Re: Safety of unprotected concurrent operations on constant objects

["J-P. Rosen" <rosen@adalog.fr>]

Le 07/05/2014 09:30, Dmitry A. Kazakov a écrit :
>> The only constraint on the user is to call Begin_Transaction at the
>> > beginning, and End_Transaction in the end, which is quite acceptable for
>> > database transactions!
> I tend to have a controlled transaction handler, which rolls back upon
> finalization [unless there was explicit Commit].
Sure, I said it was a simplified view. In practice, Begin/End
transactions are part of a limited_controlled object declared in the
callback procedure  of AWS.

> P.S. It would be nice to have a mandated way to detect if Finalize was
> called with an exception being propagated. E.g. the pending transaction is
> committed in Finalize when there is no exception and rolled back otherwise
> There is a way to do this, but it is GNAT-specific.
Since everything is handled by the task, it is easy to have a local
flag, and when you close the transaction do a rollback unless the
previous order was commit. (not the way we do it in Aegis, but for other
reasons).

> P.P.S. Monitor is not always safer, not even always possible, because some
> clients require a transaction bound to the single task. I.e. you could not
> start several transactions in the monitor on behalf of several tasks.
I don't understand your remark here. It is precisely because SQLite did
not seem to be able to handle transactions from several tasks
(concurrent or not) that I delegated all the DB interface to a single
task. This serializes all requests, but it is no big deal in our case,
since a transaction is never longer than the time to build a web page.

-- 
J-P. Rosen
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