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cache::async(n)		       In-memory caches		       cache::async(n)

______________________________________________________________________________

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 asyn‐
		     chronously 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 con‐
	      struction of the cache object is used to ask for	this  informa‐
	      tion.

	      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  dis‐
       tinct 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 oth‐
       ers, 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	  SF	     Trackers	       [http://source‐
       forge.net/tracker/?group_id=12883].   Please  also report any ideas for
       enhancements you may have for either package and/or documentation.

KEYWORDS
       asynchronous, cache, callback, synchronous

COPYRIGHT
       Copyright (c) 2008 Andreas Kupries <andreas_kupries@users.sourceforge.net>

cache				      0.3		       cache::async(n)
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