You get an AS number, and announce your own IP space. DNS failover only sort-of works.
Or your subscribe to a "GSLB" service where they do this for you for a significant fee. Or you use a "man-in-the-middle as a service" system like Cloudflare, who do this at an extremely reasonable and/or free cost.
Of course, you still have to deal with the risk of route leaks, BGP route flapping/dampening, and other things which can take your IP addresses offline despite the fact you are multihoming with different carriers in different locations.
So perhaps you setup IP addresses on different ASNs and use both DNS & IP based failover.
But then you find a bug somewhere in your software stack which makes all of this redundancy completely ineffective. So you just take your ball, go home and cry.
Or your subscribe to a "GSLB" service where they do this for you for a significant fee. Or you use a "man-in-the-middle as a service" system like Cloudflare, who do this at an extremely reasonable and/or free cost.
Of course, you still have to deal with the risk of route leaks, BGP route flapping/dampening, and other things which can take your IP addresses offline despite the fact you are multihoming with different carriers in different locations.
So perhaps you setup IP addresses on different ASNs and use both DNS & IP based failover.
But then you find a bug somewhere in your software stack which makes all of this redundancy completely ineffective. So you just take your ball, go home and cry.