Server Redundancy

Thu Aug 7 11:28:56 UTC 2003

--On 07 August 2003 08:29 +0100 Simon Lockhart <simonl at rd.bbc.co.uk> 
wrote:
>
> The gated solution sounds interesting, but doesn't automatically
> have the feedback loop of stopping advertising itself when apache
> stops responding, but the box is still up (which is a fairly common
> occurrence in our Apache2 testing).
>

It seems like a fairly trivial hack to put together a script which 
polls HTTP requests to port 80 and drops the loopback service address 
if it is consistently failing.

Then you've just got your BGP convergence time and unequal load 
balancing effects to worry about.

Whilst I'm not knocking Paul's solution in an application like 
running a root NS for which it is perfect, I'm not so sure it's 
necessarily best for every kind of service load balancing.

I've used both the route hack based and commercial NAT load 
balancers, and they both have their place.

Commercial NAT based load balancers are able to do things like 
distribute requests according to actual measured server response 
characteristics. This is great if you have clusters of servers with 
different specs but want to extract the best performance under peak 
load from the whole cluster. It also helps if you are running complex 
services where individual servers can develop a pathological slow but 
not failing response for some reason.

They are also able to do the kind of service polling as above and 
react quicker to a down server than one which relies on routing 
protocols.

Neither of the above are much advantage if you are running a cluster 
of BIND servers who's performance is equal and deterministic and 
where dropping a proportion of requests for a second or two if a 
server ever dies is no big deal.

If you are running complex web services (think expensive per server 
sw licences etc) then the investment in a pair of redundant load 
balancers for the front end to give more consistent performance under 
load as well as resilience can look very sane indeed.

--
    Rob.