Mission criticality, insane parallelism, and a few others. With modern mainframes, you can configure two processors (or more!) to run in perfect lockstep. If o

Mission criticality, insane parallelism, and a few others. With modern mainframes, you can configure two processors (or more!) to run in perfect lockstep. If one of them disagrees, the system can immediately see why and how. Run three processors in lockstep and you have quorum: One disagrees? Kill it, clone one of the others, keep going. Or, isolate it, snapshot the disagreement, hold it hostage (but alive!) and bring in a clone of the others. One of the guarantees of IBM Z/System is "Every read is perfect, every write works." You don't have 2-3 bits of ECC, you have nearly fully redundant RAM. Oh now it has to be encrypted in transit, at rest, and in process? Not a problem, it can do that without changes in code. IBM's mainframes take SIMD a step further. Let's say you're parsing JSONL. With a little work, you turn a traditional serial process into a parallel one, and what would be a massive map-reduce problem is just a matter of openMP coordination. Or better yet, since it's just lines, you can make the system pretend it's a tape full of records that happens to be interleaved by, magically, a bunch of local cores! Amazing. Oh, and now the boss says you need to have physical redundancy. Why stop there? Run your application, in lockstep, across multiple systems separated by miles. Don't need it in lockstep? That's fine, your application still thinks it's running inside one head when it's running inside many independent heads. Oh you need to take down us-west-2b? Hold on let me move that entire workload to us-west-1a. There's also Kubernetes... Here's a good talk on such things: https://www.youtube.com/watch?v=3-NC6ntYAy4 https://www.youtube.com/watch?v=3-NC6ntYAy4