Actually, the second one is basically it. (Except that I don't know the Oracle 
reference, so can't speak to that.)

Stu

> The first one is correct.
> 
> On Tue, Sep 14, 2010 at 3:22 PM, peter veentjer <alarmnum...@gmail.com> wrote:
> I have got a question about the Clojure ensure and how it actually
> works and the documentation doesn't provide enough information.
> 
> I see a few different solutions:
> 
> 1) An optimistic approach: Once a ref is 'ensured' it is included in
> the conflict detection set. This means that the approach is still
> completely optimistic because 2 concurrent transactions that have the
> same ensured ref, one of them is going to fail when the transaction
> wants to commit. This is a commit time ensure.
> 
> 2) The other approach is a more pessimistic approach where a ref can
> be protected against writes made by other transaction as soon as it is
> ensured. It still allows reads to happen, but a write/ensure is going
> to fail. It could be compared to a oracle 'select.. for update'. This
> is an encounter time ensure.
> 
> 
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