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From: evans@decvax.DEC.COM (Marc Evans)
Newsgroups: comp.lang.perl
Subject: Re: locking dbm files
Keywords: dbmopen, dbmclose, flock
Message-ID: <21131@shlump.nac.dec.com>
Date: 15 Mar 91 12:50:43 GMT
References: <21090@shlump.nac.dec.com> <1991Mar14.205242.23220@convex.com> <125585@uunet.UU.NET>
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Reply-To: evans@decvax.DEC.COM
Organization: Synergytics
Lines: 42

In article <125585@uunet.UU.NET>, rbj@uunet.UU.NET (Root Boy Jim) writes:
|> In article <1991Mar14.205242.23220@convex.com> tchrist@convex.COM (Tom Christiansen) writes:
|> ?From the keyboard of evans@decvax.DEC.COM:
|> ?This is certainly true.  dbm doesn't do locking.  To do record locking
|> ?would be hard.  You could easily enough protect it with file
|> ?locking though, but you should do this in your own code.
|> 
|> Record locking might well be impossible, dunno.
|> 
|> File locking is easy, unless, of course you are using NFS.
|> 
|> Another thing to do is run a database demon and ask it to do things for you.

Let's assume for the moment that I'm not using NFS (even though I am in reality,
and to make things even worse, locking is done because many machines want write
access to this dbm database).

In perl, dbmopen doesn't use a FILEHANDLE, and flock requires a FILEHANDLE.
We therefore end up with something like the following:

	$dbname = "foo";
	open(LOCKDB,"$dbname.pag");
	flock(LOCKDB,2);
	dbmopen(%foo,$dbname,0666);
	[...]
	dbmclose(%foo);
	flock(LOCKDB,8);

It really seems ugly to me that the script needs to understand dbm naming
conventions to perform the locking. Also, think about the exception handling
that really should be added to the above for the cases of open, flock and dbm*
functions failing. Not very pretty.

However, it would appear that short of creating a network dbm data server, this
technique is probably my best option. Any other suggestions?

- Marc
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