[Glass] Concurrency Pattern - Delete an object if a collction it contains is empty, how ?

[Martin McClure]

Marten,

It sounds like you're describing a condition where there is a semantic 
conflict, even when the conflicting sessions do not commit changes to 
the same object.

In order for GemStone to report a transaction conflict in cases like 
that, you need to have both sessions modify the same object. I will 
refer to this object as the "conflict object." This could be one of the 
objects you already have, or you might have to create it just for that 
purpose, depending on the situation.

If I understand what you've described correctly, you have an overall 
collection holding objects. Each object holds a set of attributes.
You want a transaction removing the object from the overall collection 
to conflict with a transaction adding an attribute.

If this is an accurate description of your situation, one choice would 
be to use the object itself as the "conflict object." If you modify the 
object every time you add an attribute to it, and also modify the object 
when it's removed from the collection, then concurrent transactions 
doing those operations will conflict.

IIRC, modifying an object by assigning an instvar to itself like

someInstVar := someInstVar.

is sufficient for the object to be considered written for the purposes 
of transaction conflict detection.

By using this kind of pattern, these cases *can* be handled. But when 
you run into these cases, it's worthwhile to examine your overall 
architecture to see whether there's a change that would avoid needing 
this kind of complexity, since code acquires a certain amount of 
fragility when it's required to use these patterns.

Regards,
-Martin