset of all names). Network of workstations comprises of personal workstations as well as processors not dedicated to any user. It holds a single file system. All files can be accessed via any machine in the same manner by using the same path. For a command the system can search the most appropriate workstation to execute it. Distributed System is marked by its property of appearing as a single, tenacious and coherent system to all users.
The need for Distributed Systems arises from the elegant advantages they render such as functional distribution (stand alone systems possess different functional capacities yet they may be required to share resources), economy (assortment of many stand alone systems render a better bargain than installing large mainframe), load balancing ( allotting chores to processors in a manner which optimizes performance), replication of processing capacity (individual processors functioning for the same task), augmented reliability (due to independent failure modes of standalone systems and redundant storage of critical data).
Some other examples of distributed systems are automatic banking system, and automotive systems. Key requirements for automatic banking system are: reliability, security, consistency of duplicated data, fault tolerance, and ability to support concurrent transactions. An Automotive system is a distributed real time system where critical issues are Real time communication, synchronization of physical clocks, scheduling with real time constraints, and fault tolerance. Distributed Systems come with a set of disadvantages as well. Difficulties arise while developing distributed software.
How should applications, programming languages and operating systems look? New set of problems are created by network base like overloading, loss of messages etc. Replication of data generates consistency and data security problems. Various design issues related to Distributed Systems are: Transparency, openness, heterogeneity, communication, performance, reliability, fault tolerance, security, scalability etc. Transparency concerns with accomplishing a single image of entire system (that is to create an illusion of ‘single’ and ‘simple’ machine).
For the elements of distributed system to interact, support is mandatory at two levels: Networking infrastructure and communication models and primitives. Various factors affect the performance of a distributed system like performance of solo workstations, speed of network infrastructure, extent of fault tolerance, workload assignment. Distributed Systems are typically heterogeneous in nature (different hardware and software, different protocols and devices). To handle heterogeneity aspects, an additional software layer (Middleware) is introduced which cloaks heterogeneity.
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