I failed hard so you don't have to
Meme by Justice For Tharindu
First, you need to make up your mind. If you take this project on you must proactively try to make things better. How to do this will depend on a number of factors and the support of the stakeholders, but you must actively try to make the codebase a better place, or you'd come to the end of your 2 or whatever years looking after this garbage dump and not have anything good to show for it.
Understand the system. Learning the codebase might take time, but try to figure out what the overarching concepts, the business logic, and what users actually want to achieve. What users want to do with the system and what the system currently does might be misaligned, so porting the thing with warts and all into a shiny new shell might not make sense. Use that misalignment, make it a chance to drive a wedge between the client and his software, and...
Try to get the stakeholders onboard. Try to make a point about how the shittiness of the current system is costing them customers and money. Businesses usually care about reputation and the bottom line, so you have to make your case in lingo they understand. Show them the numbers, show them how it could all be better.
Break down the system into parts. Moving everything wholesale into the new system is not going to be practical in most cases. See where you can start porting right now, and get to work. Things that could be services1 on their own, like reporting, exports, and external integrations, are good places to start.
Write tests. If I could go back in time and teach my older self anything, it's the value of writing tests first before getting down to implementations. This one thing will make your code more reliable and add value. In the case of a rewrite, this is even more straightforward: the user journeys are already concrete and you can easily write test cases for them to guide the new implementation.
Get your team excited. This has gone on long enough and I know you guys are fatigued, but we can work together and make life less shit for ourselves and our users. Try to understand their concerns and misgivings, but also don't compromise in your quest to make the world a better place. They'll thank you later.
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I always learn these lessons too late, when the chance for me to act on them has already passed. But hey, maybe it'll come in handy someday.
I don't really mean microservices here, though you could go for it if that makes sense for you. ↩