Re: ACIDity of Prevayler 2.6

It's exciting to see such a passionate discussion. :) And also good that
it's coming around to some more practical questions.

First of all, the ACID properties are ill-defined, incomplete, and
redundant, so ultimately we shouldn't take them too seriously. Yes, A and C
have some overlap, as do A and I, C and D, etc. They are a nice set of
reminders of things to talk about regarding transactions in a persistence
layer, though. And I still maintain that rollback is just one potential
mechanism to partially address some of these properties, not a fundamental
requirement for any of them.

As for journaling transactions after executing them, that would cause
queries to have the same latency as transactions, which is a pretty big
departure from Prevayler's normal performance profile. It would hardly even
be Prevayler anymore. ;)

Paul wrote:

> Klaus, you haven't answered my question. As far as I can see,
> without something to maintain consistency in the face of an
> unexpected exceptions, once that happens, the system is
> inconsistent and cannot proceed *for all users*. You must
> either a) shut down and fix the bug or b) remove the txn from
> the journal.
>
> So unlike an ordinary db, which would isolate the exception
> to a single user, and let others continue to use the
> non-problemmatic parts of the system, an exception kills the
> whole thing. That's unacceptable to me.

Why does it kill the whole thing?

The point that Klaus and I are making is really that you have to worry
about the kind of situation you're describing even if you do have a
rollback mechanism. I've seen plenty of bugs in relational database
applications that caused inconsistent data without throwing exceptions. At
some point a user notices something wonky, or some feature starts breaking
or throwing exceptions later due to the bad data.

When such inconsistencies happen, it doesn't typically mean that the system
is suddenly unusable for everyone. Maybe one entity is inaccessible, or
some calculations display incorrectly, or one user can't log in. Bugs that
take down an entire application are pretty rare, in my experience, but of
course they do occasionally happen -- again, even in systems with rollbacks.

I haven't so far addressed the question of what to do in a Prevayler-based
application when bad data creeps in; I've just clarified that you need to
be prepared for bad data regardless of rollbacks. In the worst case,
whatever your persistence layer, there's always the option to restart the
system from a nightly backup. For smaller fixes, a relational database does
provide the convenience of a built-in scripting language (some variant of
SQL) with an interactive interpreter; in a similar vein, some Prevayler
users have talked about using something like BeanShell or some other
JVM-based scripting language as a superuser feature in an application,
allowing an arbitrary script to be submitted as a transaction in a running
system. The purer approach would be to write a new fix-up transaction in
Java (possibly even with unit tests!) and either redeploy or hotswap the
app in order to execute it on the running system.

As for the history of dropping the food taster, it happened in this thread
last November, with not nearly as much excitement as in our present thread:

http://sourceforge.net/mailarchive/forum.php?thread_name=CAGs7EcU_X1%2BTE3O-CjCje2gsSoxxjnGTm8UZWb6ZkORE7%3Do77A%40mail.gmail.com&forum_name=prevayler-discussion

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