Re: Decorator Pattern - Can I use it in this case?

Responding to Weboweb...

I have the following classes:

----------------------------------------------------
Folder (class that will need logging)

File (class that will need logging)
----------------------------------------------------

Logging structure:
----------------------------------------------------
AuditBase
projectID
name

AuditFolder (inherits from Base)

AuditFile (inherits from Base)
size
serverName
----------------------------------------------------

I want to add logging functionality to both Folder and File objects.
They will both inherit from lets say "TreeItem" class.

I was thinking of using an AuditDecorator class to take a TreeItem as
a parameter in the constructor.
However, as you may see, the AuditBase class does not contain all the
properties that need to be logged. For example, AuditFile has the
size and serverName properties that would need to get logged for a
File object.

First, I am confused by the names used. Is File a problem object whose
changes need to be logged, as it seems to be in the description
immediately above)? AuditFile sounds more like an object that exists to
accept change messages and write them out to a persistent data store
(e.g., a log file).


A File object could basically be deleted, modified, downloaded,
uploaded, etc.
A Folder object could be deleted, renamed, its access modified, etc.

So are File and Folder different objects than AuditFile and AuditFolder?

Second, I am not sure things need to be that complicated. In principle
logging is pretty simple. When something changes a message is generated
and sent to a persistent store or somebody who cares about the change.
Either way, there is <usually> one "processor" of the audit information
and many client objects who have to notify that "processor" when they
change.


In this case it would not be automatic. In some cases the changes
would need to be logged, in other cases not.

OK, that just means there is some condition on sending the message (or there is some context information in the message or other objects that "processor" can access that the "processor" can use to ignore the message).

Usually the complicated part is figuring out a message format that is
easy to encode and interpret while being fairly generic. For example,
with a message format of {message ID, object ID, attribute ID, new
value}, one needs an identity scheme that allows 'new value' to be
interpreted properly for a variety of memory formats or data aggregates.

In a simple situation the "processor" might have a single logInfo
(message ID, object ID, attribute ID, value) interface method that is
invoked directly from the attribute setter of the object:

Folder::setName (name)
this.myName = name
myLogger.logInfo(FOLDER, this, NAME_CHANGE, name)

where one uses operator overloading in the Logger class to deal with
different attribute memory formats.

So my push back is: what sort of problem complexity do you have that
seems to warrant something as complex as Decorator?


In your example, the Folder class has to be aware of the Log class
(the myLogger object). I was trying to avoid the File / Folder class
being aware of any Log class.
Or to put it in another way,I am trying to not include the Log
functionality in the File / Folder classes, but just "adorn" some of
their objects at runtime if need be.

There's no free lunch. An announcement needs to be sent to somebody when something changes in Folder. That message needs to be addressed to whoever cares about the change and the 3GL type systems require substantial physical coupling to do that. [Alternatively, whoever changes Folder could also send the logging message. But that would be more fragile and one would still have the message addressing problem; it would just be in a different object.]

Also note that this is somewhat backward from what Decorator provides. Decorator allows one to add properties to an object dynamically that other objects can then access. But Folder isn't changing dynamically and nobody is accessing Folder for the logging; it initiates the message sending.

I really don't see why this shouldn't be a garden variety collaboration between Folder and whoever handles the logging details. The logical decoupling is achieved by encapsulating the logging details in the implementation of the "processor" object. Then Folder only needs to know what information to send and when to send it, which both seem intimately related to change within Folder.


Another pattern that comes to mind is a Mediator, which would know the
connection between the File / Folder class and some Log class which
actually logs the changes to some data store.

One uses Mediator when an object needs to talk to a lot of different objects about the same thing. In your case a lot of objects talk /to/ "processor", but "processor" is the only object they need to talk to for a logging collaboration.

Note that almost all of the GoF patterns, including Decorator and Mediator, address the same megathinker problem:

1 R1 collaborates with 1
[Client] ------------------------- [Service]

However, the behavior that [Service] actually provides is substituted dynamically based on the context of the collaboration. That dynamic complexity is too complex to try to capture in a single static association in the OOA/D. So the GoF patterns typically apply a combination of delegation and polymorphic dispatch to be more specific about how the substitution works. Thus the various GoF patterns represent different common contexts where such dynamic substitution is needed in a basic client/service collaboration.

My push back here is that I don't see the sorts of dynamic substitution issues in this problem that the GoF patterns resolve. File, Folder, et al simply collaborate with somebody who knows how to do audit trails. They send exactly the exactly the same message to exactly the same place for a particular kind of change every time.

[Having said that, I can certainly envision auditing situations that are more complex where the "processor" needs to be very smart and may have to gather additional context data. In that case the messages generated by Folder et al just trigger complex processing that might involve multiple objects or even an entire subsystem. Then one might encapsulate all that behind a Facade pattern that Folder et al talk to.]


*************
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H. S. Lahman
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Pathfinder Solutions
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