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A period of time for which an object displays a particular behaviour. Any one object is in a single epoch at one time, but interacting objects may be in different epochs at the time of interaction. A change of epoch may be associated with a change in the type of the object, so as to support type evolution. Alternatively, a change of epoch may be associated with a phase in the behaviour of an object of constant type. For a distributed system to function correctly, the objects composing its configuration must be consistent. Thus, as the whole system evolves through a series of epochs, the individual objects that interact must never be in epochs in which their behaviours are sufficiently different that their concurrent composition leads to a failure. This concept will support the formalisation of concepts of version and extensibility. NOTE - A specification language may need to express a) the way epochs are labelled; b) the sequence of epochs, and whether all objects need to pass through all members of the sequence; c) the rules for deriving the epoch of a composition from the epochs of its objects, particularly for configurations and complete systems; d) whether identity of the epoch of an object is necessarily part of the state of that object; e) whether objects can negotiate on the basis of their current epoch identities; f) the relation of epoch to the concepts of local and global time

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