[jbosscache-dev] Handling invalid nodes

[Manik Surtani]

Guys,

I've been thinking about behaviour with the Node API, and wonder what  
works best from a user standpoint.  Consider:

* User gets a hold of a node and holds on to the ref for performance  
reasons (Node n = cache.getNode(fqn))
* Performs tasks on the node: n.put(), n.get(), etc.
* Node checks for validity before performing any API calls, throwing a  
NodeNotValidException if the node is not valid anymore.

Nodes get marked as invalid if they are removed or moved by a  
concurrent thread, or invalidated by a remote one.  Eviction did not  
mark a node as invalid, but I think this is inconsistent and a bug:  
eviction too should mark the node as invalid.

So the question is this - with potentially likely events marking a  
node as invalid - especially with eviction - should we throw a  
NodeNotValidException?  Since (as of 2.1.0), the Node implementation  
that a user has is an instance of NodeInvocationDelegate which in turn  
has a reference to an UnversionedNode, perhaps we should handle the  
NNVE within the NodeInvocationDelegate and perform a lookup internally  
and refresh the UnversionedNode reference, and THEN throw a NNVE if we  
can't fetch the node?

Th drawback is that this is potentially two calls for what should be  
one - node gets evicted, we call cache.getNode() internally to refresh  
the reference (from a cache loader, etc), and then invoke whatever  
call the user intended to call up the interceptor chain.  The benefit  
is a smoother user experience.

Thoughts/opinions?

Cheers,
--
Manik Surtani
Lead, JBoss Cache
manik at jboss.org