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 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>