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List:       apache-modperl
Subject:    Re: Server Load Balancing: DNS Round-robin vs RewriteMap vs ????
From:       Dzuy Nguyen <dzuy () infinity-studios ! com>
Date:       2003-10-26 15:46:46
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3) Linux Virtual Server (LVS) http://www.linuxvirtualserver.org/

Nigel Hamilton wrote:

> Hi,
> 
> Imagine for a moment ... you're stuck on a deserted island and there's no hardware \
> load balancer vendor in sight. With only coconuts, five linux servers and an \
> Internet connection you need to come up with a software-based load balancing \
> scheme. 
> Which would you choose and why?
> 
> 1) Round Robin DNS
> __________________
> 
> Bind enables you to map a domain name to multiple IP addresses 
> (e.g., search.turbo10.com -> 130.54.23.24, 130.54.23.34). Setting the 
> DNS time to live (TTL) to 60 seconds helps to prevent caching the IP address.
> 
> The disadvantages of DNS round-robin include:
> * client DNS caching(?) may reduce the random distribution of traffic in the \
>                 cluster (is it that bad?)
> * it does not take account of load on any individual server
> * a server may fail and users may experience a dropped connection
> 
> The advantages of DNS round-robin include:
> * simple to implement
> * no load is born by the server to distribute the load (e.g., no need to process \
>                 Apache RewriteMap)
> * no need for a 'point' machine to accept all incoming hits
> 
> ... or ...
> 
> 2) Apache Rewrite Map
> _____________________
> This alternative can distribute the load randomly and also selectively based on the \
> URL (e.g., http://turbo10.com/?u=1212 --> 130.54.23.24, http://turbo10.com/?u=1211 \
> --> 130.54.23.25). 
> The disadvantages include:
> * a machine must act as a proxy for all requests.
> * there is an extra network socket connection required between proxy and server.
> * all HTTP headers need to be inspected and rewritten.
> * proxyReversePass is needed to send cookies back to the client.
> * more complex to implement than DNS round-robin
> 
> The advantages include:
> * the RewriteMap file can be updated based on load averages in the cluster. \
> Allowing heavily loaded machines to receive less traffic and lightly load machines \
>                 to receive more.
> * a cron job could update the map file based on load averages.
> * if a server goes down it can be removed from the RewriteMap (i.e., and the \
> cluster) immediately making the loss of a server invisible to the end user. 
> ... or ...
> 
> 3) Other Software-Based Load Balancing Solution
> _______________________________________________
> ????
> 
> 
> Regards,
> 
> 
> NIge
> 
> Nigel Hamilton
> 
> 


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