Re: Thats How Netscape does it!

["Marcus J. Ranum" <mjr @ iwi . com>]

>We would also like to load balance our web server like this and 
>have one address that could connect a user to any of 5 different 
>machines (least heavily loaded one, or some cyclic order). 

	The usual way of doing this is to use a rotating DNS record,
and have N machines, with N addresses in the DNS. Then you hack up
some code (I have some that does this) which keeps the machines
heartbeating to eachother, and if one of the N is down, you have
a backup machine quickly take its IP address *too* for a while.
BSDI and most decent versions of UNIX let you have multiple IPs
for a single interface.

	The idea of trying to calculate which is least heavily
loaded has relatively little merit, I expect. For a REALLY large
installation, luck-of-the-draw is going to tend to produce a
pretty smooth load level across all the systems. If you are
fielding 2,000 requests/minute, and have 10 machines, that's
200 requests/minute, and if you use luck-of-the-draw to let the
load balance itself, one machine might have to handle 250 hits
(which is actually a pretty large deviation - 10% off average).

	I wouldn't bother getting fancy until I had some hard
numbers that indicated getting fancy would actually benefit
you! Don't *assume* that a naive solution is inadequate: be a
scientist and *test* it, or model it.

	For some really *COOL* info on building monster web
servers, I highly recommend a paper by Dan Mosedale, which was
presented at LISA '95:
http://home.netscape.com/people/dmose/paper.ps

mjr.