- Kopparapu, Chandra.
Load
Balancing Servers, Firewalls, and Caches.
New York: John Wiley & Sons, 2002.
ISBN 0-471-41550-2.
-
Don't even think about deploying a server farm or geographically
dispersed mirror sites without reading this authoritative book. The
Internet has become such a mountain of interconnected kludges that
something as conceptually simple as spreading Web and other
Internet traffic across a collection of independent servers or sites
in the interest of increased performance and fault tolerance becomes
a matter of enormous subtlety and hideous complexity. Most of the
problems come from the need for “session persistence”: when a new
user arrives at your site, you can direct them to any available
server based on whatever load balancing algorithm you choose, but if
the user's interaction with the server involves dynamically generated
content produced by the server (for example, images generated by
Earth
and Moon Viewer, or items the user places in their
shopping cart at a commerce site), subsequent requests by the user
must be directed to the same server, as only it contains the state of
the user's session.
(Some load balancer vendors will try to
persuade you that session persistence is a design flaw in your Web
applications which you should eliminate by making them stateless or
by using a common storage pool shared by all the servers. Don't
believe this. I defy you to figure out how an application as simple
as Earth and Moon Viewer, which does nothing more
complicated than returning a custom Web page which
contains a dynamically generated embedded image, can be made
stateless. And shared backing store [for example, Network Attached
Storage servers] has its own scalability and fault tolerance
challenges.)
Almost any simple scheme you can come up with to get around the
session persistence problem will be torpedoed by one or more of the
kludges and hacks through which a user's packet traverses between
client and server: NAT, firewalls, proxy servers, content caches,
etc. Consider what at first appears to be a foolproof scheme (albeit
sub-optimal for load distribution): simply hash the client's IP
address into a set of bins, one for each server, and direct the
packets accordingly. Certainly, that would work, right? Wrong: huge
ISPs such as AOL and EarthLink have farms of proxy servers
between their customers and the sites they contact, and these proxy
servers are themselves load balanced in a non-persistent manner. So
even two TCP connections from the same browser retrieving, say, the
text and an image from a single Web page, may arrive at your site
apparently originating from different IP addresses!
This and dozens of other gotchas and ways to work around them are
described in detail in this valuable book, which is entirely
vendor-neutral, except for occasionally mentioning products to
illustrate different kinds of architectures. It's a lot better to
slap your forehead every few pages as you discover something else you
didn't think of which will sabotage your best-laid plans than pull
your hair out later after putting a clever and costly scheme into
production and discovering that it doesn't work. When I started
reading this book, I had no idea how I was going to solve the load
balancing problem for the Fourmilab site, and now I know precisely
how I'm going to proceed. This isn't a book you read for
entertainment, but if you need to know this stuff, it's a great
place to learn it.
February 2005 