Posted by Robert Janeczek on 10/21/05 11:13

Hi!

I`m currently working on PHP5-based extranet system which makes
intensive use of enterprise patterns. Some problems encountered during
developement cause serious haeadaches, but we`re going forward :)

I`m not 100% happy with architecture decisions we`ve made in the process
and therefore I came here to ask if maybe someone has better ideas than
we did.

Whole system is built on Data Mapper + Unit of Work + Identity Map +
Lazy Load base. Everything worked well until we started testing if lazy
loading is as lazy as it is supposed to be. We've used __set and __get
magic methods to automate loading data to ghost object when something
tried to access data inside. Yesterday was magic-methods-removal-day,
because we realized post-factum that using magic method sets a flag
blocking usage of another magic methods within that call. With high
encapsulation we were screwed - there was no way to assure not having
chained magic method calls (which still was far from being infinite
recurrency). __get returning null in such situations without any warning
caused many acts of disbelief in power of debugger among the developers
:] Yes, I know it`s all in comments to manual page. NOW I know that :]

Back to the architecture problem...

Inside mappers we have standard find($ID) methods returning ghosts of
real objects, which are filled at later time. Currently this mechanism
is implemented almost exacly as in Martin Fowler's sample code - by
adding notifyRead/notifyWrite in first line of public setters and
getters. This works nicely, collections of object also don`t cause
problems because they are not instantiated before we really need the
elements.

We also use separate methods for finding objects using criteria
different than just ID in database. So, some objects have
findByName($name) or findActive() methods which work very similiar to
find($ID) method, but use different queries. The problem with this
methods is the same thing which helps us very much in different parts of
application - Lazy Load.

Sample code:
$company = CompanyMapper::find(1); // we have ghost inside $company
//some other operations, but we don`t touch $company
$company2 = CompanyMapper::findByName('Big Ltd.');

Now let`s assume, that company with ID=1 has name Big Ltd. We have two
independent copies of the same object what kinda sucks :] The problem is
that both finding methods return ghosts and the second one doesn`t know
his ID yet. If there would be only ID-based searching than Identity Map
would detect it and caused returning the same instance, but it`s not...

Unfortunately, we cannot afford to loose lazy load for those searching
methods which use something different than ID. Our current solution is
putting every ghost object inside handler object, because it allows us
to switch object inside without rest of application noticing. This
however requires many classes with nothing more than bunch of methods
which transfer calls to another objects. We are working on piece of
code, which will use Reflection to autogenerate these classes, but still
it`s not very nice solution imho.

Long story short - what`s the best way to be able to load business
objects as ghosts using different criteria without problem with multiple
instances of the same object in the application.

rashid

ps. wow.. that was looong... :D

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