So break that logic up into smaller services yet, again splitting up the domain into smaller pieces. No one said a microservice can't call another microservice!

So break that logic up into smaller services yet, again splitting up the domain into smaller pieces. No one said a microservice can't call another microservice! Logins are a bad example because it really shouldn't take 100k lines to handle it, but replace 'logins' with any business domain logic that you need. And honestly, the biggest challenge often is building the simplest API possible without limiting your clients.