[OT] Re: Server Load Balancing: DNS Round-robin vs RewriteMap vs ????

[John Fricker <mo...@johnfricker.com>]

Nigel,

In building a testlab at work, I was faced with the same dilemma. 
Initially I used RRDNS to balance load between two web servers and found 
that IE cached IP addresses. This made for disappointing results.

I did find two interesting software products though. Balance 
<http://balance.sf.net/> and Pound 
<http://www.apsis.ch/pound/index.html>. I tried Balance first since it 
was written in C, had a small footprint, and had very few features (it's 
only a tcp proxy with round robin balancing and fail over). For the last 
month, I've used it successfully with intense loads and I've been 
perfectly happy with it. On a 1GHz Athlon (no doubt overkill), I can 
sustain hundreds of connections and many megabits of throughput without 
appreciable latency or cpu load. I use it to proxy http and https 
connections flawlessly. To top it off, it compiled and was usable within 
a few minutes. Reading the documentation took less time than the compile 
and configuring it was even quicker at that. As a C programmer, I 
appreciated the source code and was satisfied with the competence of the 
authors (the munich.net folks).

I'm planning on putting Pound through the paces also but since Balance 
works so well for exactly what I need I'm not terribly motivated. Pound 
has many more features and seems comparable to the Arrowpoint systems we 
use for load balancing and fail over in our production environment. So 
maybe someday I'll have a Pound v. Arrowpoint showdown.

I'd love to hear from others with similar experience.

--John

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
>  
>

Re: [OT] Re: Server Load Balancing: DNS Round-robin vs RewriteMap vs ????

[Les Mikesell <le...@comcast.net>]

From: "Nigel Hamilton" <ni...@turbo10.com>

> I was worried about DNS caching and your IE experience does not
> bode well for the round robin option. Did you set the TTL to less than 60
> seconds?

Individual IE clients will cache the DNS response indefinately regardless
of the TTL. However statistically each has an equal chance of using
each address.  A test with one or a few clients won't show it especially
if they use the same dns server and start at the same time, but it does
work with large numbers of clients.  Also if DNS returns multiple
addresses and the first choice does not respond, IE will move on to
the next in the list - other browsers may not do this as well.

---
  Les Mikesell
    lesmikesell@comcast.net

[OT] Re: Server Load Balancing: DNS Round-robin vs RewriteMap vs ????

[Nigel Hamilton <ni...@turbo10.com>]

HI John,

	Thanks for the options and sharing your experience.

	I'm only starting to realise what a can of worms this is ... and
it's certainly a topic where hearing the experience of others is
invaluable. So I hope Ged and Stas don't get too peeved if we indulge in
some [OT] load balancing banter. ;-)

	I was worried about DNS caching and your IE experience does not
bode well for the round robin option. Did you set the TTL to less than 60
seconds?
	
	Here are the suggested options so far:

	* round robin dns
	* RewriteMap
	* djbdns/tinydns allows dynamic IP address changes for round-robin
load balancing and handling a failed server
	* setting up a linux server as a load balancer - http://www.linuxvirtualserver.org
	* mod_backhand and wackamole
	* Balance (http://balance.sf.net)
	* Pound (http://www.apsis.ch/pound/index.html)
	* custom application level redirects

	I suppose there are two parts to the load balancing problem: 
directing the hits (i.e., requests), and clustering the application.

	'Directing the hits' sounds conceptually straight-forward - a bit
like watering the garden - so you would think there would be a clear 'best 
way to do it' - so I'm sure I'm not alone in wanting to hear other's 
experiences?
			

Nige

Nigel Hamilton
email:	nigel@turbo10.com
tel:	+44 (0) 207 987 5460
fax:	+44 (0) 207 987 5468
________________________________________________________________________________
http://turbo10.com		Search Deeper. Browse Faster.