Principal Java Engineer London – Hybrid (1 day per week onsite) Are you a senior/lead hands-on Engineer who thrives in event-driven, asynchronous environments? Do you enjoy building low-latency, high-throughput distributed systems where correctness, resilience, and performance really matter? This is an opportunity to work at the core of real-time payments infrastructure, helping to power instant transactions for some of the worlds largest banks. We’re hiring on behalf of a leading payments consultancy delivering mission-critical, event-driven platforms that integrate deeply with bank infrastructure to enable secure, real-time payments at global scale. What You’ll Be Doing You’ll play a senior individual-contributor role within a high-performing engineering team, building asynchronous, message-driven systems designed for scale, resilience, and near-zero downtime. Design and implement high-throughput, low-latency systems processing millions of real-time payment events Build event-driven architectures using modern Java and reactive frameworks Work extensively with non-blocking I/O, asynchronous flows, back-pressure, and concurrency Contribute to system and API design, focusing on scalability, fault tolerance, and observability Collaborate closely with architects, engineers, and client teams across Europe Operate in Agile environments (Scrum / Kanban) with a strong engineering culture Tech Stack (current & evolving): Java 17, Spring WebFlux, Akka, Kafka, reactive streams, distributed messaging, cloud-native tooling What You Bring We’re looking for engineers who enjoy thinking in events, streams, and asynchronous workflows, and who take pride in building systems that perform under pressure. Strong hands-on experience with Java 11 (ideally Java 17) in production environments Deep understanding of asynchronous and reactive programming models Proven experience with event-driven architectures and real-time messaging (Kafka, JMS, Akka, etc.) Solid grasp of distributed systems concepts: concurrency, idempotency, fault tolerance, and resilience Experience building and testing production-grade systems using TDD/BDD and CI/CD pipelines Comfortable working on complex, long-lived systems where performance are reliability are critical Bonus (not essential): Experience in banking, payments, or financial systems, or if you’ve built real-time platforms at scale. What’s In It for You £100,000 – £115,000 base salary (DOE) 20% annual bonus Work on event-driven, real-time systems with global impact Hybrid working – 1 day a week onsite (more if you prefer) Exposure to major international banks and payment networks A product-led, engineering-first environment where technical decisions matter