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?

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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  SF  Trackers
       [http://sourceforge.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>