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







cache::async - Asynchronous in-memory cache








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?












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.








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

  







  

  
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:



  

  
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.



  

  

  

  
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.

  

  

  
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.

  

  
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.










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.








asynchronous, cache, callback, synchronous








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








  

    0.3
    cache