It’s really hard. And really expensive. I used to work in five nine environments, life or death type use cases, and my rule of thumb was that you double your cost for every extra nine you add.

When we got to five nines it was multiple hot standbys with a custom control and orchestration plane - literally custom hardware we had to build. This was for local installations, so not modern cloud environments (it was over a decade ago), but many of the challenges are similar, like session handling, transmission replay and caching, locking, clashing, routing, jitter, latency etc.

6 nines is really really difficult. It’s hard to estimate costs without specific requirements, but a marketplace site with 1000 daily users means you’re expecting about 1 user per minute, which isn’t a lot. I’d imagine you could get by with the cheapest cloud hosting.

The real problem is that most major cloud providers don’t offer 6 nines. Even AWS only offers credits below 99.5%, so you’d want to not lock yourself into a single provider. My best suggestion is to have a small/cheap server with all of the big names and load balance/round robin between them.

This is such an open ended question that nobody can give you an accurate answer as there are so many factors that need to be considered.

How big is the site? How much data is being stored? What is the DB backend? How does it handle failover between DB servers in the event the primary goes down? Is it being hosted in a cloud service, your own DC or a cupboard? Does the location it is being hosted already have redundant power and internet connectivity? Are they diverse, so if one provider fails the other one will remain online? You’d need to maintain separate sites in case one location goes down due to a major event like an earthquake, so you need to replicate data in real-time to your DR location.

There are so many factors to consider and I haven’t named them all. Regardless of what your answer is, it would be very expensive to maintain any server at 6 9’s level of availability. For a marketplace website with only 1000 visitors a day there is no need for that level of availability because there would be times in the day that nobody would be on it. Only a marketplace the size of Amazon would consider that level of availability.

Sure. It just depends on how flexible your definition of "downtime" is.

Possible yes. Cost effective / valid business case probably not. Every extra 9 is diminishing returns: it'll cost you exponentially more than the previous 9 and money saved from potential downtime is reduced. Like you said 32 seconds of downtime, how much money is that for the business?

You're pretty much looking at multiple geographically diverse T4 datacenters with N+2 or even N+3 redundancy all the way up and down the stack, while also implementing diversity wherever possible so no single vendor of anything can cause you to not be operational.

Even with all that though, you'll eventually get wrecked by DNS somewhere somehow, because it's always DNS.