Editing Distributed computing (section)

==Introduction==
The word ''distributed'' in terms such as "distributed system", "distributed programming", and "[[distributed algorithm]]" originally referred to computer networks where individual computers were physically distributed within some geographical area.<ref>{{harvtxt|Lynch|1996}}, p. 1.</ref> The terms are nowadays used in a much wider sense, even referring to autonomous [[Process (computing)|processes]] that run on the same physical computer and interact with each other by message passing.<ref name="Andrews 2000"/>

While there is no single definition of a distributed system,<ref name="harvtxt|Ghosh|2007">{{harvtxt|Ghosh|2007}}, p. 10.</ref> the following defining properties are commonly used as:
* There are several autonomous computational entities (''computers'' or ''[[Node (networking)|nodes]]''), each of which has its own local [[Memory (computers)|memory]].<ref>{{harvtxt|Andrews|2000}}, pp. 8–9, 291. {{harvtxt|Dolev|2000}}, p. 5. {{harvtxt|Ghosh|2007}}, p. 3. {{harvtxt|Lynch|1996}}, p. xix, 1. {{harvtxt|Peleg|2000}}, p. xv.</ref>
* The entities communicate with each other by [[message passing]].<ref>{{harvtxt|Andrews|2000}}, p. 291. {{harvtxt|Ghosh|2007}}, p. 3. {{harvtxt|Peleg|2000}}, p. 4.</ref>

A distributed system may have a common goal, such as solving a large computational problem;<ref>{{harvtxt|Ghosh|2007}}, p. 3–4. {{harvtxt|Peleg|2000}}, p. 1.</ref> the user then perceives the collection of autonomous processors as a unit. Alternatively, each computer may have its own user with individual needs, and the purpose of the distributed system is to coordinate the use of shared resources or provide communication services to the users.<ref>{{harvtxt|Ghosh|2007}}, p. 4. {{harvtxt|Peleg|2000}}, p. 2.</ref>

Other typical properties of distributed systems include the following:
* The system has to [[Fault tolerance|tolerate failures]] in individual computers.<ref>{{harvtxt|Ghosh|2007}}, p. 4, 8. {{harvtxt|Lynch|1996}}, p. 2–3. {{harvtxt|Peleg|2000}}, p. 4.</ref>
* The structure of the system (network topology, network latency, number of computers) is not known in advance, the system may consist of different kinds of computers and network links, and the system may change during the execution of a distributed program.<ref>{{harvtxt|Lynch|1996}}, p. 2. {{harvtxt|Peleg|2000}}, p. 1.</ref>
* Each computer has only a limited, incomplete view of the system. Each computer may know only one part of the input.<ref>{{harvtxt|Ghosh|2007}}, p. 7. {{harvtxt|Lynch|1996}}, p. xix, 2. {{harvtxt|Peleg|2000}}, p. 4.</ref>