Distributed Systems
A system is said to be distributed when
- There are distinct, autonomous “computational entities”
- The autonomous entities communicate with each other by “message passing”
A distributed system may have a common goal, such as solving a large computational problem;[10] 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.[11] (wikipedia)
Time, Clocks, and the Ordering of Events in a Distributed System
This work aims to impose a total order upon systems of processes bound by a conventional metric time and composed of linear orders of discrete events.
In the opening paragraph of (Lamport 1978), Lamport lays out the following ontology as basic for the concept of a distributed system:
- The concept of (clock) time is derived from the concept of the order of event occurrence.
- Because we say “X happened at time t” if X occurred after a clock reads t and before it reads t’. So happening at a time is derived from observations situated in a after-before relationship.
- Essentially, Lamport treats time as a metric over a temporal topology.
- “our way of thinking” is founded (at least in part) on the concept of (clock) time.
- Our thinking about systems, in particular, depends on temporal ordering.
- E.g., an airline reservation system
The point of the paper is to problematize the temporal ordering of events in the kinds of systems which we call distributed.
A distributed system consists of a collection of distinct processes which are spatially separated, and which communicate with one another by exchanging messages.
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A system is distributed if the message transmission delay is not negligible compared to the time between events in a single process.
The spatial separation is non-essential. In essence, whether or not a system is distributed, according to this characterization, is a function of a threshold in a temporal metric allowing comparison of “delay” between distinct, but interrelated orders of events.
A system is understood to consist of one or more processes. Each process is a single, inherently synchronous and linear, ordering of events. These orders can interact through passing messages. When the ratio between the time taken to pass a message and the time taken between events in a process is “non-negligible”, the system is said to be “distributed”.