Senior Java Engineer - Reactive (Spring WebFlux)

Job DescriptionSenior Java Engineer - Reactive (Spring WebFlux)London - Hybrid (1 day per week onsite) Are you a senior, 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 world's 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 DoingYou'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 eventsBuild event-driven architectures using modern Java and reactive frameworksWork extensively with non-blocking I/O, asynchronous flows, back-pressure, and concurrencyContribute to system and API design, focusing on scalability, fault tolerance, and observabilityCollaborate closely with architects, engineers, and client teams across EuropeOperate 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 BringWe'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 environmentsDeep understanding of asynchronous and reactive programming modelsProven experience with event-driven architectures and real-time messaging (Kafka, JMS, Akka, etc.)Solid grasp of distributed systems concepts: concurrency, idempotency, fault tolerance, and resilienceExperience building and testing production-grade systems using TDD/BDD and CI/CD pipelinesComfortable working on complex, long-lived systems where performance and 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 bonusWork on event-driven, real-time systems with global impactHybrid working - 1 day a week onsite (more if you prefer)Exposure to major international banks and payment networksA product-led, engineering-first environment where technical decisions matter