NAME
Autocache - An automatic caching framework for Perl.
SYNOPSIS
use Autocache;
autocache 'my_slow_function';
sub my_slow_function
{
...
}DESCRIPTION
This code came about as the result of attempting to refactor, simplify and extend the caching used on a rather large website.
It provides a framework for configuring multiple caches at different levels, process, server, networked and allows you to declaratively configure which functions have their results cached, and how.
Autocache acts a lot like the Memoize module. You tell it what function you would like to have cached and if you say nothing else it will go ahead and cache all calls to that function in-process, you just specify the name of the function.
In addition to this though Autocache allows you to specify in great detail how and where function results get cached.
The module uses IoC/dependency injection from a configuration file to setup a number of Stores and Strategies. These are the basic building blocks used to determine how things get cached.
Strategies determine how a cached value should be validated, refreshed, and even whether or not the value should be stored at all.
Stores provide the actual storage for values and also specify how values get evicted from stores, if at all.
The goal here is to make it stupidly simple to start to cache certain functions, and change where and how those values get cached if you find they're in the wrong place.
CONSIDERATIONS
There are a number of considerations when using autocache, or any caching mechanism.
PURITY
Any function that is pure should have no trouble being cached.
A pure function being one;
whose value depends soley on the parameters passed to the function and no other global information, state or input from IO or external devices.
generates no side-effects (although, depending on your use, side-effects may be allowable).
If your function does depend on external state then you may or may not be able to use some form of caching. For example if your function depends on one of a number of states that may be the current one then you can always create a new function that does depend on all of that information and make the current function simply a driver for it.
For example, the function below depends on the state of a global variable $mode.
autocache 'authorised';
sub authorised
{
my ($user,$resource,$action) = @_;
if( $mode eq 'normal' )
{
...normal mode code...
}
elsif( $mode eq 'strict' )
{
...strict mode code...
}
else
{
...all other mode code...
}
}If this function is cached then autocache will cache the value generated for whatever the $mode variable is set to at the time of the first invocation, if this function is invoked again later on it may produce incorrect results since the value of $mode has changed.
We can still gain a speedup through caching but we have to rewrite it slightly to make sure we're caching a pure function.
autocache '_authorised_by_mode';
sub authorised
{
my ($user,$resource,$action) = @_;
return _authorised_by_mode($user,$resource,$action,$mode);
}
sub _authorised_by_mode
{
my ($user,$resource,$action,$mode) = @_;
if( $mode eq 'normal' )
{
...normal mode code...
}
elsif( $mode eq 'strict' )
{
...strict mode code...
}
else
{
...all other mode code...
}
}Now even though the authorised function is still dependant upon the global $mode variable the new _authorised_by_mode function is entirely dependant upon it's input parameters and nothing more and it can be cached.
NORMALISATION
Function results are cached based on the arguments to the function. For functions whose arguments are position dependant and are simple values autocache should simply do the right thing.
Two cases where autocache will require help are when the arguments to a function are provided through a hash, or more complex data structure and when objects/references are passed around that are equivalent but where the identities of the references are not.
For example, if a function 'fn' accepts a hash containing one or more parameters named 'a', 'b', 'c' and 'd' then the following calls are equivalent but autocache can't tell that.
fn( 'a', 3, 'd', 4 );
fn( 'd', 4, 'a', 3 )To overcome this you can provide a normalisation function that takes the parameters that are passed to the function and provides a canonical string version that ensures equivalent calls appear to be the same to autocache. Obviously care should be taken when designing a normalisation function where the inputs may be large. (TODO - cookbook for normalisation)
To allow autocache to automatically pick up your normalisation function it should be named the same as the function it provides normalisation for but prefixed with '_normalise_'.
For the above function we could use something like this;
sub _normalise_fn
{
my (%hash) = @_;
return join ':', map { "$_=$hash{$_}" } sort keys %hash;
}CONTEXT
Perl functions are called in one of three contexts.
scalar
list
void
Autocache automatically maintains seperate caches for each of the first two contexts that functions may be called in. Since it expects functions to be pure it understands when a function is called in void context and does nothing at all since the value will not be used.
TODO - add option to merge all contexts into one if the author knows that the same value is returned in either scalr or list context.
ARCHITECTURE
Autocache splits up the process of caching into generating the values and storing the values.
Values are generated using Strategies, which may be chained. Some examples of Strategies are Simple, Refresh and CostBased.
Values are stored using Stores. Some examples of Stores are UnboundedMemory, BoundedMemoryLRU and Memcached.
The API for both Strategies and Stores is not yet completely fixed but you should be able to quite easily take one of those that already exists and modify it to suit your needs. The configuration syntax allows you to use any custom classes you like as long as they can accept the way we perform IoC (sub-optimal right now).
TODO
Test, test, test.
BUGS
Loads, and adding more all the time. This code is yet to become stable.
LICENSE
This module is Copyright (c) 2010 Nigel Rantor. England. All rights reserved.
You may distribute under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.
SUPPORT / WARRANTY
This module is free software. IT COMES WITHOUT WARRANTY OF ANY KIND.
AUTHORS
Nigel A Rantor - <wiggly@wiggly.org>
Rajit B Singh - <rajit.b.singh@gmail.com>