GD global data array capabilities in pltcl

What would be the limitations of using the GD (global data) array in 
pltcl stored procedures ?

I mean, if I'll keep there a big cache for a warehouse using

set GD($itemId,quantity) $computedQuantity
...
then elsewhere

return $GS($itemId,quantity)

would be a problem for let's say , 15000 articles?

How would work the GD array in a heavy multi-user application?
I presume that it is shared among any backend processes, isn't it?

Are there some locking mechanisms?

In other words, is the GD array appropriate to implement some caching 
features for a heavy used application in PostgreSQL?

If not, are there any other sollutions (before going to EJB's and stuff) ?

Thanks in advance,
Constantin Teodorescu
Braila, ROMANIA