Principal Software Developer; Web

Description Flex Trade  Systems is a provider of customized multi-asset execution and order management trading solutions for buy- and sell-side financial institutions. Through deep client partnerships with some of the world's largest, most complex and demanding capital markets firms, we develop the flexible tools, technology and innovation that deliver our clients a competitive edge. Our globally distributed engineering teams focus on adaptable technology and open architecture to develop highly sophisticated trading solutions that can automate and scale with your business strategies.

At Flex Trade, we hold our values close to heart, with pride and gratitude, as they guide us in everything that we do. We are dedicated to giving our clients a competitive edge, taking ownership of our responsibilities, being flexible to adapt to ever changing environment and technology, bringing integrity to every interaction and we continue to improve, grow together and collaborate as one team.

All of these while having fun truly makes Flex Trade a wonderful place to work.

The Team We are looking for an amazing developer to join our team building Flex Trade’s OEMS platform, FlexONE, which is our state-of-the-art, fully integrated Order Management System and Execution Management System. This system was built from the ground up starting in 2013 and is written entirely in Scala.

The FlexONE team is split between our Milwaukee office, where the product was first developed, and our London office, which is now predominantly responsible for development of the UI for the application.

The London team is an experienced group of Java and Scala developers, with a deep understanding of UI and UX design. We deliver our software using agile methodologies, and we encourage best practices such as TDD, code reviews and pair programming for knowledge sharing and learning. Our team owns all aspects of the features they work on, from designing solutions to meet customer or product requirements, through implementation, QA and delivery.

Each team member works on every part of the system.

About You As Senior Web Developer, be part of our FlexONE team and own critical product surfaces across our web and mobile applications. You’ll work end-to-end across a Scala + Play Framework backend, and a React + AG-Grid front-end, using gRPC for communication with our core product’s backend systems. You will lead the technical direction as we build our Web app from an MVP to a comprehensive companion to the desktop application, as well as continuing the evolution of our Mobile app.

Responsibilities Design & Build:
Deliver robust, maintainable features across the stack (Scala/Play backend, React/AG‑Grid frontend)
Mobile Migration:
Define the path from Codename One to a JS‑compatible framework (likely React Native), including architecture, incremental migration strategy, and performance/UX parity

Quality & Reliability:
Champion code quality, testing, observability (metrics, logs, tracing), and performance across the stack

Team Growth:
Mentor engineers, assist with hiring, onboarding, and capability development as the team scales.

Collaboration:

Partner closely with the Desktop application development team to ensure feature compatibility and parity

Tech Stack Backend:
Scala, Play Framework, REST/JSON and gRPCFrontend:
React, Type Script, AG-Grid Mobile:
Current app in Codename One, target migration to React Native (or similar JS-based cross-platform framework)
Tooling:
Jenkins, Git Lab, Sonar Qube, IntelliJ IDEA Requirements

Deep Full Stack expertise

Experience building & deploying production web apps, with significant experience in React and modern Type Script Willingness  to take ownership of Scala backend (experience with Scala/Java preferred, but not…