[manpage_begin cache::async n 0.3]
[keywords asynchronous]
[keywords cache]
[keywords callback]
[keywords synchronous]
[copyright {2008 Andreas Kupries <andreas_kupries@users.sourceforge.net>}]
[moddesc   {In-memory caches}]
[titledesc {Asynchronous in-memory cache}]
[require Tcl 8.4]
[require cache::async [opt 0.3]]
[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.

[section API]

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

[list_begin definitions]

[call [cmd ::cache::async] [arg objectName] [arg commandprefix] [opt [arg options]...]]

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

[para]

The [arg 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
[const 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.

[para]

The only supported option is

[list_begin options]
[opt_def -full-async-results]

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

[list_end]
[list_end]

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

[list_begin definitions]

[call [arg objectName] [method get] [arg key] [arg donecmdprefix]]

This method requests the data for the [arg 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.

[para]

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

[list_begin enum]
[enum] The string [const set], the [arg key], and the value.
[enum] The string [const unset], and the [arg key].
[list_end]

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

[para]

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

[para]

Another important part of the cache's behaviour, as it is asynchronous
it is possible that multiple [method get] requests are issued for the
same [arg 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.

[call [arg objectName] [method set] [arg key] [arg value]]
[call [arg objectName] [method unset] [arg key]]

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

[para]

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

[para]

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

[para]

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

[call [arg objectName] [method exists] [arg key]]

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

[call [arg objectName] [method clear] [opt [arg key]]]

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

[list_end]

[vset CATEGORY cache]
[include ../doctools2base/include/feedback.inc]
[manpage_end]
