dbm locking info in the guide

[Perrin Harkins <pe...@primenet.com>]

While working on adding info on Berkeley DB to the Guide, I came across
this statement:

"If you need to access a dbm file in your mod_perl code in the read only
mode the operation would be much faster if you keep the dbm file open
(tied) all the time and therefore ready to be used. This will work with
dynamic (read/write) databases accesses as well, but you need to use
locking and data flushing to avoid data corruption."

Is anyone aware of a safe to way to do multi-process read/write access
through a dbm module other than BerkeleyDB.pm without tie-ing and
untie-ing every time?  I thought that was the only safe thing to do
because of buffering issues, but this seems to be implying that careful
use of sync calls or something similar would do the trick.  Maybe this is
just left over from before the problem with the old technique described in
the DB_File docs was discovered?  Any comments?

- Perrin

Re: dbm locking info in the guide

[Stas Bekman <st...@stason.org>]

On Mon, 19 Mar 2001, Perrin Harkins wrote:

> While working on adding info on Berkeley DB to the Guide, I came across
> this statement:
>
> "If you need to access a dbm file in your mod_perl code in the read only
> mode the operation would be much faster if you keep the dbm file open
> (tied) all the time and therefore ready to be used. This will work with
> dynamic (read/write) databases accesses as well, but you need to use
> locking and data flushing to avoid data corruption."
>
> Is anyone aware of a safe to way to do multi-process read/write access
> through a dbm module other than BerkeleyDB.pm without tie-ing and
> untie-ing every time?  I thought that was the only safe thing to do
> because of buffering issues, but this seems to be implying that careful
> use of sync calls or something similar would do the trick.  Maybe this is
> just left over from before the problem with the old technique described in
> the DB_File docs was discovered?  Any comments?

Well, I wrote this based on my experience. I've used the code that does
locking coupled with sync() and it worked fine. I know that the guide
doesn't cover the details, this is one of the things that needs to be
done.

I also suppose that this issue should be properly benchmarked, but I think
that it's safe to assume that skipping tie/untie improves the speed.

Certainly the note "would be much faster,..." is correct if the
interaction with a dbm file is very light, which makes the overhead of tie
of a significance.

_____________________________________________________________________
Stas Bekman              JAm_pH     --   Just Another mod_perl Hacker
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