Subject Database corruption...
Author Jonathan Neve

I'm posting this here instead of in the FireBird-Architect group,
because I posted it there this morning, and it still hasn't showed up...

Could anyone please give me a detailed explanation (or perhaps a paper)
about the way FireBird stores data? This may sound a bit vast, but what
I'm interested in specifically, is understanding how, when an update is
performed, it is written to the database, and how exactly the
multigenerationnal system works. If I perform a select, how does
Interbase know which version of the record to show me, etc?

The reason I would like to understand this more fully, is because I'm
wondering how it would be possible to make FireBird 100%
corruption-proof. In other words, how can we make every single disk
operation atomic? Naturally, there is already a degree of atomicity due
to the transaction system, but what would be good, would be to make
every single write atomic; that is, if it gets cut off in the middle,
have some flag somewhere, or something that can tell us that the write
wasn't completed, and that it can therefore be treated as though nothing
had happened at all.

For instance, if I perform several inserts into a table of my database,
within the scope of my transaction, these inserts are atomically grouped
together, and I either commit them all, or roll them all back.
Therefore, if the client disconnects abruptly after the first insert,
everything will get neatly rolled back. However, if, during one of the
inserts, there's a power failure, the record will only partially be
written to disk, resulting in an incorrect structure, worse than if
nothing had been written at all. We need to have a way of telling that a
certain operation was not completed successfully, and can therefore be

Of course, I also know that there is an option called ForcedWrites,
which, if it is used (which of course, it is on all my databases), makes
the database perform synchronous writes instead of asynchronous writes.
This helps a lot of course, because it greatly reduces the likelyhood of
there being unwritten data pending in the event of an abrupt shutdown.
However, it's not good enough in many cases. It's still theoretically
possible to have corruption due to a power failure. Also, a badly coded
SP of trigger can make the database to loop indefinately. In such an
event (I've had this happen to me), the only solution is to kill the
server process (killing the client isn't enough). If the SP was
performing database writes, and not mere reads, it's very likely to
corrupt the database, for obvious reasons (this is what happened in my
case). And of course, there are other ways of corruption a DB as well.

Do you think it would be possible to design the system in such a way
that every write is entirely atomic? And if so, would this entirely
solve the problem, or am I missing something. It seems to me that in
such a case, we could even afford to turn ForcedWrites off, without it
affecting the stability of the database at all. Unfortunately however,
my current understanding of the internal structure of the database is
too limited to enable me to seriously speculate in this area...

Any ideas?

Jonathan Neve.