Imagine that you are doing this at web scale. Instead of one web server you have 2000, and another 1000 middleware servers all talking to a distributed database

Imagine that you are doing this at web scale. Instead of one web server you have 2000, and another 1000 middleware servers all talking to a distributed database cluster with another 500 machines. You can probably run your web servers close to stateless sharing sessions across redis. And your middleware worker machines can all run pub/sub so as a web dev you won't have this problem much. But the developers of your distributed key/value store, your distributed pub/sub queueing system and your database are going to be very different animals. Take a look at the last 10 years of Kyle Kingsbury's work with Jepsen and you'll see what I mean (https://aphyr.com/tags/jepsen https://aphyr.com/tags/jepsen). Opportunities for corruption, unavailability and lossage abound with distributed systems.