Senior Back End Developer

A system that has run for over twenty years still underpins core production across dozens of sites worldwide. It's now at end of life, and replacing it is a funded, multi-year programme. You'd own the back-end architecture for the new one. This is business-critical in the literal sense: when the current system goes down, production stops. So the standards you set are load-bearing from week one, and the work shows up where the business actually feels it. It's greenfield in the languages that matter (Kotlin, Spring Boot, PostgreSQL, an event-driven core). The harder problem is the knowledge itself. Two decades of hard-won operational logic sits in the heads of a few domain experts and inside a legacy database nobody wants to touch. A large part of the job is building the migration that lifts that knowledge into a clean, event-driven codebase, one area at a time, without dropping a single transaction in a business that never pauses. A few things that matter: You've led back-end architecture on Kotlin or Java, because the domain-driven and event-driven patterns the rest of the team builds on will be yours to set from the start.You've run an incremental migration of a live system before, or the problem genuinely pulls at you, so keeping old and new in step without data loss is the part you're here for.You're comfortable working with non-engineers, because the people who hold the domain knowledge aren't developers, and pairing with them is how it gets captured. Remote, with roughly 1-2 days a month on-site in Scotland. Outside IR35, 12+ month project.