Unit 1 Flashcards
What are the motivations for concurrency?
Efficient use of computing resources
Models the real world
More user responsiveness
What is a distributed system?
A number of component parts (hosts or nodes) located in distinct locations but connected by a network. Component parts may be computer systems in their own right.
What does transparency refer to?
transparency is the notion the the user sees the distributed system as a single coherent system
What are the motivations for a distributed system?
Allow remote access to and sharing of resources
Scalability
Models the real world of business
Name 3characteristics/issues facing distributed systems and describe
CONCURRENT - each node can operate independently and therefore in parallel
INDIVIDUAL NODE FAILURE - this possibility introduces new issues. System should be designed to cope with this, even if it is a limited service.
LACK OF A GLOBAL CLOCK - each node has its own clock. This introduces synchronisation issues.
What is the difference in communication in a distributed and non-distributed system?
In a non-distributed multiprocessor system the CPUs are located closely together and linked via a parallel bus.
In a distributed system communications are less close coupled. Generally communication is made by passing messages along the communication link. Communication is layered in a distributed system.
Describe communication layers in a distributed system.
Lower level - streams of bits
Includes LAN, WAN and wireless
Higher or more abstract level uses TCP/IP - this specifies the detailed format of messages that may be exchanged between hosts. Allows communication among heterogenous hosts and software (many types and combinations)
What is time slicing?
In early concurrent (non distributed) systems (which for example allowed background printing to occur) time slicing allocated a fixed amount of time to each activity or user of resources.
Costs of concurrency (in a non-distributed system).
Additional complexity = new ways to fail.
More complex and expensive hardware was needed.
Deadlock may occur.
Concurrent tasks or activities may want to alter the same data, this needed to be correctly managed.
What is deadlock?
Deadlock occurs when more than one part of the system is waiting for another part to do something which will allow the other part to progress, and they never do.
What are the benefits of distributed systems?
Facilitates sharing of resources
Scalable
Efficient - can put data and processing power where needed thus minimising traffic on potentially slow, unreliable or expensive communication links.
Fault tolerance - multiple routes allow re routing eg Internet.
Cost effective as microprocessors have become cheaper eg a car.
What are the costs of a distributed system?
Reliance on network, if communications fail some nodes may become unreachable.
More points of access = more potential security issues.
May use wireless link which may not be secure.
Increased complexity of systems and software.
Got to be able to deal with heterogenous systems - INTEROPERABILITY
New ways to fail eg synchronise clock times
Motivation behind mobile systems
Allows access to computing facilities from a wide range of locations.
Wireless communication removes need to be dependant on wired networks.
Costs of mobile systems.
Communication links like 3G may be slower.
Lower bandwidth may mean less can be sent.
Security - more encryption needed.
Physical security - mobile devices can be lost.
