Often the biggest blocker on moving to a new programming language, is the cost of re-writing everything.

Cue some story here on a bank or airline somewhere still relying on cobol backend servers.

These LLM conversions really seem to make modernization of large parts software layers possible!

BadBadJellyBean an hour ago | parent | next [-]

It's not enough to do a rewrite. Someone has to maintain it. Such a huge codebase with literally zero experts is unmaintainable. There is no one who knows how the internals work.

Sure you could keep vibe coding it but I wouldn't bet my data on that. A database needs to be rock solid.

	▲	72deluxe an hour ago \| parent [-]
		This seems to be the issue with using LLMs for any code generation. Even with my own code bases that I've written entirely by hand over years, if I use AI to implement anything, I don't go through the mental model of architecting it, so I don't know how it works. I can only imagine this to be far, far worse for large code bases maintained by a team of people who are all using AI.

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CamouflagedKiwi 2 hours ago | parent | prev | next [-]

I have some familiarity with the bank situation, and while a lot of them are on some very old systems (maybe COBOL, maybe something else, either way they want off it) the cost of actually re-writing the code is far from the most significant issue.

Consider: You have a big mainframe running your tier 1 bank. Assume that you can see all the code on it, and you can feed all that to an LLM if you like. Getting it to spit out a Rust version is not what you actually want - you now have a modern language but it's still a singleton instance, so where do you run it? Most hardware doesn't give you enough uptime for what you need here, because what you actually needed was a re-architecture for distribution / failover / whatever, and while you could ask your LLM to do that you aren't going to run your bank on the result.

	▲	jiggawatts 2 hours ago \| parent [-]
		> while you could ask your LLM to do that you aren't going to run your bank on the result. Why not? I feel like we're entering a new era of prejudice against not a category of humans, but against non-human intelligences. The design patterns for distributed and fault-tolerant systems are well-known and established in the industry. Both humans and AIs are familiar with them! So if you sketch a design for the AI to follow, establish the rules in AGENTS.md, have a robust test suite, use a frontier model dialed up to eleven, etc... why not rely on the LLM output? At the end of the day, humans are not without fault either. I've been wading through some legacy "pre-AI" code recently and it has more bugs than a rainforest! Static fields used incorrectly, causing data races. Floating point types used for money amounts. JavaScript and SQL injection up the wazoo. Wildly unsafe password handling. So on, and so forth. This is the norm for most human-written software, not the exception. As a proof-of-concept, I tried an AI rewrite of one such legacy app[1], and it is not bug free, but it notably has fewer bugs than the original. Different bugs, sure, and I'll have to iron them out after a round or two of UAT, but I'm honestly more confident with what I got from the chatbot than the code inherited from humans. [1] Deals with money, but admittedly at a much lower level of risk and consequence than a banking app running on a mainframe.

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geraneum 2 hours ago | parent | prev | next [-]

> Cue some story here on a bank or airline somewhere still relying on cobol backend servers.

There's existing money and expertise in those environments to rewrite the whole thing, yet they don't. You may loan them free engineers/experts and they might still not rewrite anything.

	▲	marcus_holmes an hour ago \| parent [-]
		It's a clean-cut financial decision. The existing system works. Yes, it costs a lot to maintain, and you could definitely reduce that if you moved to a more modern system. So now you're talking payback periods. Cost of development / maintenance cost savings per year = number of years before you pay back the project. Problem is, that the cost of the development is often unclear, and the maintenance cost savings, while definitely above zero, and often unclear, and approximated the numbers usually come to a payback period in decades. And that's without the usual tech caveats; We can't promise there won't be bugs. We can't promise deadlines will be met. We can't promise the project will succeed at all. We can't promise existing functionality will be faithfully reproduced in the new system. The normal risks around any software dev project. All in all, it looks really expensive and really risky compared to just doing nothing and running the same old system for another five years. Source: I helped do some of the maths on this for a Y2K project.

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rixed 2 hours ago | parent | prev [-]

> the biggest blocker on moving to a new programming language, is the cost of re-writing everything

In 2026, not sure if it was satire. Do some people truly believe that all their software stack has to be single tech, from device drivers to end user apps? Does that extend to remotely accessed services?