Senior Backend Engineer - API Trading EMEA (F/M/D) in London

Flowdesk's mission is to build a global financial institution for digital assets, designed from the ground up for market integrity and efficiency. To achieve this in a rapidly evolving market, we apply a disciplined, first-principles approach to everything we do. This approach is embedded in our core services, from institutional liquidity provision, trading solutions, OTC execution to our comprehensive treasury management offerings. We seek individuals who are driven by this systematic approach. Joining Flowdesk means you will be a key contributor in building and scaling a more transparent and efficient financial markets infrastructure.

At Flowdesk, our APIs are how the outside world trades with us. We're opening a new role for a Senior Software Engineer - API Trading to design, build, and scale our public trading APIs to a truly competitive level. You'll own the API surface that clients integrate with (REST, WebSocket, FIX), ensure it's clear and well-documented, and work closely with trading, product, and security teams to make it a best-in-class experience. This is a hands-on, senior individual contributor role with strong technical ownership and cross-team collaboration.

• Design and build competitive trading APIs
• Architect and implement public trading APIs (REST, WebSocket, and FIX) in Rust.
• Ensure low-latency, high-throughput access to trading workflows (orders, balances, positions, executions).
• Own the API lifecycle end-to-end.
• Define and maintain OpenAPI specifications and other interface contracts.
• Keep APIs consistent, versioned, and easy to integrate for external clients.
• Drive clear, accurate, and up-to-date API documentation that external developers actually want to read.
• Build secure, robust authentication flows.
• Design and implement authentication and authorization for public trading APIs.
• Apply best practices around key management, signatures, rate limiting, replay protection, and abuse prevention.
• Collaborate across trading, product, and engineering
• Work closely with internal stakeholders to understand trading needs and constraints.
• Translate trading and product requirements into clean API designs and reliable backend flows.
• Partner with frontend/tooling teams (TypeScript) to ensure smooth end-to-end integration.
• Raise the bar on reliability and quality
• Ensure APIs are observable, well-tested, and resilient under peak load.
• Contribute to standards, patterns, and tooling for API development across the team.
• Help review code, mentor teammates, and share best practices in Rust, security, and API design.

Requirements

• English is mandatory.
• 5+ years of backend software engineering experience, ideally in trading, crypto, or high-throughput financial systems.
• Proven track record designing and shipping client-facing APIs used in production.
• Hands-on experience building and operating backend services in Rust.
• Comfortable owning services in production – performance, observability, debugging.
• Direct experience building or maintaining a public API for an exchange or trading firm (or very similar environment).
• Strong knowledge of REST and WebSocket APIs (protocol design, error handling, rate limits, versioning).
• Experience with OpenAPI (specs, code generation, compatibility management).
• FIX protocol knowledge is a big plus.
• Solid experience implementing API authentication (HMAC/key-based auth, signatures, tokens).
• Understanding of common API attack vectors and mitigations (OWASP-style threats, replay attacks, abuse control).
• Comfortable working with TypeScript for SDKs, tooling, or integration layers.
• Able to think from the integrator's perspective – how developers discover, test, and use your APIs.
• Clear, concise communicator in English (written and spoken).
• Able to work closely with trading, product, and other engineering teams, challenge requirements when needed, and converge on practical solutions.

Nice to Have

• Experience in crypto trading, digital asset exchanges, or market making.
• Exposure to matching engines, execution engines, or algorithmic trading systems (designing APIs around them is a strong plus).
• Strong interest or background in security (secure coding practices, threat modeling, cryptographic primitives).
• Contributions to open source around APIs, Rust, or security.

Benefits

• International environment (English is the main language)
• 50% of transportation costs & a sustainable mobility agreement
• Swile lunch voucher (€9.25 per day, 60% covered)
• 100% Alan Blue covered for you and your children
• Wellpass contribution to gym membership
• Team events and offsites

Here's What You Can Expect If You Apply

• HR call (30') – Get to know Flowdesk and discuss your experience.
• Introduction interview with the Head of Engineering (30')
• Technical Test
• Technical round with the Head Of Engineering (60') & Team Fit (60') onsite
• Wrap Up Interview (45') – Learn about Flowdesk's culture and how we work.
• C-Level Interview (30')

We focus on conversations, not trick questions. These discussions will help you understand how Flowdesk operates and allow you to share your journey and what you're looking for in your next role. We are committed to an inclusive and accessible recruitment process. If you require any reasonable adjustments or have specific needs to enable you to participate fully in the interview or assessment process, please contact us to discuss how we can support you.