Exercise_Week 9

Designing distributed systems

A distributed system is a collection of independent computers that appear to its users as a single coherent system.
Characteristics of distributed system

• Multiple, independent processing units
• Processors communicate via a hardware interconnect
• Processing unit failures are independent
• Manage resource sharing
• State is shared among processors

A distributed system is a collection of independent computers that are used jointly to perform a single task or to provide a single service.
Examples of distributed systems
Probably the simplest and most well known example of a distributed system is the collection of Web servers—or more precisely, servers implementing the HTTP protocol—that jointly provide the distributed database of hypertext and multimedia documents that we know as the World-Wide Web. Other examples include the computers of a local network that provide a uniform view of a distributed file system and the collection of computers on the Internet that implement the Domain Name Service (DNS).
Why do we use distributed systems?
The alternative to using a distributed system is to have a huge centralized system, such as a mainframe. For many applications there are a number of economic and technical reasons that make distributed systems much more attractive than their centralized counterparts.
Cost: Better price/performance as long as commodity hardware is used for the component computers.
Performance: By using the combined processing and storage capacity of many nodes, performance levels can be reached that are beyond the range of centralized machines.
Scalability: Resources such as processing and storage capacity can be increased incrementally.
Reliability: By having redundant components the impact of hardware and software faults on users can be reduced.
Inherent distribution: Some applications, such as email and the Web (where users are spread out over the whole world), are naturally distributed. This includes cases where users are geographically dispersed as well as when single resources (e.g., printers, data) need to be shared.

Reference:

• B. Clifford Neuman. Scale in distributed systems. In T. Casavant and M. Singhal, Readings in Distributed Computing Systems, pages 463–489, IEEE Computer Society Press, Los Alamitos, CA, USA

• Andrew S. Tanenbaum and Maarten van Steen. Distributed Systems: Principles and Paradigms, Prentice Hall, 2^nd edition, 2007