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From Theo Schlossnagle <>
Subject Re: Working on some load balancing methods
Date Sat, 08 Jan 2005 17:34:21 GMT

On Jan 7, 2005, at 4:14 PM, Sander Striker wrote:

>> From: Jim Jagielski []
>> Sent: Friday, January 07, 2005 8:52 PM
>> To:
>> Subject: Working on some load balancing methods
>> I'm currently working on code that extended the lb method within the
>> 2.1/2.2 proxy from what is basically a weighted request count to also
>> be a weighted traffic count (as measured by bytes transferred) and a
>> weighted "load" count (as measured by response time). The former is
>> further along and the methods will be selectable at runtime... This is
>> definitely a scratch I'm itching,
> I'm sure you are not the only one with that itch.

You are welcome to harvest any plumbing you like from mod_backhand.  It 
does exactly what you want only in apache 1.3.x

>> but before I spend too much (additional) time on it, I'd like some
>> feedback on whether the concept is one we can all get behind.
> FWIW, I like it.
>> I am also toying with the idea of supporting a CPU load method when
>> the origin servers are Apache via a custom response header...

"CPU load" is tricky.  It has observational bias.  System load adjusts 
two slowly.  The number of concurrent connections to each machine 
actually works pretty well as it suggests that the there are that many 
Apache "children" (adapt working as needed for Apache 2) working on the 
box and it lends itself to a current "length of the run queue".  All of 
these methods require total knowledge, otherwise you have contention 
issues and suffer from inaccuracies due to stale information.  There 
are several nice randomized approaches that work well to smooth our 
spikes due to stale data.  mod_backhand uses a simple stackable 
selection mechanism called "candidacy functions" that implement all 
this stuff (optionally as loadable modules) so they "cleverness" of the 
load balancing decisions can be decided later.

// Theo Schlossnagle
// Principal Engineer --
// OmniTI Computer Consulting, Inc. --
// Ecelerity: fastest MTA on Earth

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