Re: [Prevayler-discussion] Question about method prevayler.execute

[Justin T. S. <ju...@kr...>]

Yes, that's right. You're seeing Prevayler's rollback support in
action. The mechanism is known as the "food taster" (or sometimes
we call it the "strict transaction censor"). When this feature is
turned on, every transaction is executed twice -- once on a shadow
copy (the "food taster") of the prevalent system, and then again
on the live system (the "king"). It is only journaled once. If a
transaction throws a RuntimeException when executed against the
food taster, it is not journaled at all, and it is not executed
against the king, and the food taster itself is rolled back to the
current state of the king (by deep-copying the king).

This feature is active by default. It can be turned off for
performance (or conceptual purity) using the
setTransactionFiltering method of PrevaylerFactory. But it is very
important to understand what it is doing, and to understand that
you must not in general perform any externally-observable actions
during transaction execution, even when the food taster is turned
off.

Cheers,
Justin

P.S. All this should be on the website of course, which we're
getting back up and running real soon now!


On 8/9/06, blackboy zabaha <bla...@ya...> wrote:

> Hi,
> When I test my app, I found something I don't
> understand, at first I
> think I make the mistake my own, but when I try on the
> demo, it's result
> the same thing.
> I found that when I call
>     prevayler.execute(new
> NumberStorageTransaction(primeCandidate));
>
> this look like that it will always call
> NumberStorageTransaction.executeOn(Object
> prevalentSystem, Date ignored)
> 2 times (I just print out some text there.), is this
> intend to be so and
> nothing to worry about (but I think this might affect
> logging)? or I
> misunderstand something?
>
> ...
> Primes found: 820. Largest: 6301
> executeOn..
> <-------------  I log
> in NumberStorageTransaction.executeOn
> keep....
>  <-------------
> I log in NumberKeeper.keep
> executeOn..
> <-------------  the
> 2nd of NumberStorageTransaction.executeOn!!!
> keep....
>  <-------------
> the 2nd of NumberKeeper.keep!!!
> Primes found: 821. Largest: 6311
> executeOn..
> keep....
> executeOn..
> keep....
> Primes found: 822. Largest: 6317
> ...