[jbosscache-dev] Handling invalid nodes

[Sanne Grinovero]

Hi All,
I have been reading some time but please consider my opinion as a
complete newbie.

Still I have some experience with concurrency and would like to
highlight your attention
on the patterns used in the JDK util.concurrency; I use them often and
they provide a very clean approach
for this sort of problem:

the AtomicBoolean (for example) has
boolean compareAndSet( boolean expectedValue, boolean newValue)
the ConcurrentHashMap has
boolean putIfAbsent(Object key, Object value)

they all return false if the condition required to set the new value
is no longer valid,
so I think a clean approach would be to define the Node manipulation API
as "boolen setIfValid( ... ) and return false when the Node is no longer valid,
so a retry code could be written in clean and readable code.

This way you avoid node locking, and also you have fail-fast: if I had
to mark a node
as "do not evict" it means I'm doing some useless work, as far as I
have understood
it the result will be thrown to garbage as soon as we finished working
on the Node,
I think it would be preferrable to avoid the work at all or write it
to the most current Node.

(please just let me know if I wrote stupid things, as I said I still don't know
the internal wroking of the cache)

regards,
Sanne Grinovero

2008/5/16 Elias Ross <genman at noderunner.net>:
> On Thu, May 15, 2008 at 3:52 PM, Manik Surtani <manik at jboss.org> wrote:
>
>> 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?
>
> Something feels wrong about using exceptions as message passing events
> and flow control.
>
> You should be able to check if a node is invalid before that exception
> gets thrown. And if you do a retry, it seems possible to get stuck in
> a loop: Exception, refresh succeeds, but immediate eviction again,
> then exception again, etc. Maybe it's really unlikely, but it'd be a
> bummer.
>
> Wouldn't an evict require a write lock? If so, then this couldn't happen.
>
> Or do this as part of the interception:
> 1. Pre-fetch node, mark as "do not evict", then check again if node is
> valid (would have to use a volatile flag I think)
> 2. Do your business
> 3. Unmark
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