To redirect to another server if the primary is not available, how?

At the server level DNS this can be solved — the easiest way to give two addresses, some customers not receiving a reply from the first turn to the second, but some is not smart enough, and some for its some kind of understanding will soon be knocking on the second, so probably not for the plugs, and mirror/replica/cluster, that is, for more or less equal servers (if not equal, you can use round robin dns or other methods of balancing and/or failover).
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Well, the easiest way of seeing that the server is, and when the stand is unknown, replace the IP of the server on the IP in the stub DNS records — after a while, all clients will send to the stub.

You asked a question, the solution of which, in each case, I specialists with a salary over 100k rubles.

You asked a question, the solution of which, in each case, I specialists with a salary over 100k rubles.

only if it is "intermediary" between your VDS-coy and customers. For example, a reverse proxy (nginx, lighttpd, varnish, etc.), or a piece of iron-balancer (cisco ace, crescendo, f5, citrix, etc.), which can determine that the backend went and where to redirect any traffic. Usually on a backup server. But then the point of failure is the node. Further — only by means of BGP.