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From Michael Dürig <mdue...@apache.org>
Subject Re: enter() method for JCR implementation classes
Date Wed, 27 Feb 2013 09:51:32 GMT


On 20.2.13 21:36, Jukka Zitting wrote:

>> The problem is that SessionDelegate.needsRefresh() requires clean up after
>> the "real" method returns.
>
> We could refactor the code so that such clean up is not needed.
>
> AFAICT the mechanism is currently used to avoid repeated refreshing of
> the session from nested operations.

The problem is actually more subtle than that. The mechanism avoids 
refresh being called from reentrant calls to any session scoped 
operation (that's the reason for using a counter in perform). Otherwise 
we risk that a session is refreshed while we are in the middle of a 
session scoped operation. This can lead to very unpredictable test 
failures and bugs.


An alternative design would be to
> turn needsRefresh to a flag in SessionDelegate that could be raised by
> observation and other things like that. The enter() method would then
> trigger a refresh and clear the flag, which would prevent further
> nested operations from repeatedly refreshing until the flag gets
> raised again.

This would already work using ObservationManagerImpl.hasEvents(), which 
is exactly such a flag. However, it does not prevent us from the 
reentrant case I described above.

>
>> [...] This will at the same time address the
>> issue I brought up at our last meeting that implementing JCR method in terms
>> of each others will result in the preconditions being checked multiple times
>> instead of just once.
>
> The delegate pattern should allow us to avoid these cases. Instead of
> JCR methods calling each other, they could be calling the relevant
> delegate methods directly.

Yes and it would also solve the reentrance problem. It would however 
mean that we need to strictly disallow for any JCR session scoped method 
to call any JCR session scoped method. I'm not sure how to reliable 
communicate this since not adhering to it wouldn't immediately fail but 
rather show up in some unpredictable behaviour probably much later.

Michael

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