If radiation hits, oh say, a CPU register it might just change the value in a register. That is completely uncatchable and almost unobservable on modern hardware.

With any system on relying on hot switching to backups you must have a mechanism to detect errors. The mechanisms exist in many languages there are plenty of robust C++, Java, php, ruby, and sadly cobol systems out there. No new language gets to claim backups as a core feature of the language, unless cobol does too.

I am moving to my third job that has a cobol legacy system being replaced by something new. All three of these shops have backup systems that the business owners would never consider separating from their legacy cobol apps.

It is catchable and observable on modern hardware; mainframes have been doing it for years, with the operation also existing on some specialty Intel chips. Cores are paired, with each operation going through each of them simultaneously with the results compared. If the results don't match, they're run through again. If they still don't match, that pair of cores is shut down and operations are redirected.

Pairing like that would find this kind of error. It is also believable that mainframe shops would be willing to cut their throughput in half to get that level of reliability.

I was completely unaware of it even in a research setting. Yet I was aware of things like ECC RAM, RAID, Chipkill RAM and algorithms for continuous checking of results including some implemented in hardware to isolate faulty CPUs but they all added extra hardware or software and none reused existing commodity cores. Its so simple, of course it should be a thing.

Could you post a link to some reading on this?

This link from wikipedia has pretty much a good laundry list of descriptions different companies have taken for doing lock-stepping, etc https://en.wikipedia.org/wiki/Lockstep_%28computing%29 . It's also one of the reasons this level of machine is so expensive. Each mainframe core would be two cores in a "normal" system, plus mainframes have logic to decommission a cpu without needing a shutdown, by migrating the workload to a spare processor.

I only spoke with such confidence in my previous post because I thought I had worked on high end machines. I worked with a number of Sun and oracle machine with CPU failure detection and CPU hot swapping. But This feature was always flaky. I am eager to learn about what I missed.