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Hi Joe,
We're on parallel track somehow (for a cache
mechanism, not a design pattern), although my
implementation is very simplistic.
I have to confess that I can't really get into your
logic, don't know what the GenericCacheFactory does,
but it seems very impressive from my newbie point
of view :o)
Are you using the application, server or session
scopes? How do you implement it ?
I created a simple XML schema, that represents a
config class, when system starts I load it and
use the values (applicationName, timeout etc)
in an application tag that I store under the constructor of my cache mechanism.
Why would you want to have a sizedCachedCollection
separated from the timedCachedCollection ?
I understand the need to separate at maximum
each class, but can't you have the size
stored under some cache property on the genericCachedCollection.
A cacheCollection should know it's size and should
count the elements it holds, shouldn't it ?
Regards,
----- Original Message -----
Sent: Monday, January 10, 2005 1:26
PM
Subject: [CFCDev] Cache design using
decorators and factories - advice/criticism?
Hey folks,
I need a super-generic container in which to
keep stuff, i.e., a
cache. I need to be able to have the cache manage
its items either
based on cache size (can't grow above N items), or time
(if I ask for
an item that's past it's expiration, a specialized error is
thrown).
My first attempt at this taught me some of the evils
of
over-inheritance - I had a GenericCache that just held stuff, with
two
subclasses - TimedCache and SizedCache. As the complication level
of
some things grew, they got pretty unruly: sized cache needed
things
in the base that timed didn't, timed needed things in the base
sized
didn't, and things got tangled.
Now, I've tried to break this
apart into a cleaner system using
decorators and factories. My design
consists of the following CFCs:
GenericCacheItem - An item in the
cache
GenericCache - decorated by a GenericCacheCollection (or
subclass)
GenericCacheCollection - decorated by a
GenericCacheCollectionManager
(or
subclass)
GenericCacheCollectionManager - single method,
cacheContains(key).
When GenericCacheCollection.get(key) is called,
it first calls
cacheContains(key), which determines whether or not
the item is still
valid, and if not, removes it from the cache
I've
built a GenericCacheFactory that builds a GenericCacheCollection,
decorates
it GenericCacheCollectionManager, builds a cache, and
decorates it with the
GenericCacheCollection that's just been built.
To enable a sized cache,
I subclass GenericCacheCollection with
SizedCacheCollection, which adds
logic to the put() method that'll
remove the oldest item from the cache if
the cache is above a maximum
size (set in SizedCacheCollection's init()
method). I then built a
SizedCacheFactory that builds a
SizedCacheCollection, decorates it
with a GenericCacheCollectionManager,
builds a cache, and decorates it
with the SizedCacheCollection I've just
build.
To enable a time-based cache, I subclass
GenericCacheCollectionManager
with TimedCacheCollectionManager that's told
about a cache item's
lifespan in its init() method. The factory then
assembles these like
the GenericCacheFactory, decorating the collection
with the TimedCache
manager instead of the GenericCache
manager.
Does anyone have any advice/critique/etc. on doing things like
this?
Is this a "proper" implementation of using the decorator
pattern?
The end result, regardless, is pretty cool...I just create
the
concrete factory for the type of cache I need, call its
createCache
method, and it gives me the correct, decorated
GenericCache.
-Joe
--
For Tabs, Trees, and more,
use the jComponents:
http://clearsoftware.net/client/jComponents.cfm
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