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copied!<p><a href="http://www.ultramonkey.org/about.shtml" rel="noreferrer">ultramonkey</a> will give you a good description of how to load-balance across many servers, so you see 1 IP address but the connection gets routed to one of many servers (with fault tolerance included).</p> <p>If you don't have your web servers using the same storage (common storage can be tricky, you have to use a <a href="http://en.wikipedia.org/wiki/Storage_area_network" rel="noreferrer">SAN</a> with a 'shared' filesystem like <a href="http://en.wikipedia.org/wiki/Global_File_System" rel="noreferrer">GFS</a>, or a database) then you should enable <a href="http://www.eggheadcafe.com/conversation.aspx?messageid=31273714&threadid=31227223" rel="noreferrer">sticky sessions</a> which tell the router that each client will choose a server to communicate with the first time, and keep with that server. This is less fault-tolerant (but in the web you'll just have to refresh a broken connection to start over) but much easier to architect (and faster as each web server can remain independent)</p> <p>The other issue you'd need in a truly fault-tolerant situation is to locate the physical servers far away from each other, which raises performance issues for most sharing designs (ie you cannot put all your servers on a SAN if they're in different countries), is to use the multiple servers approach, using a single DNS name and replicate data between them regularly. <a href="http://ntrg.cs.tcd.ie/undergrad/4ba2.01/group8/DNS.html" rel="noreferrer">DNS load balancing</a> is possibly the easiest way of using multiple web servers as a single website.</p> <p>In these cases, the DB can often be a single database that all servers communicate with, or can be shared themselves, using clustering or more often log-shipping to ensure you have a backup ready to come online should the primary fail. Log-shipping is more common for backup servers than are located far away.</p>

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