In GoF, the closest pattern should be "decorator", not chain of
responsibility.
oleg shteynbuk
<[EMAIL PROTECTED] To: [EMAIL PROTECTED]
rr.com> cc:
Subject: Architecture Guide: Chain of
Responsibility, Interceptor and Composite
27/06/2003 07:00 patterns
PM
Please respond
to axis-dev
Thanks for "Axis Architecture Guide" it was very useful to me.
Below is a feedback::
1) In "Message Flow Subsystem" -> "Handlers and Chains" under the
second diagram first paragraph:
"A Chain also has similarities to the Chain of Responsibility design
pattern in which a request flows along a sequence of Handlers until it
is processed. Although an Axis Chain may process a request in stages
over a succession of Handlers, it has the same advantages as Chain of
Responsibility: flexibility and the ease with which new function can be
added."
I don't believe that there are similarities to the Chain of
Responsibility design pattern. GoF implies that each handler in the
chain either handlers the request completely and chain stops there, or
if it could not handle the request then it passes the request to its
successor without doing anything. "The Design Patterns Smalltalk
Companion" book in chapter on Chain of Responsibility discusses OO
recursion and says that examples of Chain of Responsibility is hard to
find and considers Chain of Responsibility a specialization of
Object-Oriented recursion but we do not have recursion here, it looks
more like an iteration.
Flexibility and the ease with which new function can be added are also
different. In the Chain of Responsibility handlers are chained and each
handler has a reference to the next one and it is usually used if you
want to configure chain at run time. In Axis handlers usually are
configured at design time in deployment configuration and we have more
like AOP interceptors and some processing or weaving is needed before we
can use it.
Interceptors and AOP becoming increasingly popular, and a lot of
developers are familiar with it, did i mention JBoss. Interceptor would
be also more descriptive word then handler.
BTW theserverside.com recently has a discussion "Opinion: Interceptors
should be added to J2EE".
it make sense to replace the Chain of Responsibility paragraph with:
"Handlers act like interceptors, the Interceptor pattern is in POSA 2
book."
2) The sentence before the same diagram in "Message Flow Subsystem"
-> "Handlers and Chains"
"A Chain is a composite Handler, i.e. it aggregates a collection of
Handlers as well as implementing the Handler interface as shown in the
following UML diagram:"
and after the diagram it make sense to add the following sentence to
clarify relationship to the GoF Composite pattern.
"Interface Chain acts like a Composite and interface Handler like a
Component from the GoF Composite pattern. In GoF Component is a common
abstract class; in Axis Handler is an interface that Chains and Handlers
implement so they could be treated uniformly."
I believe that Composite pattern usage is related to item 4 in the "Open
Issues" chapter.
3) The Iterator pattern could be mention too and there are other
patterns that are used internally in Axis. Maybe it make sense to have
two Architectural Guides, one simplified version for Axis users and
another one with all patterns for Axis developers.
oleg
