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I have successfully done a full rewrite. 60kLOC of VBScript/ASP to 20kLOC python/django. Added a ton of missing features that wouldn't have been possible in t

I have successfully done a full rewrite. 60kLOC of VBScript/ASP to 20kLOC python/django. Added a ton of missing features that wouldn't have been possible in the old, clipboard inheritance style. There was one page on the old site that I had tried to fix up a couple of times and failed completely at. It was ~2500 lines. New system it was nothing special, 25 lines in the view and a big but manageable template. I did this whole rewrite in about 9 months including all the work to do the database migration which took a bunch of dry runs and then the actual switchover which took nearly 12 hours to run. What's the catch? I had been working at the company for about 7 years at that point as the sole caretaker of the application so I knew the business needs inside and out, I knew where all the problem areas were and I had plenty of ideas about what features we desperately wanted to add but couldn't. I didn't necessarily add all of them up-front but I was able to make good schema decisions to support those later features. Rewrites aren't impossible by any means, but I suspect my story is more of the exception than the rule.

Eridrus 11y ago
I've done two rewrites at the startup I joined 2.5 years ago of codebases that were our core competitive advantage and that were no longer maintained when I joined. Both of the rewrites were successful, but they were small codebases (5k & 10k LoC) and I was the expert on what they were meant to be doing. The first one was our client-side analytics product that I ported from a single 5k LoC CoffeeScript file to TypeScript, the second was a Node.JS/CoffeeScript application that I ported to Java/Groovy. I think the move to TypeScript was probably the one with the least clear benefit; Our lack of experience with CoffeeScript and the compiler's lax semantics were causing us problems, but we could have moved to pure JavaScript. I like TypeScript and I feel more confident refactoring and merging with a statically typed language, but it's hard to quantify. This took about 6 weeks, though most of my attention going to fighting customer-facing fires during this period. Launching this was excruciatingly stressful though because there was a bug we could only tell existed through aggregate A/B test statistics. The second one from Node.JS/CoffeeScript to Java/Groovy has seen more clear benefits from being on the JVM since our team has more experience with it overall, both in Dev & Ops, and a direct port boosted perf 3x and we've benefited from the more extensive library support and interoperability with other JVM languages we use elsewhere in our system. This took about 2 months to go from nothing to prod. It was also useful organizationally since we'd talked about rewriting it for so long that we'd been treating the existing system as legacy and not wanting to invest any extra time in that codebase for that reason. Releasing this was comparatively easy since it essentially acted as a streaming transformer, taking JSON in from one queue and emitting JSON out another queue without any state and I could confirm that the new system had only irrelevant changes. Still, half of this time was spent on testing.

Eridrus 11y ago

I've done two rewrites at the startup I joined 2.5 years ago of codebases that were our core competitive advantage and that were no longer maintained when I joined. Both of the rewrites were successful, but they were small codebases (5k & 10k LoC) and I was the expert on what they were meant to be doing. The first one was our client-side analytics product that I ported from a single 5k LoC CoffeeScript file to TypeScript, the second was a Node.JS/CoffeeScript application that I ported to Java/Groovy. I think the move to TypeScript was probably the one with the least clear benefit; Our lack of experience with CoffeeScript and the compiler's lax semantics were causing us problems, but we could have moved to pure JavaScript. I like TypeScript and I feel more confident refactoring and merging with a statically typed language, but it's hard to quantify. This took about 6 weeks, though most of my attention going to fighting customer-facing fires during this period. Launching this was excruciatingly stressful though because there was a bug we could only tell existed through aggregate A/B test statistics. The second one from Node.JS/CoffeeScript to Java/Groovy has seen more clear benefits from being on the JVM since our team has more experience with it overall, both in Dev & Ops, and a direct port boosted perf 3x and we've benefited from the more extensive library support and interoperability with other JVM languages we use elsewhere in our system. This took about 2 months to go from nothing to prod. It was also useful organizationally since we'd talked about rewriting it for so long that we'd been treating the existing system as legacy and not wanting to invest any extra time in that codebase for that reason. Releasing this was comparatively easy since it essentially acted as a streaming transformer, taking JSON in from one queue and emitting JSON out another queue without any state and I could confirm that the new system had only irrelevant changes. Still, half of this time was spent on testing.