Subject: nsswitch implementation questions...
To: None <current-users@NetBSD.ORG>
From: Luke Mewburn <email@example.com>
Date: 01/16/1996 15:22:01
I'm in the process of working my nsswitch implementation into
NetBSD-current, and I have an issue that I'd like to resolve before I
go any further.
In the present implementation, an external program parses
/etc/nsswitch.conf into /etc/nsswitch.db, so that programs have fast
Some people aren't happy with this, so I'll outline the different
methods of interpreting the configuration file that I know of, and
the various pros and cons of each.
1. Parsing the file upon process invocation (e.g, Solaris <=2.4)
Pros: - easy to edit config file.
Cons: - need to restart daemons so that changes are picked up.
- if a process starts parsing whilst an editor is writing
out the file, it could read a partial config file.
2. Parsing the file at each lookup if it has changed (e.g, ULTRIX /etc/svc.conf)
Pros: - changes are picked up immediately
Cons: - slowest method due to parsing overhead
- potential to read partial config file (as above).
3. Talking via IPC to a daemon which restarts when a new parse is
necessary (e.g, Solaris 2.5's nscd)
Pros: - faster than method 1
- changes are picked up immediately? (not sure on this)
Cons: - need to have extra daemon running, with the various
overhead of ensuring that it's running, etc.
- same parsing race condition as previous two methods
4. Reading a pre-parsed database on each lookup (e.g, my implementation)
Pros: - fastest lookup method
- atomic update (via rename of temp file) of database
prevents race condition of other methods
- all processes pick up changes to methods when
database is updated - no need to restart
Cons: - requires user to run 'ns_mkdb' when update is made.
(which does, however, provide the atomicity
I prefer method 4 (of course ;). I feel that because the nsswitch
will be used all the time, it needs to be fast. That is also why the
passwd database was converted to a 'db' database in 4.4BSD.
Can anyone outline any Real Good Reasons why method 4 is inferior to
method 1 or 2? Note that ULTRIX's svc.conf, which uses method 2, has a
much simpler syntax than nsswitch.conf, and is much easier (and
quicker) to parse. I'm not even going to bother contemplate method 3;
I list it here for comparison only.
Luke Mewburn <firstname.lastname@example.org>
I'd rather be a UNIX wizard than a Web serf.