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From Thomas Müller <thomas.muel...@day.com>
Subject Re: [jr3] Synchronized sessions
Date Thu, 25 Feb 2010 21:24:13 GMT

> http://issues.apache.org/jira/browse/JCR-2443.

Unfortunately this bug doesn't have a test case. Also I didn't find a
thread dump that shows what the problem was exactly. I can't say what
was the problem there.

Observation is definitely an area where synchronization can
potentially lead to deadlocks. Maybe observation needs to use its own
session(s) so that it can't block. This is not a new issue however:
most writes are already synchronized (not all writes however).

I'm hesitant to change synchronization with the current
implementation: doing that would very likely lead to Java level
deadlocks. We need to make sure synchronization is always done on the
same level, and in the same order. With the current implementation,
that's challenging.

Of course performance and concurrency is very important. But the
current approach (mutable data structures, some writes are
synchronized) is quite dangerous. Instead, immutable data structures
should be used, at least for values and objects in the shared cache.
Everything else should be properly synchronized if mutable, or - if
that's too slow - the proper data structures should be used, for
example ConcurrentHashMap, CopyOnWriteArrayList, CopyOnWriteArraySet.


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