In an ideal world, you get what you want all the time. The shopping lines are always short, the traffic is always light, and there are more jobs than there are people to fill them. Unfortunately, we do not live in an ideal world, and just like a street can come to a stand-still when there is too much traffic, so can a server.
If you have a single server and its working, you might never suspect you would have a problem, but if too many people visit your server, some visitors could start getting denied access. Even worse, too much congestion can even bring a server down. With redundant servers, you can spread the load over two or more identical servers. The information across both is the same, so it does not matter which one a user gets when they access your site.
The other, perhaps more critical advantage of server redundancy is that even if one server goes down, the other one will continue to work. The more redundant servers you have, the more reliable your site will be. Generally speaking redundancy is probably not necessary for small, low-traffic sites, but for big sites that see huge volumes of regular users, it could be a life saver.
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