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# cache::async(n) 0.3 tcllib "In-memory caches"

## Name

cache::async - Asynchronous in-memory cache

## Synopsis

- package require Tcl 8.4
- package require cache::async ?0.3?

- ::cache::async objectName commandprefix ?options...?
- objectName get key donecmdprefix
- objectName set key value
- objectName unset key
- objectName exists key
- objectName clear ?key?

## Description

This package provides objects which cache data in memory, and operate asynchronously with regard to request and responses. The objects are agnostic with regard to cache keys and values, and unknown methods are delegated to the provider of cached data. These two properties make it easy to use caches as a facade for any data provider.

## API

The package exports a class, cache::async, as specified below.

::cache::async objectName commandprefix ?options...?

  The command creates a new cache object with an associated global Tcl command whose name is objectName. This command may be used to invoke various operations on the object.

  The commandprefix is the action to perform when an user asks for data in the cache and the cache doesn't yet know about the key. When run the commandprefix is given three additional arguments, the string get, the key requested, and the cache object itself, in the form of its object command, in this order. The execution of the action is done in an idle-handler, decoupling it from the original request.

  The only supported option is

  -full-async-results

    This option defines the behaviour of the cache for when requested keys are known to the cache at the time of get request. By default such requeste are responded to asynchronously as well. Setting this option to false forces the cache to respond to them synchronuously, although still through the specified callback.

The object commands created by the class commands above have the form:

objectName get key donecmdprefix

  This method requests the data for the key from the cache. If the data is not yet known the command prefix specified during construction of the cache object is used to ask for this information.

  Whenever the information is/becomes available the donecmdprefix will be run to transfer the result to the caller. This command prefix is invoked with either 2 or 3 arguments, i.e.

  1. The string set, the key, and the value.

  2. The string unset, and the key.

  These two possibilities are used to either signal the value for the key, or that the key has no value defined for it. The latter is distinct from the cache not knowing about the key.

  For a cache object configured to be fully asynchronous (default) the donecmdprefix is always run in an idle-handler, decoupling it from the request. Otherwise the callback will be invoked synchronously when the key is known to the cache at the time of the invokation.

  Another important part of the cache's behaviour, as it is asynchronous it is possible that multiple get requests are issued for the same key before it can respond. In that case the cache will issue only one data request to the provider, for the first of these, and suspend the others, and then notify all of them when the data becomes available.

objectName set key value
objectName unset key

  These two methods are provided to allow users of the cache to make keys known to the cache, as either having a value, or as undefined.

  It is expected that the data provider (see commandprefix of the constructor) uses them in response to data requests for unknown keys.

  Note how this matches the cache's own API towards its caller, calling the donecmd of get-requests issued to itself with either "set key value" or "unset key", versus issuing get-requests to its own provider with itself in the place of the donecmd, expecting to be called with either "set key value" or "unset key".

  This also means that these methods invoke the donecmd of all get-requests waiting for information about the modified key.

objectName exists key

  This method queries the cache for knowledge about the key and returns a boolean value. The result is true if the key is known, and false otherwise.

objectName clear ?key?

  This method resets the state of either the specified key or of all keys known to the cache, making it unkown. This forces future get-requests to reload the information from the provider.

## Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category cache of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

## Keywords

asynchronous, cache, callback, synchronous

## Copyright

Copyright © 2008 Andreas Kupries <andreas_kupries@users.sourceforge.net>