developing concurrent distributed systems 1

from simple to complex

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application service provider (ASP)

An organisation offering access to application software run on centralised servers and accessed over the internet using a browser or specialised client software.

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application software

software designed to carry out a function for the user eg a payroll application or a word processing program (as opposed to systems software like the operating system or middle-ware).

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the data transfer capacity of a communications link, usually measured for digital communications in units of bits per second.

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Central Processing Unit / processor

the part of a computer system that carries out the instructions of a software program in order to process data.

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centralised computer system

a computer system where all, or almost all, of the processing is carried out at one central location.

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computer bureau

the clients of a computer bureau pay for the computer time needed to run their business software on the bureau's computers - a similar approach to current application service providers, how ever this idea predates the internet so data transfer was via magnetic disks or tapes.

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concurrent system

a system which may have a number of activities active at the same time and allows all of its activities to make progress.

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a state of a concurrent system where the whole system stops because two or more componenets are waiting for each other to continue (and they never do).

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distributed system

a system containing a number of distinct components at different locations, where each component is, in some sense, a computer system itself and the components are linked by a network.  the system may demonstrate transparency to varying degrees.

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dumb terminal

a computer terminal with very little processing power - simply a keyboard and screen together with some simple communications links.

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embedded system

a computer system that is physically part of a larger system (often a real time system) and may monitor or control the system of which it forms a part.

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a large organisation, such as an industrial company, a bank or a government department.

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enterprise software

software that runs on enterprise systems.

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enterprise system

a computing system suitable for the requirements on an enterprise.

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fault tolerant system

a system that can continue normally (or perhaps with reduced functionality) after a hardware or software error has occurred.

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grid computing

a way of harnessing the power of many computers distributed across the internet to solve problems requiring great processing powers.

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host / node

an individual component of a distributed system that has some processing capability.

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the ability of systems to work across a variety of different environments or platforms.

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local area network (LAN)

a computer network where all the hosts are physically close together, often in the same building, and normally entirely controlled by one organisation or department (in contrast to a WAN).

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location-aware system

a mobile system that can take advantage of satellite location services (such as gps) to determine its location and hence adapt its behavior depending on its physical location.

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short-term rapidly accessed computer hardware for storing data - often but not always, volatile (looses data if powered off) eg RAM.

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mobile system

a distributed computer system whose components are not fixed in their location eg a mobile phone network.

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monolithic architecture

a design of computer system in which all, or almost all, the processing is carried out at one central location eg a mainframe computer.

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a design of CPU chip with more than one processing unit (or 'execution core') that can improve performance by operating in parallel - the special case of two processing units is called dual-core.

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operating system

software that interacts with the underlying hardware of a computer system and provides a layer or platform on which other software, such as application software, can run without needing detailed knowledge of the hardware.

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parallel bus

a set of parallel wiring connections that can link closely positioned CPUs in a multiprocessor system.

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parallel system

a concurrent system that can really carry out a number of activities simultaneously because it has more than one processor.

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personal digital assistant (PDA)

a small handheld computing device, usually with relatively limited resources of display, memory and storage.

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pseudo-parallel system

a concurrent system which shares processor time between a number of activities but only one activity is actually running at one given time.

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random access memory (RAM)

short-term, rapidly accessed hardware for storing data - normally in the form of volatile semiconductor devices (looses data if looses power).

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real-time system

a system which must guarantee to respond correctly to inputs within a specified time period.

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acceptably fast to respond to a request for action or information - the definition of acceptability depends on the context.

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scalable system

a systems whose capacity can be increased, or less commonly decreased, to match changes in demand (where demand may be measured in number of users or amount of processing).

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allocation of processor (ie CPU) time between the various activities in a concurrent system.

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sequential system

a system in which each activity must complete execution before another activity can start.

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computer hardware for holding data in the long term (even after being powered down) - typically uses devices such as magnetic or optical disks that are slower but cheaper per unit of data than memory.

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TCP (Transmission Communications Protocol) and IP (Internet Protocol) are the most commonly used specifications of the detailed format of messages exchanged between internet hosts.

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a device for accessing a centralized computer system via local cabling or, if more remote, via the telephone system.  Some terminals have limited processing ability, often to aid in graphical display, but the bulk of the processing in the system is carried out on the central processor.

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a rather dated term for a concurrent system with a number of users that allocates time to each user, so that each has the impression that the computer is responding to just their request.

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a scheduling approach for concurrent systems in which each activity gets a fixed and equal amount of processor time before the processor switches to the next activity.

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the extent to which a distributed system appears to its users as a coherant system providing a range of functions (so that the user does not need to know that the system is distributed).

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ubiquitous computing

a situation where computing devices are so small and cheap that they occur in huge numbers throughout everyday enviroments.

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a computer system with a single central processing unit.

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unacceptably slow to respond to a request for action or information - the definition of acceptability depends on the context.

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wide area network (WAN)

A computer network than spans a large geographical area such as a region, a country or the world and which typically relies on public providers of telecoms or other long-distance services for part of its communication path (in contrast to a LAN)

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