An ephemeron is a data structure that solves two related problems in garbage collected systems. On the one hand, an ephemeron provides a notification when some object is about to be collected. On the other hand, an ephemeron allows data to be associated with some object without creating a reference to that object that will prevent the object from being collected. An ephemeron is a key-value pair, where the key is the object that the ephemeron guards, notifying the system when that object is collectable, and the value can be any data associated with the object such as a property list, and which may be empty.
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Lua does not contain a separate ephemeron construct, but its table data structures may be set to holds its keys, values, or both in a weak fashion. If the keys are held weakly, but values are held strongly, the table will act like an ephemeron.
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Since the elements of the property list may refer back to the key, they may prevent collection of that key. But the ephemeron is treated specially by the garbage collector. The value field is not traced until the key is found to be reachable from the system roots other than through ephemeron keys. The set of ephemerons whose keys are only reachable from ephemeron keys are then holding onto keys that are ready to be collected; these objects are not reachable from the roots except through ephemerons.
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An implementation of an OCaml ephemeron type was presented in 2014 and added to the standard library in release 4.03.
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Languages such as C#, F#, and VB.NET, as of .NET Framework 4.0, have support in the ConditionalWeakTable class. The underlying ephemeron mechanism (DependentHandle) is private.
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While this problem can always be solved in any given concrete situation by using one of the various weak association types, choosing the 'right' kind of association depends on a variety of factors some of which can change dynamically. Ephemerons solve this problem by defining that the 'contents' (value) of an ephemeron will be held strongly until the key is known to be garbage collected. From then on, the contents of the ephemeron will be held weakly. Therefore, the contents of an ephemeron can become eligible for garbage collection if and only if the key is garbage collectable which is the exact behavior which we would observe for an instance variable of the object.
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In addition to Earthed, Kilbey published a book of poetry in 1998 entitled Nineveh/The Ephemeron; Kilbey later republished a hard copy version that contained both books and a limited number of 50 copies was released. In August 2013, Uncollected, described as, "A deluxe edition of his books - Earthed, The Ephemeron, Nineveh, Fruit Machine and other selected work", was released on his own Time Being label. Kilbey's inaugural autobiography, Something Quite Peculiar, was then published by Hardie Grant on 1 November 2014.
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Ephemera () is a noun, the plural neuter of ephemeron and ephemeros, Greek and New Latin for epi () – "on, for" and hemera () – "day". The ancient sense extended to the mayfly and other short lived insects and flowers, and for something which lasts a day or a short period of time.
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For weak references, the garbage collector always follows the value in the key- value pair. For ephemerons, instead, the garbage collector doesn't follow the value but queues the ephemeron for further observation at a second stage: after the first tracing phase is done, it runs through the queue looking at each ephemeron and if its key was seen, then it follows its value. This subtle difference impacts in graphs with some kinds of cycles, where weak pairs do not describe correctly that an object ought to be "almost collectable". For example, consider a key-value pair with weak references where the key is an object and the value is a set of properties attached to the object.
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Terry Randolph Hummer (born August 7, 1950) is an American poet, critic, essayist, editor, and professor. His most recent books of poetry are After the Afterlife (Acre Books) and the three linked volumes Ephemeron, Skandalon, and Eon (Louisiana State University Press). He has published poems in literary journals and magazines including The New Yorker, Harper's, Atlantic Monthly, The Literati Quarterly, Paris Review, and Georgia Review. His honors include a Guggenheim Fellowship inclusion in the 1995 edition of Best American Poetry, and two Pushcart Prizes.
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It is expected that when the object is ready to be collected, the properties will also go away. But if the value, possibly transitively, maps to its own key (the object), then the object will never be collected. If an ephemeron was used instead, the value wouldn't have been followed unless the object was proved alive, solving the cycle. Ephemerons are similar to weak pairs, but an object in an ephemeron's key field may be classed as "almost collectable" even if it is reachable from the ephemeron's value fields.
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An ephemeron is an object which refers strongly to its contents as long as the ephemeron's key is not garbage collected, and weakly from then on. Ephemerons solve a problem which is commonly found when trying to "attach" properties to objects by using a registry. When some property should be attached to an object, the property should (in terms of GC behavior) typically have the life-time that an instance variable of this object would have. However, this is complicated by having an external association between the object and its property such as: property --------- registry --------- association --------- object Here, the registry (a third party) will hold onto the association itself which would require manual removal from the registry (instead of automated garbage collection).
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