Senior Software Engineer, Ledger Store in London

Are you curious, motivated, and forward-thinking? At FIS, you’ll work on some of the most complex and impactful challenges in modern banking, helping build next-generation, AI-enabled platforms used by financial institutions worldwide. We foster an environment that values ownership, autonomy, and end-to-end responsibility, where engineers are encouraged to take initiative, make meaningful technical decisions, and ship solutions directly into production. We believe in open, collaborative, and ownership-driven teams with a strong focus on real-world impact.

About the team:

OpenCoreOS is building an AI-enabled banking platform for Tier-1 financial institutions within FIS. Our foundation is a thin ledger: a fast, reliable system built on open technologies that gives banks the freedom to build and evolve their services. We combine the autonomy and technical depth of a startup with the scale and long-term commitment of FIS. We use AI throughout engineering and operations, with engineers accountable for design, validation, security and production outcomes.

About the Role:

You will build, operate and scale Ledger Store, the replicated double-entry ledger that records every money movement on the platform. The engineering challenge is precise: each transfer must apply exactly once, survive crashes and produce consistent state across replicas. These guarantees must hold at low latency and high throughput, across cloud environments and, where customers require it, on-premises deployments.

We look for engineers with experience in distributed or durable systems, strong judgement about correctness and performance, and ownership from design through production operation.

What You Will Work On:

You will build, operate and scale Ledger Store, the replicated double-entry ledger that records every money movement on the platform. The engineering challenge is precise: each transfer must apply exactly once, survive crashes and produce consistent state across replicas. These guarantees must hold at low latency and high throughput, across cloud environments and, where customers require it, on-premises deployments. We look for engineers with experience in distributed or durable systems, strong judgement about correctness and performance, and ownership from design through production operation.

Core Responsibilities:

• Own systems through design, implementation, testing, deployment and production operation.
• Make trade-offs between correctness, latency, throughput and operational complexity explicit.
• Establish observability and alerting that help engineers detect failures and understand their causes.
• Take part in on-call and incident response. Address root causes and prevent related failures.
• Deliver small, traceable changes supported by tests and measured results.
• Use AI-assisted development, MCP servers and agent workflows where they improve engineering or operational outcomes.
• Mentor colleagues and write clear design notes, reviews and operational documentation.

What We Look For:

You do not need every item below. Depth in a few areas matters more than familiarity with every technology.

• Systems experience: You have built and operated a distributed or durable system in production.
• Correctness: You can reason about concurrency, consistency, idempotency and failure modes.
• Performance: You understand how memory management, allocation, copying and concurrency affect runtime behavior. You find bottlenecks by measuring.
• Programming depth: We are language agnostic for this role; Rust experience is welcome. You can learn the tools the problem requires.
• Production ownership: You have debugged critical systems and participated in on-call or incident response.
• Engineering judgement: You evaluate alternatives and explain their trade-offs.
• Communication: You explain complex systems clearly in discussions, design notes and reviews.
• AI-assisted engineering: You use AI thoughtfully and take responsibility for the quality and safety of the result.

Also Valuable:

• Financial ledgers, payments, core banking or banking modernisation.
• Low-latency systems, high-throughput workloads or tail-latency optimisation.
• Deterministic simulation, fuzzing or property-based testing.
• Multi-cloud, hybrid or on-premises deployments.
• Capacity planning, deployment safety and disaster recovery.
• Security and compliance in regulated environments.

How We Work:

We reason from evidence, reproduce failures before fixing them and measure before optimizing. We build safeguards into systems and use tests to challenge our assumptions. The work involves real constraints: network distance between replicas, strict correctness requirements and customer security and platform processes. You will help make these constraints explicit and design systems that operate reliably within them.

Why Join Us?

Work on the system that records money movement at the foundation of banking infrastructure. Join a small, senior team with high autonomy, substantial technical ownership and responsibility for production outcomes.