Several years ago, our team refactored a large Java control system into a Ruby based one. The Ruby actually performed faster, scaled better, and was much easier to understand and maintain. The ergonomics of Ruby enabled clearer thinking about the problem, leading to the better results.
More recently, we've replaced Python, Ruby, and Java based systems with golang based ones. Not having to lug around a VM and associated other parts (jars, gems, ...) is a huge win. Performance is better across the board, and we've reduced the amount of hardware needed. There's also much better understanding of the code across the whole team.
Out of interest, to what degree do you think the improvement in comprehensibility is from the new ecosystem vs performing a rewrite with learnings from the original write?
In the first case, we inherited the codebase. However, it was clear that too much attention was paid to low level stuff such as threading, etc. Ruby allowed us to more easily focus on the problem with more of a micro services approach. The Java monolith we replaced was emblematic of its time. Frameworks piled on libraries piled on other things. A lot of complexity to achieve the intended function.
The move to golang has been really interesting. The language really does seem to scale well with teams. Its great to be able to pick up even code that you would expect to be complicated, such as the golang tls library, and be able to understand it. The much easier to use concurrency and fantastic standard library mean there's a lot less looking around for which framework to use or which approach to apply.
I want to explicitly call out an important point here. They inherited the original codebase and were able to understand the domain problems and what the original solution got wrong AND what it got right. Pretty much any language or framework they chose to rewrite the application in should have seen similar benefits (even if rewritten in Java). I've seen several projects where a rewrite was given to a entirely new team and those projects tend to be as complex or more so than the original (and tend to take a long time to ship)
Often, it's politically possible to achieve a complete rewrite by switching to another language, while staying in the same language would cause pressure to reuse parts of the old system or refactor the old system. Which is a bit sad.
That's a good point - a could top level comment could be:
"We switched from x to y. It was important because it justified rewriting the thing, which improved performance. Would have loved to rewrite it in x, but support just wasn't there."
I'm slowly converting all my old homegrown Ruby / Perl / Python projects to Go and enjoying it. I never wrote complicated code anyway but Go really pushes you into simple readable code that's much easier to reason about.
More recently, we've replaced Python, Ruby, and Java based systems with golang based ones. Not having to lug around a VM and associated other parts (jars, gems, ...) is a huge win. Performance is better across the board, and we've reduced the amount of hardware needed. There's also much better understanding of the code across the whole team.